# A multiwavelength view of the protostellar binary IRAS 04325+2402: a case for turbulent fragmentation

A. Scholz, K. Wood, D. Wilner, R. Jayawardhana, P. Delorme, A. Caratti O

Garatti, V. D. Ivanov, I. Saviane, B. Whitney

► To cite this version:

A. Scholz, K. Wood, D. Wilner, R. Jayawardhana, P. Delorme, et al. A multiwavelength view of the protostellar binary IRAS 04325+2402: a case for turbulent fragmentation. Monthly Notices of the Royal Astronomical Society, 2010, 409, pp.1557. hal-00632020

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# A multiwavelength view of the protostellar binary IRAS 04325+2402: a case for turbulent fragmentation

A. Scholz,<fnref n="1" />,<fnref n="2" />* K. Wood,<fnref n="2" /> D. Wilner,<fnref n="3" /> R. Jayawardhana,<fnref n="4" /> P. Delorme,<fnref n="2" />

A. Caratti o Garatti,<fnref n="1" /> V. D. Ivanov,<fnref n="5" /> I. Saviane<fnref n="5" /> and B. Whitney<fnref n="6" />

Accepted 2010 July 21. Received 2010 July 16; in original form 2010 April 1

## ABSTRACT

IRAS 04325+2402 (hereinafter IRAS 04325) is a complex protostellar system hosting two young stellar objects (AB and C) at a separation of 1250 au. Here, we present a new deep Gemini imaging and spectroscopy for the system covering the wavelength regime from 1 to 12 µm as well as Submillimeter Array interferometry at 870 µm, in combination with *Spitzer* and literature data. Based on this rich data set, we provide a comprehensive picture of IRAS 04325 over scales from a few au to several parsec. Object AB is a low-mass star with a disc/envelope system and an outflow cavity, which is prominently seen in infrared images. Object C, previously suspected to be a brown dwarf, is likely a very low mass star, with an effective temperature of ~3400 K. It features an edge-on disc and an elongated envelope, and shows strong indications for accretion and ejection activities. Both objects are likely to drive parsec-scale molecular outflows. The two objects are embedded in an isolated, dense molecular cloud core. High extinction, lack of X-ray emission, and relatively high bolometric luminosity argue for a very young age below 1 Myr. The disc/outflow systems of objects AB and C are misaligned by ~60° against each other and by 80° and 40° against the orbital plane of the binary. The system might be a good case for primordial misalignment, as opposed to misalignment caused by dynamical interactions, because the outflow direction is constant and the realignment time-scale is likely larger than the system age. This favours turbulent fragmentation, rather than rotational fragmentation, as the formation scenario. We show that the spectral energy distributions and images for the two objects can be reproduced with radiative transfer models for disc/envelope systems. Our analysis provides reassurance in the established paradigm for the structure and early evolution of young stellar objects, but stresses the importance of developing three-dimensional models with sophisticated dust chemistry.

**Key words:** circumstellar matter – stars: formation – stars: low-mass – stars: pre-main-sequence – stars: protostars.

## 1 INTRODUCTION

The study of protostars and their environment requires the analysis of radiation across a broad range of wavelengths, including the infrared (IR) and the submillimetre/millimetre (submm/mm) regime. At the same time, it is desirable to achieve high spatial resolution to characterize the small-scale structures close to the sources (see re-

view by Watson et al. 2007). However, only a few objects have been studied at uniform and high resolution over the full wavelength domain of relevance (1–1000 µm). Testing the current paradigms for collapse, accretion/outflow, and disc evolution with high-resolution multiwavelength studies of individual objects is thus of major importance.

In this paper, we present a case study for the protostellar system IRAS 04325+2402 (hereinafter IRAS 04325), shown in Fig. 1. Originally identified as LDN 1535 in the catalogue of dark clouds by Lynds (1962), it has been studied extensively over three

<footnote n="1">¹School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland</footnote>

<footnote n="2">²SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS</footnote>

<footnote n="3">³Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138, USA</footnote>

<footnote n="4">⁴Department of Astronomy & Astrophysics, University of Toronto, 50 St. George Street Toronto, ON M5S 3H4, Canada</footnote>

<footnote n="5">⁵European Southern Observatory, Ave. Alonso de Cordova 3107, Casilla 19, Santiago 19001, Chile</footnote>

<footnote n="6">⁶Space Science Institute, 4750 Walnut St. Suite 205, Boulder, CO 80301, USA</footnote>

<footnote>*E-mail: aleks@cp.dias.ie</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1.** K-band close-up view of IRAS 04325 obtained with Gemini-North and NIRI. The image is shown in two different intensity scales: the left-hand panel shows the point sources and bright extended emission close to the southern source AB and the right-hand panel illustrates the faint structure between AB and the northern source C.</figcaption></figure>

decades (117 publications according to SIMBAD). A number of publications starting from 1980s have focused on the molecular gas in L1535 and the large-scale environment (e.g. Ungerechts, Winnewisser & Walmsley 1982; Goldsmith & Sernyak 1984). After the identification of an IRAS source associated with this cloud (Parker 1988, 1991), the object has been analysed in the IR domain (e.g. Hodapp 1994) and in the submm/mm continuum (e.g. Ladd et al. 1991; Ohashi et al. 1996; Saraceno et al. 1996), tracing the dust in the system. IRAS 04325 was found to harbour a protostar with subsolar luminosity and a disc seen at high inclination (Kenyon, Calvet & Hartmann 1993a,b), embedded in a cloud core with a diameter of several tens of thousands of au. In addition, large-scale outflow activity has been identified (e.g. Heyer et al. 1987; Moriarty-Schieven et al. 1992; Wang et al. 2001). The source is established as a prototypical small-scale site of star formation.

Until 1999, the structures within the IRAS beam have not been resolved, which results in various problems. The near-IR images clearly show a bright nebulosity, indicating that the fluxes from IRAS and single-dish observations are contaminated by extended emission. As a result, the properties of the central source and its close environment are poorly constrained. Confusingly, the direction of one of the large-scale outflows does not agree with the orientation of the IR nebulosity, interpreted as outflow cavity, as pointed out by Kenyon et al. (1993a). Hartmann et al. (1999) presented HST imaging of the object, revealing complex substructure. In particular, the images show the presence of a second well-separated point source (IRAS 04325C), which has a resolved disc seen edge-on. The main light source IRAS 04325AB is speculated to be a binary. In addition, there are various scattered light features.

Originally fuelled by the suspicion that the second source IRAS 04325C might be a proto brown dwarf, we have conducted our own observations of the system. In our first paper (Scholz et al. 2008), we detect the second component with 1.3-mm interferometry and show the first IR spectrum for this object. Here, we present a comprehensive data set for the full IRAS 04325 system, including new near- and mid-IR imaging, near-IR spectroscopy for both components, and submillimeter interferometry. We consistently assume

a distance of 150 pc for the Taurus star forming region (Loinard et al. 2007), when converting from apparent to physical separations. All physical distances are given in projection against the plane of the sky and should be seen as lower limits to the actual distances.

## 2 OBSERVATIONS AND DATA REDUCTION

### 2.1 Near-IR imaging

Near-IR images in the *J*, *H* and *K'* bands were taken on the 2008 August 23 with the NIRI camera (Hodapp et al. 2003) at the Gemini-North 8.1-m telescope.<fnref n="1" /> All Gemini observations for IRAS 04325 were taken in queue mode in the band-2 program GN-2008B-Q-73. We used the f/6 camera with a scale of 0.12 arcsec pixel⁻<fnref n="1" /> and 120 × 120-arcsec² field of view (FOV). A seven-step dither pattern was carried out in each band with 15-arcsec offsets and 10 s integration time per position. The total on-source time in each band was 70 s. The seeing measured from one to three well-detected point sources outside IRAS 04325 was 0.5 arcsec.

The reduction of the NIRI *J*, *H*- and *K'*-band images proceeded as follows: a sky image for each filter was created using the dithered science images. The sky was then subtracted to each science image using the ESO eclipse package (Devillard 1997). We carried out a flat-field correction using domeflats. The alignment and co-addition of the images was done with the programs Scamp (Bertin 2006) and Swarp (Bertin et al. 2002) to produce the final science stacks.

In the same Gemini program, we obtained *L*'- (3.8 µm) and *M*'-band (4.7 µm) images with NIRI's f/32 camera (0.02 arcsec pixel⁻<fnref n="1" />) and a FOV of 22.4 × 22.4 arcsec². In the *L*-band, a five-step dither pattern with 3-arcsec offsets was executed twice with 20 co-added 1-s exposures at each position, which gives a total on-source time of 200 s. The same pattern was carried out 13 times in the *M* band with 30 co-added 0.5-fs exposures, in total 25 min. These observations were obtained on the 2008 December 3, including a set of images for the bright A0 star HD36719 for calibration purposes. All data were taken at low airmass (<1.2); the airmass difference between the science and calibration target was ≲0.1.

For the reduction of the *L*- and *M*-band data, we followed the standard recipes based on the tools in the Gemini/NIRI package within IRAF. Flats and sky images were created based on the dithered science images, discarding the first frame. In the *L* band, all nine available images were used for the sky frame. The *M*-band sequence is long enough for the sky to change significantly. For each individual frame, we used the nine frames taken closest in time to create the sky image. After subtracting the sky, the science frames were divided by the normalized flat-field. The resulting images were mostly flat with the exception of the first and last two frames in the *M*-band suffering from imperfect sky subtraction. These images with an uneven background were discarded. To co-add the individual images, we used the standard combine routine in IRAF. The images of the standard star for *L*- and *M*-band were handled in exactly the same way.

<footnote n="1">¹ The Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministrio da Cincia e Tecnologia (Brazil) and Ministerio de Ciencia, Tecnologa e Innovacin Productiva (Argentina).</footnote>
## 2.2 Mid-IR imaging

IRAS 04325 was observed with Gemini-North and Michelle (Bryson, Glasse & Atad-Ettedgui 1994) in the N' band with a central wavelength of 11.2 µm on 2008 November 3. To sample the sky properly, the standard procedure, including chopping and nodding, was used. In short, the pointing was chopped at 4.2 Hz between the target and an adjacent position 1 arcsec to the east by moving the secondary mirror. In addition, the whole telescope was nodded in the same direction in larger time-intervals. The chop throw was chosen to avoid the extended emission from IRAS 04325 from the sky field. The exposure time per frame was 0.02 s; in total, we integrated 2 × 590 s on source. The airmass for the two science integrations was 1.07 and 1.02. In addition, the standard star HD20893 was observed at airmass 1.02.

For the standard reduction, we used the Gemini/MIDIR package in IRAF and followed the 'cookbook' for Michelle provided by Gemini. This includes the tasks miregister to prepare and reformat the data cubes and mistack to average by co-adding the signal from each nod position and dividing by the number of frames. A residual pattern was removed by fitting each column with a constant and subtracting this fit. Source C is weakly visible only in one of the resulting stacked frames – the one taken at lower airmass. The peak intensity for object C is about three times the noise in the sky background. The signal does not improve significantly when co-adding the two images. Only the image with detection was used for the photometry. The standard star frames were reduced in the same fashion as the science images.

## 2.3 Near-IR spectroscopy

Object AB was observed with SofI (Morwood 1997) in spectroscopy mode at the ESO/NTT in La Silla. We obtained this spectrum and the associated calibration files from the ESO archive. The low-resolution Red Grism was used with coverage from 1.5 to 2.5 µm and a 0.6-arcsec slit, which results in a resolution of *R* ~ 1000. Two pairs of nodded integrations (ABBA) were observed with a nodding step of 20 arcsec and 225-s exposure time per position. The total on-source time was 900 s. A telluric standard star (HIP 23719, spectral type G1V) was observed immediately after the science target.

We carried out a standard reduction, including subtraction of the nodded pairs (A–B) to remove the sky and flat-field correction. The four spectra were extracted with a 1.2-arcsec aperture, wavelength-calibrated using Xenon arc spectra, and co-added. The telluric spectrum was treated in the same way. The combined spectrum for IRAS 04325AB was then divided by the telluric spectrum and multiplied by the solar spectrum. The final science spectrum is shown in Fig. 4 (shown later).

We obtained a spectrum for object C on 2008 October 14/15 with Gemini-North and NIRI, using the f/6 camera, the 6-pixel slit mask (slit width = 0.72 arcsec), and the H- and K-band grisms. This gave a resolution of *R* ~ 500 over a wavelength range of 1.4–2.5 µm. The slit was aligned in the east–west direction. The total on-source time of 3000 s per band was split in 10 single integrations. To correct for the variable background emission, the object was moved along the slit in steps of 2 arcsec between the single exposures. For telluric correction, we observed the early A stars, HIP17791 and HIP23088, in the same observing nights.

The NIRI spectra were extracted and calibrated using our own IDL procedures. The frames were flat-fielded using an internal flat taken immediately after the science frames. Consecutive exposures are subtracted to remove the sky background. The 10 individual

spectra are then extracted and median-combined. The same operation is performed for the telluric calibration stars. From the standard spectra, we derived a telluric spectrum by dividing through a blackbody spectrum with a temperature of 10 000 K and interpolating over the hydrogen lines. The science spectrum was divided by the telluric spectrum. The wavelength calibration was carried out based on the bright OH lines, which are visible in the (unreduced) science images. The final H- and K-band spectra are shown in Fig. 5 (shown later).

## 2.4 Archival images

To complement our data set, we used archival images for the Taurus region. Our target and its environment is covered by the TAP Survey, carried out with the near-IR camera WFCAM at the United Kingdom Infrared Telescope<fnref n="2" /> (Davis et al. 2008). The survey provides a data base of near-IR images in the K-band and the narrow-band filter centred on the 1–0 S(1) hydrogen emission feature at 2.122 µm (hereinafter called H₂ band), which we use to look for shocked emission caused by molecular outflows.

In addition, we downloaded the pipeline-reduced PBCD Spitzer images from the 3rd delivery (DR3) of the Taurus Spitzer Legacy project (PI: Deborah Padgett, program id #3584) from the NASA/IPAC Infrared Science Archive. This includes the IRAC data in the four channels at 3.6, 4.5, 5.8 and 8.0 µm, as well as the MIPS images at 24 and 70 µm.

## 2.5 Submillimeter Array interferometry

A field centred on IRAS 04325 was observed with the Submillimeter Array (SMA)<fnref n="3" /> on 2008 October 20 using eight antennas in a compact configuration that provided baseline lengths of 11–77 m. The weather was good, with system temperatures of 200–250 K (DSB) near transit. The phase centre was RA = 04°35′35.29, Dec. = +24°08′27.6 (J2000), which is the position of IRAS 04325C. The correlator was configured for the full 2 GHz bandwidth, including one spectral chunk of 82 MHz in the middle of the upper sideband devoted to the CO J = 3–2 line at 345.786 GHz with a channel spacing of 0.20 MHz (0.176 km s⁻¹). The SMA primary beam full width at half-maximum is 35 arcsec at this frequency. Calibration of complex gains was performed by interleaving 3-min observations of the quasars 3C111 (4.30 Jy) and J0530+135 (0.84 Jy) with 15-min observations of IRAS 04325 over the hour angle range −5 to +5. The flux scale was set using observations of Uranus, with an estimated uncertainty better than 20 per cent. The passband was calibrated using observations of the strong sources 3C454.3 and Uranus. The data calibration was done with the MIR/IDL software, followed by standard imaging and deconvolution tasks within the MIRIAD package. For the continuum image, the beam size (robust = 0) was 2.0 × 1.6 arcsec<fnref n="2" /> at PA = −68°, and the rms noise 2.2 mJy beam⁻¹, after combining the lower and upper sidebands (effective bandwidth ~4 GHz). For the CO line images, the beam size (natural weight) was 2.1 × 1.8 arcsec<fnref n="2" /> at PA = −56°, with rms

<footnote n="2">² The TAP survey is hosted by the Joint Astronomy Center. Image reduction was carried out by the Cambridge Astronomical Survey Unit for the WFCAM Science Archive. For more information, see http://www.jach.hawaii.edu/UKIRT/TAP/</footnote>

<footnote n="3">³ The SMA is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica.</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** SMA 870-µm dust continuum image showing the two sources AB and C and associated extended emission. The ellipse in the lower left-hand corner shows the $2.0 \times 1.6$-arcsec$^2$, PA = -68° beam. The contours are in steps of 4.4 mJy beam$^{-1}$ (twice the rms noise level). Negative contours are shown by the dashed lines.</figcaption></figure>

noise $0.26$ Jy beam$^{-1}$ in each spectral channel. The continuum image is shown in Fig. 2; the CO line maps for a sequence of velocity bins are plotted in Fig. 3. Since the interferometer observations are sensitive only to small-scale structures ($\lesssim 10$ arcsec), the sources seen in the CO maps are not isolated and should be seen as local overdensities and temperature enhancements.

# 3 QUANTITATIVE ANALYSIS

## 3.1 Object AB: near-IR spectroscopy

The spectrum of the source AB is red and entirely featureless (see Fig. 4), without the broad water absorption bands usually seen in late-type objects (Cushing, Rayner & Vacca 2005). The slope from

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Spectrum for AB obtained with NTT/SOFI. Overplotted with the dashed lines are the spectra for a K7V star (above) and a M4V star (below the science spectrum), from the IRTF/SpEx Spectral Library (Rayner et al. 2009). Both comparison spectra are reddened with $A_V = 30$ mag.</figcaption></figure>

1.5 to 2.3 µm is well approximated by a straight line. There may be a hint of CO absorption bands at $>2.3$ µm.

The lack of features precludes a detailed spectral analysis; instead, we simply compare the slope with reddened spectra for late-type objects from the IRTF/SpEx Spectral Library (Rayner, Cushing & Vacca 2009). G-, K- and early M-type spectra reddenend with $A_V \sim 30$ provide a good match. As an example, we show in Fig. 4 the spectrum for the K7V star HD237903. Towards mid-M spectral types, the fit becomes progressively worse, no matter which extinction is chosen. In particular, the slope over H and K band is no longer constant, due to the water absorption feature between the two bands. In addition, the CO bands and water absorption cause the flux level to drop at $>2.2$ µm. The mismatch is shown in Fig. 4, where we compare with the spectrum of the M4V star Gl 299 reddened with $A_V = 30$ mag. Thus, a spectral type later than early M seems unlikely.

This limit on the spectral type, however, needs confirmation, particularly because the spectrum may be affected by additional continuum emission from the disc or accretion. The slit orientation in the east-west direction avoids the scattered light features north and south of the source (see Fig. 7 shown later), but veiling from

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Contour maps of the continuum-subtracted CO line observations with the SMA. The contours are in steps of 1.5 Jy beam$^{-1}$; negative contours are shown by the dashed lines. The LSR velocity in km s$^{-1}$ is indicated for each panel. The positions of the compact sources AB and C, as measured in the continuum map, are marked with crosses.</figcaption></figure>
the inner disc (within 0.5 arcsec) cannot be excluded. Lacking high-resolution spectroscopy, we cannot put limits on the veiling. Typical excess continuum in T Tauri stars makes the near-IR spectrum redder (Folha & Emerson 1999), that is, the spectral type may be earlier than estimated or the extinction may be overestimated.

## 3.2 Object C: near-IR spectroscopy

The spectrum of object C shows features of the photosphere, in particular, a flattening at the red end of the *H* band and a triangular shape in the *K* band (Fig. 5). The spectra in the *H* and *K* bands were compared separately to model spectra. We neglect that the spectra might be affected by scattering and veiling. Both effects are expected to be smoothly and slowly varying functions of wavelength; thus, they should not affect the spectrum in individual bands significantly.

For the comparison, we used a series of AMES-DUSTY model spectra (Allard et al. 2001) with effective temperatures ranging from 2000 to 4000 K and $log g = 3.5$ or 4.0, as expected for young stars. We varied $T_{\text{eff}}$ and $A_V$ and aimed to obtain a consistent solution for the two bands. We find a decent match between observed and model spectra for $T_{\text{eff}} = 3400 \pm 200$ K and extinction $A_V = 22 \pm 2$ mag. As shown in Fig. 5, this fits *H* and *K* bands equally well.

It is impossible to match the observed spectrum with a model for an object close or below the substellar boundary ($T_{\text{eff}} \lesssim 3000$ K), as claimed by Scholz et al. (2008), particularly not in the *H* band. Young brown dwarfs exhibit a characteristic triangular-shaped *H*-band spectrum due to water absorption (Brandeker et al. 2006; Kirkpatrick et al. 2006; Scholz et al. 2009), which is not seen in our data. We conclude that IRAS 04325C is unlikely to be a brown dwarf; its spectrum is more consistent with being a very low mass star with an effective temperature around 3400 K or spectral type of M3–M4 (Mentuch et al. 2008).

Various emission features associated with molecular hydrogen are clearly detected in the *K* band, particularly 1–0 S(2) at 2.034, 1–0 S(1) at 2.122, 1–0 S(0) at 2.223, 1–0 Q(1) at 2.407 and 1–0 Q(3) at 2.424 µm. In addition, there is a Brγ emission feature at 2.166 µm, commonly associated with ongoing accretion (e.g. Natta, Testi & Randich 2006). The *H* band shows the [Fe II] lines at 1.644, 1.669, 1.677 and 1.748 µm (the latter might be blended with a hydrogen feature). While the lines of molecular hydrogen can originate in the disc or in a jet, the [Fe II] emission can only occur in the jet.

Thus, our spectra prove ongoing accretion and outflow activity in IRAS 04325C. From the way we have analysed the spectra, it is certain that the emission originates in an area within the point spread function (PSF) of the point source, that is, within a radius of 0.5 arcsec (75 au) around the source.

The [Fe II] lines provide a constraint on the electron density $n_e$ in the jet (e.g. Nisini et al. 2002; Garcia Lopez et al. 2008). The flux ratio between the well-detected lines at 1.644 and 1.677 µm is 2.9, which gives $n_e > 10^5$ cm$^{-3}$. Note that the [Fe II] line ratio saturates for larger densities. Consistently, the lower limits to the flux ratios of 1.544 versus 1.644 µm and 1.600 versus 1.644 µm yield densities of $>0.5 \times 10^5$ and $>1.0 \times 10^4$ cm$^{-3}$, respectively. Such high values are usually found at the base of the jet, while the density typically drops farther away from the source (e.g. Takami et al. 2006; Garcia Lopez et al. 2008). Thus, the analysis confirms that the [Fe II] emission originates in a jet close to IRAS 04325C.

The Brγ line is used to derive an estimate for the mass accretion rate, following Natta et al. (2006). The approximate equivalent width in this line is $7 \pm 2$ Å. Scaling with the *K*-band magnitude of 8.5 mag for a 3400-K star at 1 Myr from Baraffe et al. (1998) gives a line luminosity of $log(L_{\text{Br}\gamma}/L_{\odot}) \sim -4.1$. With the empirical relations by Calvet et al. (2004), this translates into an accretion luminosity of $log(L_{\text{acc}}/L_{\odot}) \sim -0.8$ (see also Muzerolle, Hartmann & Calvet 1998). The mass accretion rate is then $\dot{M} = L_{\text{acc}}R/GM$. Assuming a mass of $0.4 M_{\odot}$ and a radius of $1.5 R_{\odot}$ results in an accretion rate in the range of $(1.0 \pm 0.3) \times 10^{-8} M_{\odot} \text{ yr}^{-1}$, comparable to accretion rates derived for T Tauri stars but higher than those for young brown dwarfs (Mohanty, Jayawardhana & Basri 2005).

## 3.3 Photometry of point sources

We derived magnitudes/fluxes for the two components of IRAS 04325 from the Gemini near- and mid-IR images as well as from the submm continuum map.

For the JHK images, we adopted an aperture of 1.2 arcsec (10 pixel) and a background annulus between 1.2 and 1.8 arcsec (10–15 pixel). The field contains one 2MASS source outside IRAS 04325 with $J = 17.98$, $H = 16.41$ and $K = 14.64$, which was used to shift the instrumental magnitudes into the 2MASS system. The resulting values are listed in Table 1. In the *J* band, there

<figure><img src="image_7.png" /><figcaption>**Figure 5.** *H-* and *K*-band spectrum from Gemini/NIRI (solid line) in comparison with a model (DUSTY, 3400 K, $log g = 4.0$, $A_V = 22$ mag, see Allard et al. 2001). The model (dashed line) is shifted in the *y*-direction for clarity. The emission lines detected on the $3\sigma$ level are marked with the dot–dashed lines (see text for more information).</figcaption></figure>

<figure><img src="image_8.png" /></figure>
<table><caption>**Table 1.** IR photometry for the point sources in IRAS 04325. If not specified, the uncertainty in the fluxes is ~10 per cent; this includes the calibration error. For more details, see Section 3.3. The fluxes at 1300 µm are from Scholz et al. (2008).</caption><thead><tr><th>Band</th><th>λ (µm)</th><th>AB</th><th>C</th><th>Unit</th><th>Aperture (arcsec)</th></tr></thead><tbody><tr><td>J</td><td>1.25</td><td>16.54</td><td>&lt;18.6</td><td>mag</td><td>1.2</td></tr><tr><td>H</td><td>1.65</td><td>14.37</td><td>16.92</td><td>mag</td><td>1.2</td></tr><tr><td>K'</td><td>2.15</td><td>11.87</td><td>14.75</td><td>mag</td><td>1.2</td></tr><tr><td>L'</td><td>3.78</td><td>10.36</td><td>13.4 ± 0.2</td><td>mag</td><td>1.0</td></tr><tr><td>M'</td><td>4.68</td><td>9.37</td><td>12.3 ± 0.4</td><td>mag</td><td>1.0</td></tr><tr><td>N'</td><td>11.2</td><td>65</td><td>3.4 ± 1.1</td><td>mJy</td><td>1.2</td></tr><tr><td></td><td>870</td><td>55</td><td>41</td><td>mJy</td><td>3.0</td></tr><tr><td></td><td>1300</td><td>10 ± <sup>3</sup>&frasl;<sub>1</sub></td><td>10 ± <sup>3</sup>&frasl;<sub>1</sub></td><td>mJy</td><td>2.0</td></tr></tbody></table>

is diffuse emission at the position of object C, but no clear point source; we consider the flux measurement to be an upper limit. The previously published photometry of IRAS 04325 has been obtained with larger apertures (Hartmann et al. 1999; Connelley, Reiputh & Tokunaga 2008). Hence, these literature fluxes are significantly larger than ours, because they include extended emission.

For the L'-, M'-, and N'-band photometry, we used apertures of 1.0 or 1.2 arcsec and a sky annulus similar to the one for JHK. In these bands, the extended emission is less problematic; thus, the choice of the sky annulus does not significantly affect the results. The L'/M'-band magnitudes were shifted to the standard system based on the values for the calibration star HD36719, which has 5.4 mag throughout this wavelength regime (Leggett et al. 2003). For the N'-band magnitudes, we used the star HD20893 for calibration, for which an N'-band flux of 4.3 Jy is reported on the Gemini website,<fnref n="4" /> based on Cohen et al. (1999).

The fluxes in the 870-µm continuum map, measured within a 3.0-arcsec aperture, are 55 and 41 mJy for AB and C, respectively.

The uncertainties were estimated by varying the size of the aperture and the sky annulus and checking the changes in the fluxes. Combined with the uncertainty in the fluxes used for the calibration, this mostly yields errors ≲10 per cent. The exceptions are the L'- and M'-band images for object C, where the errors are substantially larger.

In Fig. 6, we analyse the near-IR colours and magnitudes of the two sources (filled symbols). In addition, we estimated the photospheric colours and magnitudes by comparing the constraints on the effective temperatures with the 1-Myr evolutionary track by Baraffe et al. (1998) (empty symbols). For source C, we used $T_{\text{eff}} = 3400$ K, as derived by comparing the spectrum to models (Section 3.2). For object AB, we only have a limit on the spectral type (≪early M), which corresponds to $T_{\text{eff}} \gtrsim 3600$ K (Mentuch et al. 2008). Thus, the plotted flux constitutes a lower limit as well. These photospheric data points may be uncertain in the y-axis by up to ±1 mag due to the unknown age and the uncertainty in $T_{\text{eff}}$, but this does not affect the following interpretation significantly.

For object AB (square symbols), the colours are well explained by the extinction path (dotted line), indicating a line-of-sight extinction of $A_V \sim 30$ mag, in agreement with our constraint from the spectrum (Section 3.1). Thus, both spectroscopy and photometry are consistent with an extincted photosphere of a single low-mass star. For object C (octagons), however, it is clear that extinction alone cannot explain the H-K colour. Object C appears to be 6 mag fainter in the K-band image than expected from the photospheric

<figure><img src="image_9.png" /><figcaption>**Figure 6.** Colour-magnitude diagram illustrating the interpretation of the near-IR photometry. The filled symbols show measured photometry; empty symbols show the approximate photospheric fluxes obtained by comparing the constraints on $T_{\text{eff}}$ with the 1-Myr evolutionary track by Baraffe et al. (1998). The squares correspond to object AB and octagons correspond to object C. The dotted line is the reddening path for $A_V = 1-33$ with the Mathis (1990) extinction law. The dashed line shows the standard scattering model MRN12 from Kenyon et al. (1993b) with inclinations ranging from $\cos i = 0.975$ to 0.0. The solid line is the scattering model plus extinction.</figcaption></figure>

fluxes derived from the spectrum. Scattered light models provide a much better fit. The solid line shows the model MRN12 from Kenyon et al. (1993b) plus an extinction of $A_V = 15$ mag, which allows to reproduce the K-band flux and the colour simultaneously. This analysis confirms that the object is mostly seen in scattered light through an edge-on disc.

In summary, the information from photometry and spectroscopy indicates that objects AB and C are both low-mass stars. Since the mass of object AB is likely to be higher than the mass of object C and object AB is located closer to the core of the nebula, object AB is probably the most relevant centre of infall in the IRAS 04325 system.

## 3.4 PSF fitting

It has been speculated that object AB might be a binary (hence the name). Based on the HST images, Hartmann et al. (1999) argued that ‘there is a double structure, possibly indicating two sources or possibly indicating a dark absorption lane running roughly east-west across the object.’ In the HST images, the separation between the two components is in the range of 0.2 arcsec roughly in the north-south direction, corresponding to ~30 au. The ‘dark lane’ would indicate an absorption feature in front of the object, possibly a disc with diameter of a few tens au.

Based on our K-band image, we test for the presence of a second source in AB by constructing a model PSF from the three well-detected field stars outside IRAS 04325. All sources in the image are fitted with this model PSF, using daophot within IRAF. The $\chi^2$ of the fit is <2 for all field stars, 3.2 for object C and 12.0 for object AB. The contour plot of the residuals (Fig. 7), after subtracting one PSF, does not show any evidence for a second point source, and fitting two PSFs instead of one does not improve the fit. The high $\chi^2$ is most likely caused by the strongly uneven background and not by a stellar companion.

Thus, we prefer to interpret the double structure seen in the HST image as an indication for a disc seen at high inclination that bisects the image of the star, rather than the presence of a resolved

<footnote n="4">⁴ http://www.gemini.edu/sciops/instruments/mir/MIRStdFluxes.txt</footnote>
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Contour maps of the *K*-band image from Gemini/NIRI around the source IRAS 04325. One small tickmark corresponds to 0.6 arcsec (5 pixel). The left-hand panel shows a small area around source AB after subtracting one point source centred at the peak of the emission. The total image size is 6 × 6 arcsec². The two extended structures D and E to the south and north of the point source are clearly visible. The right-hand panel shows the northern part of the source around object C; the image size is 10 × 10 arcsec². The dashed lines mark the light streaks connecting AB and C and extending further to the north. Note that the dark lane from the disc around C, as seen in the *HST* image, is not resolved here.</figcaption></figure>

companion. This finding is supported by the combined information from photometry and spectroscopy; for a binary, we would expect a mismatch (i.e. a later spectral type than expected from photometry). The orientation of the 'lane' is roughly perpendicular to the outflow emanating from AB (Section 4).

In the submm continuum image, both AB and C are spatially resolved. The spatial structure is studied after discarding the data from baselines shorter than 44 m, which effectively removes the structures larger than a few arcsec. We used the task uvfit in the *MIRIAD* package to fit the visibilities of the sources. Various combinations of point sources and Gaussian sources were tried for both objects.

The northern source C is well matched by an elliptical Gaussian with axes of 1.2 and 0.2 arcsec and a position angle (PA) of ~50° (Fig. 2). Note that the PA of the beam is from south-east to north-west, that is, perpendicular to the orientation of the source. The parameters of the Gaussian have large uncertainties, but it is safe to conclude that the PA is consistent with the orientation of the disc, as inferred from the *HST* images (PA = 30°, Hartmann et al. 1999). The same *HST* images also constrain the radius of the disc to 0.2 arcsec (~30 au), which is not resolved by the SMA. The elongated structure seen in the submm data might be caused by a cold outer disc or by an elongated circumstellar envelope with a diameter of ~1–2 arcsec, that is, 150–300 au. This size matches well with the constraint from the spectral energy distribution (SED) modelling (Section 5.2).

The southern AB source contains a compact component that is well matched by a point source, likely caused by a small-scale disc. In addition, there is a contribution of spatially extended emission, mostly oriented in the east-west direction over an area of 3–5 arcsec.

## 3.5 Astrometry

Based on the PSF fit discussed in Section 3.4, the relative positions of AB and C were determined with an accuracy in the range of 0.1 arcsec (1 pixel). The distance between the two sources measured

from the peak position of the model PSF is 8.32 arcsec. Simply measuring the positions of the emission peaks also gives a consistent distance of 8.3 arcsec. From the coordinates given by Hartmann et al. (1999), based on images from 1997 November, we infer a distance between AB and C of 8.23 arcsec. Within the error bar, both measurements are consistent.<fnref n="5" /> The PAs from the available near-IR images agree well: we measure 351°±8 (east of north), while the literature values are 351°±4 (Connelley et al. 2008) and 351°±6 (Hartmann et al. 1999), all with uncertainties of ≲1°.

These measurements constrain the relative proper motion between AB and C to <9 mas yr⁻¹. For comparison, the average proper motion of young stars within 5° of IRAS 04325 is 23 mas yr⁻¹ (Ducourant et al. 2005; Luhman et al. 2009), with a standard deviation of 10 mas yr⁻¹. While the constraints on the proper motion for AB and C do not prove that the object is a physically bound system, they do confirm common membership in the Taurus association.

The angular distance between AB and C corresponds to a separation of 1250 au and a relative movement ≲15 au over 11 yr, which results in an upper velocity limit of 6 km s⁻¹ between AB and C. This does not rule out the possibility that C has been much closer to AB in the past: with a velocity of 6 km s⁻¹, it would have taken only 10³ yr for C to move to its current position from a starting point close to AB. It is thus conceivable that C has been ejected by a close dynamical encounter with AB in the early evolution of the system.

# 4 MORPHOLOGY OF IRAS 04325

## 4.1 Structure within IRAS 04325

The IRAS source is located within the small dark cloud L1535, a part of the B18 cloud complex in Taurus. The few field stars visible in

<footnote n="5">⁵ For completeness, the relative distance from the astrometry given by Connelley et al. (2008), based on imaging from 2003-2005, is 8.03 arcsec; the accuracy of their astrometry is not clear.</footnote>
both *H* and *K* band have *H* − *K* colours of 2–3 mag, corresponding to extinctions of 25–35 mag, which indicates substantial amounts of gas and dust along the line of sight.

The near- and mid-IR images of IRAS 04325 show a bright elongated emission nebula (Hodapp 1994; Hartmann et al. 1999; Connelley, Reipurth & Tokunaga 2007). In our images in the *JHK* bands, the dimensions of this nebulosity are about 10 × 20 arcsec², corresponding to 1500 × 3000 au, with a PA of ~15°–20°. The nebula is divided into two lobes. While the northern lobe is brighter, the southern one is more extended with two long light streaks visible up to 15 arcsec to the south of the nebulosity centre. The structure is best explained as light from object AB scattered by dust at the edges of an outflow cavity; indeed, the orientation fits with various outflow related features (Section 4.2).

Apart from the two compact sources, the multiwavelength images reveal a complex substructure in IRAS 04325 [see the contour plot in Fig. 7, where we have subtracted a point source at the position of AB (see Section 3.4)]. We note that the PSF subtraction affects only the area within 0.5 arcsec radius of AB and does not alter the structure seen in the figure. There are two extended features in close proximity to AB (Fig. 7, left-hand panel). The southern feature, called *D* in the following, has a peak 0.7 arcsec south-west of AB and appears in the *K* band as a half-ring broadly covering the south part of the point source. This is best explained by scattered light from an envelope around the central source, with a radius of about 100 au. The position and structure of this feature agree well with extended CO emission seen in the submm regime at radial velocities of 6.5–8 km s⁻¹, indicating the presence of cold gas. In the CO map, the maximum intensity is seen at 6.9 km s⁻¹ with a peak at 0.8 arcsec south of AB (Fig. 3).

The northern feature, called *E* in the following, appears in the *K* band as an open cone with an apex located 1.2 arcsec north of AB, corresponding to 180 au (see Figs 1 and 7). Two curved light streaks extend this structure towards the east and west. The same extensions are seen in the submm continuum image (Fig. 2). The position of feature *E* also coincides with a CO lobe seen in a velocity regime between 2 and 5 km s⁻¹ (see Fig. 3). It is most pronounced at 4–4.5 km s⁻¹, with a peak position at about 1.0 arcsec north of AB. Feature *E* is orientated at a PA of ~15° with respect to AB, coinciding with the orientation of the emission nebula. The morphology and orientation fit well with an interpretation as part of the outflow cavity.

Our own *K*-band image and the one from *HST* (Hartmann et al. 1999) show a faint S-shaped structure connecting the point sources AB and C and extending farther to the north (Figs 1 and 7, right-hand panel). This is most likely scattered light from dust features. It is possible that this 'bridge' between AB and C is coincidental; we see the extension of the outflow-related feature *E* (see above) and light streaks originating from source *C* in superposition in the plane of the sky. Hartmann et al. (1999) argued that the two faint features near *C* are scattered light from the cavity edges of an outflow from object *C*. Their direction is 60°–90° offset from the orientation of the disc seen in the *HST* images, which fits into the outflow interpretation. However, in our CO observations, there is no evidence for outflow lobes at the same positions (see below).

Therefore, we prefer a scenario in which these structures belong to a physical 'bridge' between AB and C. Such a 'bridge' could be indicative of gravitational interaction in the early evolution of the system, but the long-term stability of the outflows (Section 4.2) argues against this interpretation. Instead, we propose to explain the structure as dust features from interacting accretion streams (as speculated by Hartmann et al. 1999). The S-shaped morphology

indeed resembles the density maps in accreting binaries from the SPH simulations by Bate & Bonnell (1997).

Two CO lobes are detected in close proximity to source C in a narrow velocity range of 6.5–7.5 km s⁻¹ (see Fig. 3). The strongest one is almost exactly coinciding with the position of the compact source seen in the continuum image: the offset between peak of the CO intensity and peak in the continuum is <1.0 arcsec. The PA for the elongation of this CO lobe is roughly 300°. The second CO blob is located roughly 5 arcsec away at PA ~290° with respect to C. A third CO detection is seen in the velocity range of 4.4–4.8 km s⁻¹ 5 arcsec west of object C at PA ~270°. The orientation of these three CO features is roughly perpendicular to the alignment of the disc (PA = 30°). Thus, they could be associated with an outflow from source C.

## 4.2 Structure outside IRAS 04325

A region of diffuse near-IR emission is found about 40 arcsec north and 15 arcsec east of IRAS 04325AB, corresponding to a PA of 20° (see Fig. 8). Thus, this area is located in the extension of the axis defined by the elongated emission nebula. The feature has been mentioned before in the literature, for example, by Park & Kenyon (2002), and is most pronounced at 2–5 µm. In our *K*-band image, the feature has a size of 20 × 20 arcsec². The proximity to IRAS 04325 and the location along the outflow axis lead us to believe that the feature is physically associated with IRAS 04325. Under this assumption, the physical distance from IRAS 04325 would be ~6000 au. This patch of emission could be scattered light at the northern end of the outflow cavity.

In the IRAC bands at 3.6 and 4.5 µm, a 10–15 arcsec wide, dark band is visible between the IRAS 04325 nebula and the emission region farther north (Fig. 8, left-hand panel), possibly a foreground cloud blocking the view to the source.

At 8 µm, the bright diffuse patch in the north of IRAS 04325 is not visible; instead, the image shows a roughly circular, 1.5 × 1.5-arcmin² absorption feature centred 70 arcsec north and 30 arcsec east of IRAS 04325 (PA = 23°), that is, just north of the bright patch (Fig. 8, right-hand panel). This dark feature has fainter extensions to the east and south, overlapping with IRAS 04325. Its centre is located along the outflow axis of IRAS 04325AB. The position agrees well with the core L1535 N-SMM seen in the submm maps of IRAS 04325 presented by Hogerheijde & Sandell (2000), indicating that this feature represents an overdensity of dust. It is possible that the northern lobe of the outflow is blocked by this core, explaining the lack of large-scale outflow features further to the NE (see below).

<figure><img src="image_11.png" /><figcaption>**Figure 8.** Spitzer IRAC images centred on the nebulosity north of IRAS 04325 at 4.5 (left-hand panel) and 8.0 µm (right-hand panel). IRAS 04325 is marked with a black circle. Noteworthy are (a) the emission knot seen at 4.5 µm (left-hand panel, marked with an arrow) at the northern end of the nebula and (b) the dark feature visible at 8 µm.</figcaption></figure>
The dark feature is only visible at 8 µm, and not at any other wavelength in the mid-IR. In the 8-µm image, it is the only such feature within a radius of 1°. This may be related to the presence of strong polycyclic aromatic hydrocarbon (PAH) emission bands in the other IRAC bands (e.g. Peeters et al. 2002). It is conceivable that we see a region of strong absorption projected against the bright PAH background, as they are often found in the Galactic plane (e.g. Egan et al. 1998). In fact, the combination of submm emission and 8-µm absorption has been observed for a number of cores in IR dark clouds (Vasyunina et al. 2009).

IRAS 04325 has long been known to drive molecular outflows (Heyer et al. 1987; Terebey, Vogel & Myers 1989; Moriarty-Schieven et al. 1992). Specifically, a large body of evidence is available for an outflow driven by source AB at a PA of 10°–20°. Features associated with this outflow are the elongated emission nebula (also seen in HCO+, Hogerheijde et al. 1998), the scattered light feature E (Section 4.1), the HH objects 434–436 about 1° south-west (2.7 pc) of the IRAS source at PA = 17.5–19.5 (Wang et al. 2001) and HH703 30 arcmin south of IRAS 04325 (1.4 pc) at PA = 16° (Sun et al. 2003).

We searched in the H₂ images as well as in the IRAC images for more shock features along the outflow axis. In particular, the IRAC band 2 (4.5 µm) is known to contain a number of bright emission lines of shocked H₂ (see Ybarra & Lada 2009, and references therein). The group of Herbig—Haro objects 434–436, so far only detected in the [S II] line and possibly part of a bow shock (Wang et al. 2001), is well visible in H₂ and in all four IRAC bands. In the 4.5-µm image, we find an additional knot about 1.1 arcmin north of AB (10 000 au) at PA = 14° [α = 4h35m36s.5, δ = +24°09'25.75 (J2000), Fig. 8, left-hand panel]. This feature is clearly extended and not seen at any other wavelength, which argues for an association with the outflow. No other obvious feature was found for PA of 15°–20° and separation from IRAS 04325 up to 1° in both directions.

There is a second large-scale outflow emanating from IRAS 04325, identified by Heyer et al. (1987). The outflow is seen in integrated CO emission spanning 12 arcmin, that is, 0.5 pc, if related to IRAS 04325. It is described as well-collimated, redshifted and monopolar. In clear disagreement to the first outflow discussed above, this second flow is oriented from the IRAS source towards the north-west at a PA around ~310°, roughly perpendicular to the disc of source C.

The most likely source for this second outflow is object C. This star clearly has jet activity, proven by the variety of outflow related emission lines in the near-IR spectrum. Three CO lobes are seen around this object with PA of 270°–300°, roughly matching the orientation reported by Heyer et al. (1987), which could belong to the outflow as well. We do not find any additional counterparts of this outflow in the H₂ and the IRAC images.

## 5 SPECTRAL ENERGY DISTRIBUTION MODELLING

We aim to reproduce the SEDs for objects AB and C with a Monte Carlo radiative transfer code. In addition to the data presented here and in Scholz et al. (2008), we make use of the Spitzer/IRS spectrum available for object AB (Furlan et al. 2008). Only the Short-Low (SL) exposure is used, which is obtained through a 3.6-arcsec-wide slit oriented roughly in the north–south direction and covers the wavelength range from 5.2 to 14 µm. This spectrum matches well with the photometry at 11.2 µm. We do not use the Long-Low (LL)

spectrum (obtained through a 10.6-arcsec slit) and the IRAC/MIPS photometry, due to their poor spatial resolution.

Our model code computes thermal radiation and non-spherical scattering from dust in a two-dimensional axisymmetric system with a central source, flared disc and larger scale flattened envelope with a narrow evacuated bipolar outflow cavity (e.g. Whitney et al. 2003a,b). The central photospheres are approximated with NextGen model atmospheres (Hauschildt, Allard & Baron 1999; Allard et al. 2001) with $log g = 4$. The main free parameters for the disc are mass, outer radius, inner radius, accretion rate, scaleheight factor $h_0$ and flaring power $\beta$. The latter two are defined as $h = h_0(r/R_*)^\beta$, with $h$ the scaleheight of the disc and $r$ the distance from the star. The parameters for the envelope include envelope accretion rate, mass, outer radius and outflow cavity angle. In addition, the model fits the ambient density and the interstellar extinction between the source and the observer.

For the envelope, we assume dust with an interstellar-like size distribution, using the model by Kim, Martin & Hendry (1994). In the disc, we use a dust model, including silicates and carbonaceous grains with solar abundances. Here, the grain size distribution is a power law with an exponential decay for particles with sizes above 50 µm and a formal maximum grain size of 1 mm (Wood et al. 2002). This dust model thus allows for the presence of larger grains than in the interstellar medium. For more details and references about the radiation transfer code, see Scholz, Jayawardhana & Wood (2006) and Robitaille et al. (2006). The code has recently been extended to treat the non-thermal reprocessing of photons by PAH molecules and very small grains (VSGs, Wood et al. 2008), using opacities, abundances and re-emission templates from Draine & Li (2007).

Given the large number of free parameters and the numerous simplifying assumptions (e.g. two-dimensional disc, axisymmetric system, no variability), the modelling is subject to degeneracies (e.g. Chiang et al. 2001; Robitaille et al. 2007). The models presented here are by-eye fits to the SED and do not represent a unique solution. We do not claim to have explored the full parameter space. Instead, we use the model to verify if the SEDs can, in principle, be understood in the commonly used framework of disc/envelope models. In addition, this will provide an independent test for the plausability of the constraints for the physical parameters of the system. The parameter best constrained by the modelling is the circumstellar mass, as it depends only on the submm/mm data points. On the other hand, the mass distribution in disc and envelope, the disc inclination, as well as the extinction are poorly constrained.

### 5.1 Object AB

Fig. 9 shows a model for object AB computed without the inclusion of PAH/VSGs. We find that the two-dimensional model with disc, envelope and outflow cavity is able to reproduce many of the features of the observed SED. One matching SED model is shown in Fig. 9. This specific model is based on the following parameters, consistent with the observational constraints: stellar mass $1 M_\odot$, stellar radius $2 R_\odot$, effective temperature 3800 K, disc inclination 78°, disc mass $10^{-3} M_\odot$, disc radius 50 au, disc accretion rate $10^{-8} M_\odot \text{ yr}^{-1}$, scaleheight factor $0.01 R_*$, flaring power 1.25, envelope accretion rate $3 \times 10^{-6} M_\odot \text{ yr}^{-1}$ and cavity half-opening angle 10°. The total circumstellar mass is $0.18 M_\odot$ for the full scale of the simulation of $10^4$ au.

These results are consistent with previous attempts to model the source. For example, Whitney, Kenyon & Gomez (1997) found a centrifugal disc radius of 50 au, an inclination of 72°–90° and a
<figure><img src="image_12.png" /><figcaption>**Figure 9.** SED fitting for IRAS 04325AB. Shown with the solid line is a model calculated for a square aperture of 200 au, that is, comparable with the observations. The dot-dashed line shows the same model for the full scale of the simulation ($10^5$ au). The photospheric spectrum is plotted as the dashed line. The observed fluxes are plotted with the triangles, except for the IRS spectrum, which is seen as a thick solid line between 5 and 14 µm. The models assume an external (i.e. outside disc and envelope) extinction of $A_V = 3$ mag.</figcaption></figure>

total infall rate of $5 \times 10^{-6} \text{ M}_\odot \text{ yr}^{-1}$ based on near-IR imaging polarimetry. The model presented by Furlan et al. (2008) aims to reproduce the full IRS spectrum and broad-band photometry from the near-IR to the mm regime and results in a centrifugal disc radius of 100 au, a cavity semiopening angle of 15° and a disc inclination angle of 80°.

The largest discrepancies between model and observed SED occur in the near- and mid-IR. This part of the SED is sensitive to small changes in extinction, stellar parameters, disc inclination and to non-axisymmetric geometries (Indebetouw et al. 2006). The images of this object show a complex geometry, that is, three-dimensional effects will likely affect the mid-IR emission.

The IRS spectrum for this object, shown in detail in Fig. 10, is not well reproduced by our model. The deep absorption trough around 9 µm is most likely caused by the amorphous silicate absorption band at 9.7 µm, as seen in many other protostars (Furlan et al.

<figure><img src="image_13.png" /><figcaption>**Figure 10.** Modelling the IRS spectrum (solid line) for IRAS 04325AB. The dotted line is a model without PAH, that is, the same as in Fig. 9. For the dot-dashed line model, we include PAH in the disc. The dashed line has PAH in the disc and envelope. The triangles show the photometric data points. All models are calculated using the same parameters as in Fig. 9. They assume an external (i.e. outside disc and envelope) extinction of $A_V = 3$ mag.</figcaption></figure>

2008). In addition, the spectrum could be affected by the frequently observed ice absorption bands at 6.0 and 6.8 µm (e.g. van Dishoeck 2004).

On the other hand, the peaks at 6.2 and 7.6 µm coincide with two of the most prominent features of PAH emission spectra (Peeters et al. 2002). We made an attempt to model the spectrum by including treatment of PAH/VSGs in the radiative transfer simulations. In Fig. 10, we show the model with PAH only in the disc (dot-dashed line) and PAH in the disc and envelope (dashed line). Qualitatively, the shape of the two peaks is matched by the model. The absolute flux levels, however, are not adequately reproduced by these PAH models. The strength of the peaks is highly sensitive to the location of the PAH; in this case, a model with PAH only in the disc provides the best match.

PAH emission is generally found to be rare in discs around young stellar objects, particularly in late-type objects (Furlan et al. 2006; Geers et al. 2006), with detection rates below 10 per cent. Given the rarity of PAH features and the ubiquity of ice features in low-mass protostars, we consider it more likely that ices are predominantly responsible for the shape of the mid-IR spectrum in IRAS 04325AB. As our analysis has shown, the presence of PAH provides an alternative plausible interpretation for the peaks. To disentangle these effects, it is necessary to include ice opacities in the models (see e.g. Crapsi et al. 2008).

## 5.2 Object C

In Fig. 11, we show the SED for object C and a model SED that matches most of the data points. In the modelling, we fix the effective temperature of the central object (3400 K, see Section 3.2), the accretion rate ($10^{-8} \text{ M}_\odot \text{ yr}^{-1}$, see Section 3.2) and the outer disc radius (30 au, Hartmann et al. 1999). In addition, we require a high disc inclination ($i > 80°$). An edge-on or close to edge-on geometry is necessary to reproduce the appearance of the *HST* image and to explain why photometry and spectroscopy cannot be simultaneously reproduced by an extincted photosphere (see Section 3.3).

The model shown in Fig. 11 is calculated for $R_* = 1.8 R_\odot$, $i = 85°$, $h_0 = 0.01 R_*$ and $\beta = 1.25$, that is, the disc is assumed to be affected by relatively strong flaring. The total mass in disc and envelope has to be between 0.001 and 0.01 $\text{M}_\odot$ to reproduce the submm/mm data points (assuming the canonical gas-to-dust ratio

<figure><img src="image_14.png" /><figcaption>**Figure 11.** SED fitting for IRAS 04325C. Shown is a model with $A_V = 17.5$ mag that matches all available independent constraints (effective temperature, disc radius, accretion rate). Note that the J-band flux is an upper limit. The dashed line shows the photospheric spectrum and the dotted line shows the intrinsic SED without external extinction.</figcaption></figure>
of 100). The model predicts most of the circumstellar mass to be in the disc (0.004 M☉), with only a thin envelope ($6 \times 10^{-5}$ M☉), but other configurations are conceivable. The envelope has an outer radius of 300 au, an accretion rate of $10^{-7}$ M☉ yr$^{-1}$ and an outflow cavity with a half-opening angle of 15°.

The model assumes interstellar extinction of $A_V = 17.5$ mag. This does not include the extinction caused by the disc and envelope, which is extremely high for this object due to the edge-on disc. The value for the extinction agrees well with the estimate we obtain from the combined information of near-IR spectroscopy and photometry (Section 3.3). It differs significantly from the extinction used in the SED modelling for object AB ($A_V = 3$ mag, Section 5.1), which is implausible because outside the disc/envelope systems the extinction should be comparable. As noted before, the constraint for $A_V$ obtained from SED modelling is subject to large uncertainties, due to the numerous degeneracies in the near-IR SED.

As seen in Fig. 11, the photometric data point in the $M'$ band at 4.7 µm is difficult to reproduce with these models. This does not depend on the particular choice of disc/envelope properties; the data point is an outlier for all reasonable combinations of parameters. Most models predict a decreasing SED (in $\lambda F_\lambda$) between 3 and 5 µm, whereas the data point at 4.7 µm is 30 per cent higher than the one at 3.6 µm. One possible explanation is additional flux generated by the outflow, which is not part of the model. The $M'$ band contains a number of strong H$_2$ emission lines typical for shocks in protostellar outflows (Ybarra & Lada 2009), in particular, the 0–0 S(9) transition at 4.69 µm. Shocked H$_2$ emission, as seen in the near-IR for this object (Section 3.2), could contribute significantly to the $M'$-band flux.

# 6 SUMMARY AND CONCLUSIONS

We present a multiwavelength study for the embedded protostellar system IRAS 04325. IRAS 04325 is one of very few known multiple young stellar objects with at least one resolved disc and two protostellar jets. New imaging with subarcsecond resolution has been obtained in the IR covering the wavelength range 1–12 µm and in the submm at 870 µm. This is combined with low-resolution near-IR spectroscopy for the two compact sources in the system and with literature data. In the following, we summarize the constraints on the properties of IRAS 04325:

(i) We show that the both components in the system are low-mass stars. Object C is an M dwarf, probably with an effective temperature around 3400 K, while AB is likely of earlier type and more massive. Object C is unlikely to be a brown dwarf, as speculated previously.

(ii) Despite the wealth of available data, there are enormous difficulties in estimating the fundamental parameters of the central sources. In particular, the presence of veiling, discs and strong line-of-sight extinction puts limits on our ability to reliably derive effective temperatures from spectra. The poorly constrained age hampers the conversion to masses.

(iii) The near-IR emission-line spectrum for object C provides strong evidence for ongoing accretion and jet activity.

(iv) We argue that the previously seen double structure of object AB is not due to the presence of a close stellar companion. Instead, it is best explained by an absorption lane in front of the object, most likely caused by a disc.

(v) Comparing our astrometry with the positions reported by Hartmann et al. (1999), we put a limit of <9 mas yr$^{-1}$ on the relative proper motion between AB and C. This confirms a common membership in the Taurus star forming region.

(vi) The morphology and SED for object AB is consistent with the presence of a small disc (~50 au radius) and a larger envelope (≤100 au radius).

(vii) Object C has been found to have an edge-on disc with radius 30 au by Hartmann et al. (1999). Our observations confirm that the object is mostly seen in scattered light due to the high inclination. Based on submm imaging and SED analysis, we identify an elongated envelope or outer disc with a radius of ≤300 au around this object.

(viii) The mass of circumstellar material in the system is well constrained by the submm/mm data points. While the disc masses are quite small (0.001–0.01 M☉), the total mass within 10 000 au is in the range of 0.2 M☉.

(ix) Many of the features in the SED and the multiwavelength images can be understood in the framework of commonly used two-dimensional disc and envelope models. The remaining inconsistencies may well be caused by the three-dimensional structure of the system, which is visible in the images but not accounted for in the models. The mid-IR spectrum for source AB shows spectral features that cannot be matched with a standard model using a single population of dust grains; instead, they require the inclusion of silicates, ices, and possibly PAHs.

(x) We identify several new features associated with a parsec-scale outflow driven by object AB with a PA of 10°–20°. Object C likely has its own large-scale outflow at a PA of 270°–310°.

(xi) Various pieces of evidence indicate an early evolutionary state for IRAS 04325. The star–disc systems are embedded in a core with a large line-of-sight extinction of $A_V \sim 30$ mag. The object is undetected in the Taurus XMM X-ray survey (Güdel et al. 2007), which is unusual for Class II objects (15 per cent undetected), but common for protostars (60 per cent). The bolometric luminosity of the system, as determined by fitting the unresolved SED (Froebrich 2005), is around $1.0 L_☉$. According to the pre-main-sequence tracks by D’Antona & Mazzitelli (1994, update 1998), two objects with effective temperatures around 3400 K would have a total luminosity of 1.3, 1.0 and $0.9 L_☉$ at $10^5$, $2 \times 10^5$ and $5 \times 10^5$ yr, respectively. Thus, an age of a few times $10^5$ yr seems reasonable.

(xii) The PAs of discs, envelopes and outflows put useful limits on the geometry of the system. For the AB disc/envelope, C disc/envelope and binary orbit, the PAs are approximately 90°, 30° and 350°, respectively. Thus, the disc/envelope systems in IRAS 04325 are strongly misaligned with respect to each other (60°) as well as with respect to the orbital plane of the binary (80° and 40°). The discs are neither aligned nor coplanar.

The last finding, first stated by Hartmann et al. (1999) and bolstered here by more evidence, warrants some further comments. From polarimetry studies, it is known that the dominant majority (80–90 per cent) of young binaries have disc orientations within 30° of the orbital plane (Jensen et al. 2004; Monin, Ménard & Peretto 2006). There are exceptions, found by imaging the jets (e.g. Davis, Mundt & Eisloffel 1994; Lim & Takakuwa 2006) or discs (Kang et al. 2008), but many of them seem to be higher order systems, where one of the components is again a close binary (see the review by Monin et al. 2007). This could be the case for IRAS 04325 as well, although we argue against the presence of an additional component.

Misalignments can be either primordial or arise during the early evolution, due to dynamical interactions in a multiple system. In the case of IRAS 04325, the presence of parsec-scale outflows with constant directions to within a few degrees seems to exclude strong alignment changes in the recent history. Typical dynamical
time-scales for parsec-scale jets are $10^4$ yr (e.g. Eislöffel & Mundt 1997). For the north-south jet driven by object AB, we estimate a lifetime of $10^4-10^5$ yr, assuming typical velocities of the order of a few 100 km s$^{-1}$. The dynamical time-scale for the north-west jet has been determined to be $2 \times 10^5$ yr (Heyer et al. 1987). Thus, the system has been stable over a substantial fraction of its early history. This is confirmed by the absence of strong relative motion between the two components, as it might be expected after recent gravitational encounters.

In general, the misalignments between discs and orbits in binary systems will decay with time; the time-scale for this readjustment is expected to be of the order of 20 orbital periods or $10^6$ yr for IRAS 04325 (Bate et al. 2000; Jensen et al. 2004), which might be longer than its lifetime. For higher order systems, the time-scales for realignment are much longer, because the various orbital planes have to be realigned, not only the discs. In addition, the disc radii in IRAS 04325 are significantly smaller than the orbital separation (<50 au versus 1250 au). In such cases, it is possible that the tilt between discs and orbital planes grows with time and an initial misalignment is enhanced (Lubow & Ogilvie 2000). Thus, IRAS 04325 might be a good case for a system where we see primordial misalignment between discs and orbital planes.

For a wide, isolated system, like IRAS 04325, there are two main possible avenues of formation: fragmentation from a centrifugally supported core (rotational fragmentation) or from turbulent material (turbulent fragmentation) – see Goodwin et al. (2007) for a review. As discussed extensively by Bate et al. (2000), primordial misalignments are easy to produce in a turbulent fragmentation scenario, for example, due to spatial variations in the direction of the angular momentum within the core. On the other hand, rotational fragmentation predicts a strong alignment between discs and orbits. The picture presented here for IRAS 04325 shows that strong misalignments can be present even in small-scale star-forming environments and favours turbulent fragmentation as the formation scenario.

## ACKNOWLEDGMENTS

We thank Ian Bonnell for instructive discussions related to subjects discussed in this paper and Dirk Froebrich for pointing us to the publicly available H$_2$ images for this region. We received the IRS spectrum from Elise Furlan, who also gave helpful comments on an early version of this paper. Her help is greatly appreciated. The careful and constructive review by the anonymous referee helped to improve this paper significantly. AS would like to acknowledge financial support from the Scottish Universities of Physics Alliance SUPA under travel grant APA1-AS110X. ACoG acknowledges financial support from the Science Foundation Ireland, grant 07/RFP/PHYF790.

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