Toward Understanding the B[e] Phenomenon. VIII.
Nature and Variability of IRAS 07080+0605

S. A. Khokhlov, A. S. Miroshnichenko, S. V. Zharikov, K. N. Grankin, O. V.

Zakhozhay, N. Manset, A. A. Arkharov, N. Efimova, S. Klimanov, V. M.

Larionov, et al.

► To cite this version:

S. A. Khokhlov, A. S. Miroshnichenko, S. V. Zharikov, K. N. Grankin, O. V. Zakhozhay, et al. Toward Understanding the B[e] Phenomenon. VIII. Nature and Variability of IRAS 07080+0605. The Astrophysical Journal, 2022, 932, 10.3847/1538-4357/ac6de0. insu-04863676

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# Toward Understanding the B[e] Phenomenon. VIII. Nature and Variability of IRAS 07080+0605

S. A. Khokhlov<fnref n="1" />, A. S. Miroshnichenko<fnref n="1" />,<fnref n="2" />,<fnref n="3" />,<fnref n="4" />, S. V. Zharikov<fnref n="1" />,<fnref n="5" />, K. N. Grankin<fnref n="6" />, O. V. Zakhozhay<fnref n="7" />, N. Manset<fnref n="8" />, A. A. Arkharov<fnref n="4" />, N. Efimova<fnref n="4" />, S. Klimanov<fnref n="4" />, V M. Larionov<fnref n="4" />,<fnref n="9" />, A. A. Khokhlov<fnref n="1" />, A. V. Kusakin<fnref n="3" />, C. T. Omarov<fnref n="3" />, R. I. Kokumbaeva<fnref n="3" />, I. V. Reva<fnref n="3" />, and A. T. Agishev<fnref n="1" />

Received 2021 July 22; revised 2022 May 5; accepted 2022 May 6; published 2022 June 14

## Abstract

We report the results of spectroscopic and photometric observations of the emission-line object IRAS 07080+0605 carried out in 2004–2021. We found that the object is significantly underluminous for its spectroscopic properties ($T_{eff} = 8500 \pm 500$ K, $log g = 2.0 \pm 0.5$), if a strong visual attenuation by a factor of ~43 found through the spectral energy distribution modeling is not taken into account. Visual brightness variations with a stable period of 190 days but a variable amplitude of ~0.2 mag were found in the ASAS SN data and attributed to variable circumstellar extinction in the dusty disk. We also found that the observed behavior of IRAS 07080+0605 is similar to that of the protoplanetary nebula Red Rectangle. The dusty disk of IRAS 07080+0605 shows the presence of carbonaceous particles (~10%–20% of the total dust content) and polycyclic aromatic hydrocarbon emission bands. However, IRAS 07080+0605 shows no obvious signs of the refractory element depletion, which is common in post-asymptotic giant branch (AGB) dusty binaries, or of a visual nebula. Absorption-line positions vary with an amplitude of ~25 km s⁻<fnref n="1" />, suggesting the presence of a secondary component. Spectroscopic monitoring on a timescale from days to months is needed to search for regular variations. We conclude that IRAS 07080+0605 is most likely a binary system with an A-type component on its way toward the post-AGB evolutionary stage, as binarity is capable of explaining most of the observed features.

*Unified Astronomy Thesaurus concepts: A stars (5); Binary stars (154); Spectroscopic binary stars (1557); Be stars (142); Emission line stars (460)*

## 1. Introduction

The B[e] phenomenon is defined as the presence of forbidden (in addition to permitted) line emission in the spectra of mostly B-type stars along with large infrared (IR) excess due to circumstellar dust. Allen & Swings (1976) discovered 65 Galactic objects with these properties and suggested possible explanations for the phenomenon that included formation of a planetary nebula, interaction of an OB star with a cool component, and a direct ejection of matter by a massive OB star. Lamers et al. (1998) summarized the results of the first 20 yr of studies of such objects and found the presence of the B[e] phenomenon in four groups of stars with known evolutionary status by that time: pre-main-sequence Herbig Ae/Be (HAeBe) stars, symbiotic binaries, compact planetary nebulae, and a small subgroup of supergiants. At the same time, these authors were unable to assign about half of the objects from the original list of B[e] stars to any of the known groups and called them unclassified objects with the B[e] phenomenon.

Miroshnichenko (2007) suggested that most unclassified objects were neither HAeBe stars nor B[e] supergiants. Their main common properties include emission-line spectra, which are usually much stronger than those of other emission-line stars of similar spectral types (e.g., classical Be stars, HAeBe stars, and even luminosity type Ia supergiants) and strong IR excesses, whose flux peaks at wavelengths ~10–30 µm and exhibits a fast decrease toward longer wavelengths. The latter property suggests a compact distribution of the circumstellar dust. As the emission-line strengths in these objects can hardly be explained by radiation-driven stellar winds from single stars with the luminosity range ($2.5 \le (\log L/L_{\odot}) \le 4.5$) determined for a sample of them, the main hypothesis about the origin of their circumstellar matter became the transfer of matter between components in binary systems. The group was named FS CMa-type objects (Miroshnichenko 2007) after the prototype of all objects with the B[e] phenomenon (Swings 2006).

The FS CMa group has been expanded from ~20 to ~70 objects, most of which were found in various IR sky surveys (e.g., IRAS) and identified with optical sources (Miroshnichenko et al. 2007, 2011; Kuratova et al. 2017). At the same time, only about a dozen binaries have been confirmed in the sample (Miroshnichenko & Zharikov 2015), which is mostly due to sparse observations and the complex nature of the objects. Although recent observing campaigns allowed us to prove

<figure><img src="image_3.png" /></figure>

Original content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹Al-Farabi Kazakh National University, Al-Farabi Ave., 71, 050040, Almaty, Kazakhstan; skhokh88@gmail.com</footnote>

<footnote n="2">²Department of Physics and Astronomy, University of North Carolina at Greensboro, P.O. Box 26170, Greensboro, NC 27402-6170, USA</footnote>

<footnote n="3">³Fesenkov Astrophysical Institute, Observatory, 23, Almaty, 050020, Kazakhstan</footnote>

<footnote n="4">⁴Main (Pulkovo) Astronomical Observatory of the Russian Academy of Sciences, Pulkovskoye chosse 65-1, 196140, Saint-Petersburg, Russia</footnote>

<footnote n="5">⁵Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 106, Ensenada 22800, BC, México</footnote>

<footnote n="6">⁶Crimean Astrophysical Observatory, Scientific Research Institute, 298409, Nauchny, Russia</footnote>

<footnote n="7">⁷Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany</footnote>

<footnote n="8">⁸CFHT Corporation, Kamuela, HI 96743, USA</footnote>

<footnote n="9">⁹V.V. Sobolev Astronomical Institute of the St.-Petersburg State University, St.-Petersburg, 198504, Russia</footnote>
<table><caption>**Table 1**
Summary of the Spectroscopic Observations</caption><thead><tr><th>Date</th><th>JD</th><th>Exp. (s)</th><th>Range (Å)</th><th>ID</th><th>Date</th><th>JD</th><th>Exp. (s)</th><th>Range (Å)</th><th>ID</th></tr><tr><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th></tr></thead><tbody><tr><td>02/13/2004</td><td>3048.919</td><td>2400</td><td><sup>a</sup></td><td>CFHT</td><td>11/21/2010</td><td>5522.160</td><td>1680</td><td>3900-10000</td><td>CFHT</td></tr><tr><td>12/24/2004</td><td>3363.944</td><td>3600</td><td>3900-10000</td><td>CFHT</td><td>12/17/2010</td><td>5548.171</td><td>1680</td><td>3900-10000</td><td>CFHT</td></tr><tr><td>10/12/2005</td><td>3655.985</td><td>900</td><td>4400-6700</td><td>SPM</td><td>11/17/2012</td><td>6250.019</td><td>3600</td><td>3900-7300</td><td>SPM</td></tr><tr><td>12/20/2005</td><td>3724.922</td><td>1200</td><td>3900-10150</td><td>McD</td><td>01/28/2013</td><td>6320.831</td><td>7200</td><td>3900-7300</td><td>SPM</td></tr><tr><td>12/12/2006</td><td>4082.039</td><td>1800</td><td>3900-6600</td><td>SPM</td><td>01/30/2013</td><td>6322.826</td><td>7200</td><td>3900-7300</td><td>SPM</td></tr><tr><td>12/14/2006</td><td>4083.934</td><td>3600</td><td>3900-6600</td><td>SPM</td><td>10/19/2013</td><td>6584.997</td><td>2400</td><td>4600-8100</td><td>SPM</td></tr><tr><td>12/15/2006</td><td>4084.952</td><td>8400</td><td>3900-6600</td><td>SPM</td><td>10/21/2013</td><td>6587.000</td><td>2400</td><td>4600-8100</td><td>SPM</td></tr><tr><td>12/16/2006</td><td>4085.963</td><td>4800</td><td>3900-6600</td><td>SPM</td><td>10/04/2015</td><td>7299.987</td><td>3600</td><td>3900-7300</td><td>SPM</td></tr><tr><td>12/27/2006</td><td>4096.911</td><td>6000</td><td>3900-10150</td><td>McD</td><td>12/07/2015</td><td>7363.818</td><td>3000</td><td>3900-9200</td><td>ESO</td></tr><tr><td>11/11/2007</td><td>4416.008</td><td>3600</td><td>3900-6600</td><td>SPM</td><td>12/06/2017</td><td>8093.973</td><td>3600</td><td>3900-7300</td><td>SPM</td></tr><tr><td>11/15/2007</td><td>4419.989</td><td>1200</td><td>3900-6600</td><td>SPM</td><td>11/20/2018</td><td>8442.940</td><td>3600</td><td>3900-10000</td><td>CFHT</td></tr><tr><td>11/16/2007</td><td>4420.893</td><td>6000</td><td>3900-6600</td><td>SPM</td><td>11/24/2018</td><td>8447.040</td><td>2400</td><td>3900-7300</td><td>SPM</td></tr><tr><td>10/6/2008</td><td>4745.996</td><td>2700</td><td>3900-6600</td><td>SPM</td><td>11/08/2019</td><td>8796.939</td><td>3600</td><td>3900-7300</td><td>SPM</td></tr><tr><td>10/10/2008</td><td>4749.974</td><td>3600</td><td>3900-6600</td><td>SPM</td><td>11/10/2019</td><td>8799.002</td><td>3600</td><td>3900-7300</td><td>SPM</td></tr><tr><td>12/15/2008</td><td>4815.972</td><td>3600</td><td>3900-10150</td><td>McD</td><td>11/26/2020</td><td>9180.978</td><td>4200</td><td>3900-10000</td><td>CFHT</td></tr><tr><td>11/5/2009</td><td>5140.957</td><td>4800</td><td>3900-6600</td><td>SPM</td><td>12/07/2020</td><td>9192.149</td><td>4400</td><td>3900-10000</td><td>CFHT</td></tr><tr><td>11/9/2009</td><td>5144.961</td><td>6000</td><td>3900-6600</td><td>SPM</td><td>11/25/2021</td><td>9543.613</td><td>2700</td><td>3900-9000</td><td>CFHT</td></tr><tr><td>10/17/2010</td><td>5486.996</td><td>3600</td><td>3900-7300</td><td>SPM</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Notes. Column (1): observing date (Month/Day/Year). Column (2): Julian Date (JD = 2,450,000). Column (3): total exposure time in seconds. Column (4): spectral range in Å. Column (5): Observatory ID. SPM—Observatorio Astronómico Nacional San Pedro Martir; CFHT—Canada–France–Hawaii Telescope; McD—McDonald Observatory.

binarity of some group members (e.g., Khokhlov et al. 2018; Miroshnichenko et al. 2020), binarity of some others still remains elusive (e.g., Khokhlov et al. 2017).

This paper is devoted to a study of IRAS 07080+0605 ($V \sim 12\,m\,1$), which has one of the strongest IR excesses among the objects with the B[e] phenomenon. IRAS 07080+0605 was detected by Kohoutek & Wehmeyer (1999) in a survey for emission-line stars and designated as HBHA 717–01. Later Miroshnichenko et al. (2007) included it in the FS CMa group and suggested a binary nature. Arias et al. (2018) suggested that IRAS 07080+0605 is a pre-main-sequence A0-type binary still embedded in a warm CO molecular cloud, although signs of star-forming regions were not detected nearby by visual inspection of optical and WISE images. A simple model for the spectral energy distribution (SED) proposed in this paper suggests the presence of a significant circumstellar extinction but does not fit well the optical part of the SED. However, neither Miroshnichenko et al. (2007) nor a recent study by Condori et al. (2019) found any direct evidence for a stellar component. Additionally, the latter authors suggested that IRAS 07080+0605 is an A[e] star with an uncertain classification, such as either a main-sequence or a pre-main-sequence object. These three studies were based on single spectroscopic observations in narrow wavelength regions except for one spectrum taken with the FEROS spectrograph in the entire optical range, parts of which are shown by Condori et al. (2019) and which we use in this paper, that hampered a thorough inspection of enough spectral features to determine accurate fundamental parameters of the underlying star.

In this paper we study a large collection of high-resolution optical spectra of IRAS 07080+0605, model its SED in the optical and IR region, and present our own multicolor optical and near-IR photometry to refine its fundamental parameters and reinvestigate its nature and evolutionary status.

## 2. Observations

Thirty-five high-resolution optical spectra of IRAS 07080+0605 were obtained between 2004 February and 2021 November at several facilities. They include the 2.1 m telescope of the Observatorio Astronómico Nacional San Pedro Martir (OAN SPM, Baja California, México) with a REOSC spectrograph (spectral resolving power R = 18,000), the 2.7 m Harlan J. Smith telescope of the McDonald Observatory (Texas, USA) with the Tull coudé spectrograph TS2 (R = 60,000; Tull et al. 1995), and the 3.6 m Canada–France–Hawaii Telescope (CFHT; Maunakea, HI, USA) with the spectrograph Gecko (R = 100,000; Baudrand & Vitry 2000) and the spectropolarimeter ESPaDOnS (R = 65,000; Manset & Donati 2003). One spectrum taken with the échelle spectrograph FEROS (R = 48,000) mounted at the 2.2 m telescope of the European Southern Observatory (ESO) was retrieved from the ESO archive. The log of the high-resolution spectroscopic observations is shown in Table 1. A low-resolution IR spectrum of IRAS 07080+0605 was taken with the spectrograph IRC (Houck et al. 2004) at the Spitzer Space Observatory in a range between 5.3 and 38 µm in 2010. We describe and discuss it in detail below in Section 4.

The OAN SPM and McDonald data were reduced in a standard way with the *echelle* package in IRAF.<fnref n="10" /> Observations obtained at CFHT were reduced with the Upena and LibreESPRIT software packages (Donati et al. 1997). Parts of a CFHT spectrum of IRAS 07080+0605 with a representative set of identified spectral lines are presented in Figure 1.

Photometric *BVR* data in the Johnson–Cousins system of a 10' × 10' field around IRAS 07080+0605 were obtained on 10

<footnote><sup>a</sup> Four narrow wavelength regions around several lines (Na I D, Si II λλ6347 and 6371, Hα, and O I λλ7772–7775) taken with the spectrograph Gecko (R = 100,000).</footnote>

<footnote n="10">¹⁰ IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation.</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Parts of the optical high-resolution spectrum of IRAS 07080+0605 taken at CFHT on 2018/11/20. The intensity is normalized to the continuum, and the wavelength scale is heliocentric.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 2.** A 15' × 15' field around IRAS 07080+0605 from the Digital Sky Survey (DSS). The J2000 coordinates are shown along the axes. The object is the circled star in the middle of the field. Stars are marked with numbers.</figcaption></figure>

<table><caption>**Table 2**
Optical Photometry of IRAS 07080+0605</caption><thead><tr><th>Date</th><th>JD</th><th>V</th><th>B-V</th><th>V-R<sub>c</sub></th></tr></thead><tbody><tr><td>12/11/2014</td><td>7003.271</td><td>12.11</td><td>0.11</td><td>0.12</td></tr><tr><td>12/14/2014</td><td>7006.352</td><td>12.11</td><td>0.13</td><td>0.12</td></tr><tr><td>2/4/2015</td><td>7058.192</td><td>12.16</td><td>0.11</td><td>0.12</td></tr><tr><td>2/5/2015</td><td>7059.125</td><td>12.14</td><td>0.13</td><td>0.12</td></tr><tr><td>2/26/2015</td><td>7080.239</td><td>12.13</td><td>0.12</td><td>0.12</td></tr><tr><td>4/4/2016</td><td>7483.146</td><td>12.02</td><td>0.12</td><td>0.12</td></tr><tr><td>4/7/2016</td><td>7486.108</td><td>12.02</td><td>0.13</td><td>0.12</td></tr><tr><td>2/6/2017</td><td>7791.241</td><td>11.94</td><td>0.11</td><td>0.12</td></tr><tr><td>10/12/2020</td><td>9135.467</td><td>12.08</td><td>0.13</td><td>0.12</td></tr><tr><td>10/19/2020</td><td>9142.444</td><td>12.07</td><td>0.13</td><td>0.12</td></tr></tbody></table>

**Note.** The $BV(R_c)$ data obtained at TShAO. Column (1): observing date (MM/DD/YYYY). Column (2): Julian Date (JD −2,450,000). Columns (3)–(5): the object’s brightness in the Johnson–Cousins photometric system. The average measurement uncertainty does not exceed 0.02 mag in any band.

nights at a 1 m telescope of the Tien-Shan Observatory (TShAO) of the Fesenkov Astrophysical Institute (Kazakhstan) between 2015 January and 2019 January. A 3056 × 3056 Apogee F9000 D9 CCD camera with 12 µm pixels and a three-step cooling was used with a set of $BVR_c$ filters. The TShAO data were reduced using the Maxim-DL software.<fnref n="11" /> The brightness of IRAS 07080+0605 and field stars marked in Figure 2 were measured using the IRAF package *phot*. The TShAO instrumental magnitudes were converted into the Johnson–Cousins *BVR* system by measuring the brightness of standard stars in several open clusters (e.g., NGC 2169) and using photometric data for the IRAS 07080+0605 field stars listed in the UCAC4 catalog (Zacharias et al. 2012). The

<table><caption>**Table 3**
Near-IR Photometry of IRAS 07080+0605</caption><thead><tr><th>JD</th><th>J</th><th>H</th><th>K</th></tr></thead><tbody><tr><td>4885.05</td><td>9.85</td><td>8.30</td><td>7.03</td></tr><tr><td>4889.36</td><td>9.82</td><td>8.30</td><td>6.97</td></tr><tr><td>4890.34</td><td>9.84</td><td>8.30</td><td>7.01</td></tr><tr><td>4904.35</td><td>9.81</td><td>8.28</td><td>7.00</td></tr><tr><td>4909.30</td><td>9.86</td><td>8.30</td><td>7.02</td></tr><tr><td>5144.01*</td><td>9.88</td><td>8.30</td><td>7.00</td></tr><tr><td>5307.32</td><td>9.88</td><td>8.35</td><td>7.05</td></tr><tr><td>5313.29</td><td>9.89</td><td>8.38</td><td>7.09</td></tr><tr><td>5482.02*</td><td>9.88</td><td>8.35</td><td>7.14</td></tr><tr><td>5868.67</td><td>9.87</td><td>8.34</td><td>7.07</td></tr><tr><td>5880.64</td><td>9.86</td><td>8.32</td><td>7.03</td></tr><tr><td>6650.58</td><td>9.87</td><td>8.34</td><td>7.05</td></tr><tr><td>6726.37</td><td>9.81</td><td>8.28</td><td>7.01</td></tr><tr><td>6729.37</td><td>9.79</td><td>8.29</td><td>7.01</td></tr><tr><td>6737.37</td><td>9.84</td><td>8.30</td><td>7.01</td></tr><tr><td>6960.70</td><td>9.87</td><td>8.31</td><td>7.07</td></tr><tr><td>6963.68</td><td>9.89</td><td>8.33</td><td>7.05</td></tr></tbody></table>

**Note.** The near-IR *JHK* data obtained at Campo Imperatore and OAN SPM (marked with an asterisk). Column (1): Julian Date (JD −2,450,000). Column (2): *J*-band brightness. Column (3): *H*-band brightness. Column (4): *K*-band brightness. The average measurement uncertainty is between 0.01 and 0.02 mag in all the bands.

measurement results in the standard system are presented in Table 2.

Near-IR *JHK* photometry was taken with the 1.1 m telescope AZT-24 at Campo Imperatore (Italy) in 2008–2014 with imager/spectrometer SWIRCAM (D’Alessio et al. 2000). The brightness of IRAS 07080+0605 was measured by comparison with Two Micron All Sky Survey (2MASS) data (Cutri et al. 2003) for several field stars. Additionally, two *JHK* observations were carried out at the 0.84 and 2.12 m telescopes of OAN SPM in 2009 and 2010. The IR camera CAMILA with a NICMOS 3 HgCdTe detector (Cruz-Gonzales et al. 1994) was used at this observing site. The results are presented in Table 3. Additionally, BVR magnitudes measured on the TShAO images and other parameters of nine stars from the 15' × 15' region around the object are presented in Table 4.

# 3. Data Analysis

## 3.1. Absorption Lines

The absorption-line spectrum is represented by weak lines of He I, Mg I, Cr II, Si II, Fe II, Ti II, and O I, as well as by most hydrogen lines of the Balmer and Paschen series. The Balmer line widths, along with moderately strong Si II $\lambda\lambda6347$ and 6371 lines (equivalent width (EW) ~0.4 and ~0.2 Å, respectively; see Figure 1), suggest a luminosity that is larger than those of A-type dwarfs but smaller than those of A-type supergiants. At the same time, the O I $\lambda\lambda7772-7775$ triplet is very strong (EW ~2.7 Å; Figure 1). Such a strong feature is usually observed in the spectra of the most luminous normal (with no emission lines except for $H\alpha$) B- to early F-type supergiants (Arellano Ferro et al. 2003).

All these features suggest that the underlying star has an early A spectral type with a luminosity higher than that on the main sequence. In a search for the best theoretical fit to the absorption-line spectrum, we calculated a set of synthetic

<footnote n="11">¹¹ http://diffractionlimited.com/product/maxim-dl/</footnote>
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Left panel: comparison of a part of the spectrum of IRAS 07080+0605 taken at CFHT (R ~ 65,000) with that of a model atmosphere ($T_{eff}$ = 8500 K, log g = 2.0, and a projected rotational velocity of v sin i = 60 km s⁻¹) calculated using the code SPECTRUM (Gray & Corbally 1994). Heliocentric wavelengths are shown in Å, and intensity is normalized to that of a nearby continuum. The spectra are shifted by 0.2 $I_{cont}$ with respect to each other for easier comparison. Right panel: comparison of the spectrum (the Balmer Hα and Hδ lines, 3955–4130 Å) of IRAS 07080+0605 with the same model atmosphere as in the left panel. The wavelength and intensity are shown in the same units as in the left panel, but the spectra are shifted by 0.35 $I_{cont}$.</figcaption></figure>

<table><caption>**Table 4**
Photometry of Stars in the Field of IRAS 07080+0605</caption><thead><tr><th>N</th><th>R.A. (J2000)</th><th>Decl. (J2000)</th><th>V</th><th>B-V</th><th>V-R</th><th>T<sub>eff</sub></th><th>A<sub>V</sub></th><th>D</th></tr></thead><tbody><tr><td>1</td><td>7:10:33.8</td><td>+05:55:02.1</td><td>11.02</td><td>0.43</td><td>0.29</td><td>6500</td><td>0.19 ± 0.06</td><td>406 ± 2</td></tr><tr><td>2</td><td>7:10:33.7</td><td>+06:01:48.3</td><td>12.68</td><td>0.45</td><td>0.28</td><td>6500</td><td>0.45 ± 0.07</td><td>761 ± 8</td></tr><tr><td>3</td><td>7:10:39.9</td><td>+06:04:54.8</td><td>11.35</td><td>0.47</td><td>0.29</td><td>6500</td><td>0.35 ± 0.06</td><td>707 ± 9</td></tr><tr><td>4</td><td>7:10:38.7</td><td>+06:05:27.3</td><td>12.74</td><td>0.52</td><td>0.31</td><td>6000</td><td>0.44 ± 0.07</td><td>785 ± 7</td></tr><tr><td>5</td><td>7:10:47.2</td><td>+05:59:22.2</td><td>12.69</td><td>0.62</td><td>0.35</td><td>6000</td><td>0.32 ± 0.09</td><td>402 ± 2</td></tr><tr><td>6</td><td>7:11:13.5</td><td>+06:06:22.0</td><td>9.96</td><td>0.41</td><td>0.27</td><td>6250</td><td>0.15 ± 0.08</td><td>199 ± 2</td></tr><tr><td>7</td><td>7:10:48.3</td><td>+06:00:18.1</td><td>13.13</td><td>0.50</td><td>0.28</td><td>6250</td><td>0.38 ± 0.04</td><td>601 ± 8</td></tr><tr><td>8</td><td>7:10:57.6</td><td>+06:07:37.6</td><td>13.10</td><td>0.40</td><td>0.27</td><td>7000</td><td>0.48 ± 0.08</td><td>676 ± 9</td></tr><tr><td>9</td><td>7:10:52.6</td><td>+05:56:00.0</td><td>13.02</td><td>0.42</td><td>0.27</td><td>7000</td><td>0.53 ± 0.06</td><td>762 ± 14</td></tr></tbody></table>

Note. N is a sequential number of the stars marked in Figure 2. The equatorial coordinates are taken from the UCAC4 catalog (Zacharias et al. 2012). $T_{eff}$ in K is determined by comparison with theoretical SEDs from Kurucz (1998). Distance D in pc is from the Gaia EDR3 (Bailer-Jones et al. 2021).

spectra based on model atmospheres provided by F. Castelli<fnref n="12" /> using the program SPECTRUM (Gray & Corbally 1994) and applied various rotational broadening to them. As a result, we found a reasonably good fit of the metallic line strengths and Balmer line widths with a synthetic spectrum with the effective temperature $T_{eff}$ = 8500 K, surface gravity log g = 2.0, and a projected rotational velocity of v sin i ~ 60 km s⁻¹. The latter value agrees well with that derived by Arias et al. (2018). The low surface gravity also agrees with the strong luminosity-dependent Si II and O I lines mentioned above. A representative spectral region of 4520–4590 Å, which contains mostly absorption lines formed in the star’s atmosphere and was used for determining the average RV through cross-correlation (see Section 3.3), is shown in Figure 3.

We can put some constraints on the accuracy of these parameters, which are limited by possible effects of the circumstellar gas and stellar evolution (e.g., refractory element depletion; see Section 4). In particular, $T_{eff}$ seems to be accurate

to ~±500 K because the derived value is consistent with the photometric data and the interstellar extinction (Section 3.5). The surface gravity seems to be accurate to ~±0.5, as a higher gravity would imply a lower luminosity, which is inconsistent with the large observed IR excess (Section 4.1), while a lower gravity would contradict the relatively large v sin i.

Both the derived $T_{eff}$ and log g are different from those previously published. In particular, the neutral helium lines would be significantly stronger at $T_{eff}$ = 13,000 K and log g = 4.5 as suggested by Arias et al. (2018), while the Balmer line profiles barely affected by emission (e.g., Hα and Hδ; see right panel of Figure 3) would be noticeably wider at $T_{eff}$ = 10,100 ± 700 K and log g ~ 4.0 as suggested by Condori et al. (2019). We estimated the surface gravity from the object’s position in the H-R diagram shown in the latter paper. The high values of log g are inconsistent with the strong Si II lines and the oxygen triplet mentioned above. Additionally, $T_{eff}$ = 13,000 K is inconsistent with the optical photometry shown in Figure 13 from Arias et al. (2018), and the object’s position near main sequence shown in Figure 6

<footnote n="12">¹² https://wwwuser.oats.inaf.it/castelli/grids gridp00k2odfnew/ap00k2tab.html</footnote>
<table><caption>**Table 5**
Parameters of the Hα Line in the Spectra of IRAS 07080+0605</caption><thead><tr><th>JD</th><th>RV<sub>blue</sub></th><th>RV<sub>red</sub></th><th>RV<sub>centr</sub></th><th>I<sub>blue</sub></th><th>I<sub>red</sub></th><th>I<sub>centr</sub></th><th>EW</th><th>JD</th><th>RV<sub>blue</sub></th><th>RV<sub>red</sub></th><th>RV<sub>centr</sub></th><th>I<sub>blue</sub></th><th>I<sub>red</sub></th><th>I<sub>centr</sub></th><th>EW</th></tr></thead><tbody><tr><td>3048.919</td><td>-49.8</td><td>49.4</td><td>-5.9</td><td>6.0</td><td>8.4</td><td>6.9</td><td>-29.0</td><td>5522.160</td><td>-37.5<sup>a</sup></td><td>48.0</td><td>4.6</td><td>10.5<sup>a</sup></td><td>14.0</td><td>13.0</td><td>-51.7</td></tr><tr><td>3363.944</td><td>-36.8<sup>a</sup></td><td>41.1</td><td>6.4<sup>a</sup></td><td>11.0<sup>a</sup></td><td>19.01</td><td>13.3<sup>a</sup></td><td>-56.0</td><td>5548.171</td><td>-45.7</td><td>48.0</td><td>-4.1</td><td>13.3</td><td>12.9</td><td>13.5</td><td>-51.6</td></tr><tr><td>3655.985</td><td></td><td>25.1<sup>b</sup></td><td></td><td></td><td>10.5<sup>b</sup></td><td></td><td>-50.3</td><td>6250.019</td><td>-41.1</td><td>43.8</td><td></td><td>11.3</td><td>14.2</td><td></td><td>-62.1</td></tr><tr><td>3724.922</td><td>-49.4</td><td>31.1</td><td>3.7</td><td>13.9</td><td>16.3</td><td>12.1</td><td>-57.3</td><td>6320.831</td><td>-52.6</td><td>16.0</td><td>-24.2</td><td>12.5</td><td>14.6</td><td>12.7</td><td>-56.2</td></tr><tr><td>4082.039</td><td>-37.5<sup>a</sup></td><td>39.3</td><td></td><td>10.9<sup>a</sup></td><td>9.7</td><td></td><td>-56.9</td><td>6322.826</td><td>-33.8</td><td>34.7</td><td></td><td>13.4</td><td>15.7</td><td></td><td>-59.0</td></tr><tr><td>4083.934</td><td></td><td>39.3<sup>b</sup></td><td></td><td></td><td>14.0<sup>b</sup></td><td></td><td>-59.1</td><td>6584.997</td><td>-37.0<sup>a</sup></td><td>52.6</td><td>-0.9</td><td>12.3<sup>a</sup></td><td>14.4</td><td>12.3<sup>a</sup></td><td>-63.8</td></tr><tr><td>4084.952</td><td></td><td>33.4<sup>b</sup></td><td></td><td></td><td>13.1<sup>b</sup></td><td></td><td>-59.8</td><td>6587.000</td><td>-39.3<sup>a</sup></td><td>52.6</td><td>7.3<sup>a</sup></td><td>11.1<sup>a</sup></td><td>13.8</td><td>12.0<sup>a</sup></td><td>-58.8</td></tr><tr><td>4085.963</td><td></td><td>27.4<sup>b</sup></td><td></td><td></td><td>13.0<sup>b</sup></td><td></td><td>-59.8</td><td>7299.987</td><td>-38.8<sup>a</sup></td><td>50.3</td><td>8.2</td><td>9.2<sup>a</sup></td><td>13.4</td><td>11.8</td><td>-54.1</td></tr><tr><td>4096.911</td><td>-6.9</td><td>33.8</td><td>8.7<sup>a</sup></td><td>16.5</td><td>17.2</td><td>15.4<sup>a</sup></td><td>-62.8</td><td>7363.818</td><td>-33.4</td><td>58.5</td><td>18.7</td><td>9.0</td><td>13.0</td><td>10.0</td><td>-50.8</td></tr><tr><td>4416.008</td><td>-27.4<sup>a</sup></td><td>31.5</td><td></td><td>11.2<sup>a</sup></td><td>10.1</td><td></td><td>-51.7</td><td>8093.973</td><td>-29.7</td><td>41.1</td><td></td><td>11.5</td><td>13.0</td><td></td><td>-52.3</td></tr><tr><td>4419.989</td><td></td><td>43.9<sup>b</sup></td><td></td><td></td><td>11<sup>b</sup></td><td></td><td>-51.4</td><td>8442.940</td><td>-38.9</td><td>42.1</td><td>5.9</td><td>12.0</td><td>14.9</td><td>9.5</td><td>-52.6</td></tr><tr><td>4420.893</td><td>-28.8<sup>a</sup></td><td>31.5</td><td></td><td>9.8</td><td>11.8</td><td></td><td>-55.2</td><td>8447.040</td><td>-29.3</td><td>47.1</td><td></td><td>11.5</td><td>12.9</td><td></td><td>-51.4</td></tr><tr><td>4745.996</td><td>-57.6<sup>a</sup></td><td>38.4</td><td>5.0<sup>a</sup></td><td>6.9<sup>a</sup></td><td>11.5</td><td>10.1<sup>a</sup></td><td>-47.5</td><td>8796.939</td><td>-37.0</td><td>59.4</td><td>-10.5<sup>a</sup></td><td>11.5</td><td>15.1</td><td>11.3<sup>a</sup></td><td>-64.8</td></tr><tr><td>4749.974</td><td>-69.0<sup>a</sup></td><td>41.14</td><td>-5.0<sup>a</sup></td><td>6.9<sup>a</sup></td><td>12.0</td><td>10.5<sup>a</sup></td><td>-51.3</td><td>8799.002</td><td>-41.1</td><td>58.1</td><td></td><td>11.8</td><td>14.7</td><td></td><td>-66.7</td></tr><tr><td>4815.972</td><td>-57.6</td><td>41.14</td><td>-15.1</td><td>11.2</td><td>17.4</td><td>11.0</td><td>-57.0</td><td>9180.978</td><td>-51.2<sup>a</sup></td><td>48.5</td><td>2.3</td><td>7.6<sup>a</sup></td><td>10.9</td><td>10.6</td><td>-43.4</td></tr><tr><td>5140.957</td><td>-7.8</td><td>57.14</td><td></td><td>13.1</td><td>13.6</td><td></td><td>-58.1</td><td>9192.149</td><td>-48.9<sup>a</sup></td><td>45.7</td><td>2.7</td><td>6.6<sup>a</sup></td><td>11.5</td><td>11.3</td><td>-45.6</td></tr><tr><td>5144.961</td><td>-13.3</td><td>53.9</td><td></td><td>14.2</td><td>16.2</td><td></td><td>-64.3</td><td>9543.613</td><td>-38.3</td><td>50.4</td><td>0.1</td><td>13.8</td><td>10.4</td><td>11.4</td><td>-48.9</td></tr><tr><td>5486.996</td><td>-40.7</td><td>54.9</td><td>16.9</td><td>13.5</td><td>11.5</td><td>12.1</td><td>-53.2</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Notes. Column (1): The observing date (JD = 2,450,000). Columns (2)–(4): the heliocentric RVs or the blue (RV<sub>blue</sub>), red (RV<sub>red</sub>), and an additional, central (RV<sub>add</sub>) emission peak. Columns (5)–(7): their intensities in the continuum units. Column (8): the EWs of the line emission component, given in Å. The profiles with uncertain peak positions (see text) are marked with

<sup>a</sup> The apparently single-peaked profiles are marked with.

<sup>b</sup> The peak positions were measured by Gaussian fitting avoiding asymmetric parts of the profile.

from Condori et al. (2019) is inconsistent with the luminosity type II given in Table 5 from this paper.

Interstellar absorption features detected in the spectrum of IRAS 07080+0605 are limited to the lines of neutral sodium at 5889 and 5995 Å (the narrowest component) and neutral potassium at 7699 Å, which are located at a radial velocity (RV) of 8 km s⁻¹. The sodium line profiles also contain several absorption and emission components of the circumstellar origin (see Figure 1). Diffuse interstellar bands (DIBs) are extremely weak, suggesting a low interstellar reddening (see discussion in Section 3.5).

## 3.2. Emission Lines

The emission-line spectrum of IRAS 07080+0605 includes hydrogen lines of the Balmer series (Hα and Hβ), a few permitted lines of Ca II and Na I, emission components of some Fe II lines, and forbidden lines of [O I], [Fe II], [Ca II], [N II], [S II], [Cr II], and [Ni II].

The Na I lines and some of the Fe II lines have complicated profiles with redshifted absorption components (e.g., 4923, 5018, and 5169 Å; Figure 1). The forbidden lines are typically single or double peaked. However, the [O I] lines sometimes contain a central peak (similar to that in the Hα line) and exhibit a variable peak intensity ratio (Figure 4). The Hα line profile varies between single peaked and triple peaked (Figure 4). The appearance of double-peaked forbidden lines in the object’s optical spectrum was earlier mentioned by Miroshnichenko et al. (2007), while Condori et al. (2019) mentioned both double- and triple-peaked emission lines.

The central (third) peak may not show up in the lower-resolution spectra (e.g., taken at OAN SPM). In addition, a single-peaked appearance seems to be due to the peaks blending even at the highest resolution, making the peak position measurement difficult. In such cases we were only able to estimate the peak position, which is marked in Table 5.

Similar triple-peaked Hα line profiles have been observed in the spectra of some other FS CMa-type objects, such as MWC 728 (Miroshnichenko et al. 2015) and HD 85567 (Khokhlov et al. 2018).

## 3.3. Absorption-line Variations

Measurements of the RVs of individual lines and their groups show variations, which exceed a typical measurement accuracy (1–2 km s⁻¹). In particular, cross-correlation of a set of metallic absorption lines in a wavelength range of 4520–4590 Å that contains ~10 lines (mostly Fe II; see left panel of Figure 3) shows RV variations with an amplitude of ~25 km s⁻¹. We used 19 spectra with the highest signal-to-noise ratio in the local continuum (≥30). The derived RV values with uncertainties are shown in Table 6.

The small number of spectra taken over such a long period of time turned out to be insufficient to reliably detect any regular periodicity in the RV variations. However, their existence may be a sign of a secondary component present in the system. More data need to be obtained on a shorter timescale to search for a stable period.

## 3.4. Brightness Variations

The visual brightness of IRAS 07080+0605 was measured for the first time in the 1960s in the course of the Hamburg survey of emission-line stars (Kohoutek & Wehmeyer 1999, $m_V = 13.6$ mag). The coordinates of IRAS 07080+0605 positionally coincide with those of the visual star TYC 175–3772–1, whose brightness in the Tycho photometric system measured in 1989–1993 in the course of the HIPPARCOS mission was $B_T = 12.27 \pm 0.17$ mag and $V_T = 12.69 \pm 0.28$ mag (Høg et al. 2000). Neither of these data is consistent with the brightness level ($V = 11.9–12.5$ mag) detected in several automatic monitoring surveys, such as NSVS (Woźniak et al. 2004), ASAS-3 (Pojmanski 2002), and ASAS SN (Kochanek et al. 2017).
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Variations of the $H\alpha$ and [O I] $\lambda$6300 lines in the spectrum of IRAS 07080+0605. The following features are shown in the panels from left to right: the $H\alpha$ line profiles with a central peak, the $H\alpha$ line profile variations on a timescale of a few days, the double-peaked $H\alpha$ line profiles, and variations of the [O I] $\lambda$6300 line. The observatory names are shown along with the observing dates for each spectrum. Intensity is normalized to the local continuum, while RV is shown in units of 100 km s$^{-1}$.</figcaption></figure>

<table><caption>**Table 6**
RVs of the Absorption Lines in the 4520–4590 Å Region</caption><thead><tr><th>JD</th><th>RV</th><th>σ(RV)</th></tr></thead><tbody><tr><td>2,453,363.944</td><td>7.2</td><td>1.8</td></tr><tr><td>2,454,082.033</td><td>4.2</td><td>3.5</td></tr><tr><td>2,454,083.927</td><td>2.1</td><td>2.6</td></tr><tr><td>2,454,084.945</td><td>1.1</td><td>2.4</td></tr><tr><td>2,454,085.956</td><td>-6.2</td><td>3.0</td></tr><tr><td>2,454,420.886</td><td>-0.4</td><td>3.0</td></tr><tr><td>2,455,522.160</td><td>5.8</td><td>0.8</td></tr><tr><td>2,455,548.171</td><td>10.5</td><td>1.3</td></tr><tr><td>2,456,250.012</td><td>-8.7</td><td>1.2</td></tr><tr><td>2,456,322.826</td><td>-4.9</td><td>2.0</td></tr><tr><td>2,457,299.987</td><td>3.2</td><td>1.1</td></tr><tr><td>2,457,363.818</td><td>12.5</td><td>0.8</td></tr><tr><td>2,458,093.973</td><td>-9.1</td><td>1.5</td></tr><tr><td>2,458,442.940</td><td>14.0</td><td>0.8</td></tr><tr><td>2,458,447.040</td><td>9.2</td><td>1.9</td></tr><tr><td>2,458,796.939</td><td>5.1</td><td>1.2</td></tr><tr><td>2,459,180.978</td><td>10.9</td><td>2.2</td></tr><tr><td>2,459,192.149</td><td>-8.1</td><td>0.7</td></tr><tr><td>2,459,543.613</td><td>10.1</td><td>1.1</td></tr></tbody></table>

**Note.** Column (1): Julian date of the observation. Column (2): RV of the absorption lines in the spectral region 4520–4590 Å in km s$^{-1}$. Column (3): uncertainty of the RV in km s$^{-1}$. The RV scale is shown relative to that of a template averaged from several CFHT spectra with close RVs. The heliocentric RV in the template spectrum is 26.9 ± 2.8 km s$^{-1}$.

The object's light curve from all three latter surveys, as well as our own data reported in Miroshnichenko et al. (2007) and those from Table 2, is shown in Figure 5.

<figure><img src="image_8.png" /><figcaption>**Figure 5.** Brightness variations of IRAS 07080+0605. Top panel: the V-band data are taken from the NSVS (plus signs), ASAS-3 (filled circles), and ASAS SN (open circles—V-band data) surveys, as well as taken by us (triangles). The ASAS SN data taken in the g band (diamonds) are shifted by -0.07 mag to match the V-band brightness level. The NSVS data are transformed into the standard V-band system using the equation from Woźniak et al. (2004). Bottom panel: K-band data from the 2MASS catalog (the leftmost point) and Table 3. The symbol size shows the average measurement uncertainty.</figcaption></figure>

The light curve is suggestive of several cycles. A periodogram analysis of the ASAS-3 data prompted Condori et al. (2019) to propose that the most probable period in the brightness variations is 72 days, although they also found that the highest peak in the Fourier power spectrum corresponds to a period of 248.2 days. Nevertheless, the data folded with both periods show a very scattered distribution around the average sinusoidal curves.
<figure><img src="image_9.png" /><figcaption>**Figure 6.** The V-band brightness variations IRAS 07080+0605 in 2014–2019 from the ASAS SN data. The top panel shows the light curve with the actual data (open circles) and the running average (dashed line) used to determine the brightness trend. The bottom panel shows a detrended V-band brightness with respect to the running average.</figcaption></figure>

We averaged the data points from both ASAS-3 and ASAS SN taken within every 1-day period and found that the ASAS SN data have much smaller uncertainties. This is most likely due to higher brightness measurement uncertainties of the ASAS-3 data compared to those of ASAS SN. This prompted us to do a periodogram analysis based on the ASAS SN data only. This data set began in the second half of 2014 and is ongoing. First, we analyzed the data from different observing seasons after removing a long-term trend. Then, we used the phase dispersion minimization (PDM) algorithm (Stellingwerf 1978) to search for the most probable brightness variation period.

We found that the light curves during the 2015, 2016, and 2018 seasons resemble each other very closely. We conclude that the ASAS SN light curve is very suggestive of a period of 190 days (Figure 6). However, this period is not revealed in the 2014 and 2019 seasons. The entire set of ASAS SN photometry taken in both filters (V and g) also shows the highest peak in the Fourier power spectrum for the period of 189 days. The peak is rather wide, and folding the light curve with this period produces a single-maximum phase curve with a relatively large scatter (~0.2 mag) but a well-defined minimum. It is very similar to the phase curve shown in Arias et al. (2018) for a period of 246.7 days found in the ASAS-3 data. Such a behavior suggests the presence of a quasi-periodic process with a variable amplitude that is taking place in the dusty disk. A longer cycle with a characteristic time of ~2000 days is also noticeable in the optical brightness variations shown in Figure 5, as well as in the ASAS-3 data shown by Arias et al. (2018).

Our own photometric data in both the optical and near-IR domains show very small variations of the color indices, implying a gray character of the circumstellar extinction (see Section 4). The JHK brightness of IRAS 07080+0605 in our data set is very close to that of the 2MASS observation taken on 1999 December 5.

### 3.5. Interstellar Extinction Law in the Direction of IRAS 07080+0605

To constrain the interstellar part of the total extinction to IRAS 07080+0605, we constructed a relationship of the

<figure><img src="image_10.png" /><figcaption>**Figure 7.** Interstellar extinction law in the direction of IRAS 07080+0605. Filled circles show data for stars from the 15' area around the object labeled in Figure 2. The open circle shows data for HR 2633 (see text). All the distances are taken from the Gaia EDR3 catalog. The thick line shows a linear fit to the star data. The vertical thin lines show the object's distance uncertainty.</figcaption></figure>

interstellar extinction with distance from the Sun in its direction. We measured the brightness of some stars in a 15' × 15' field around IRAS 07080+0605 (Figure 2) using our BVR data (see Section 2) and retrieved their distances from the Gaia EDR3 catalog (Bailer-Jones et al. 2021). Finally, we added the 2MASS JHK and g-band data from the UCAC4 catalog (Zacharias et al. 2012) to our optical photometry and determined $T_{\text{eff}}$ of these stars by comparison with theoretical SEDs from Kurucz (1998) affected by a standard interstellar extinction law with a total-to-selective extinction ratio of $R_V = 3.1$ (e.g., Savage & Mathis 1979).

The resulting interstellar extinction law is shown in Figure 7. Based on this relationship, the interstellar extinction at the object's Gaia EDR3 distance of $542 \pm 10$ pc is $A_V = 0.34 \pm 0.05$ mag, which corresponds to an interstellar reddening of $E(B-V) = 0.10 \pm 0.02$ mag.

According to a relationship between the EW of the DIB at 5780 Å, one of the strongest DIBs in the optical region, and the color excess $E(B-V)$ by Herbig (1993), this DIB would have an EW = $0.05 \pm 0.05$ Å for an $E(B-V) = 0.10$ mag. We might not have detected it in our spectra owing to a moderate signal-to-noise ratio and the broadness of this DIB's profile. Another possibility is a weakness of DIBs along this line of sight, which was not explored in a number of DIB studies (e.g., Raimond et al. 2012; Kos & Zwitter 2013). In order to check how strong DIBs in the object's direction can be, we took an optical spectrum of HR 2633, a reddened B2 star ($E(B-V) = 0.16 \pm 0.01$ mag) located at an angular distance of $2^{\circ}2$ from IRAS 07080+0605. Three spectra (each combined form three 15-minute-long exposures) taken in a range of 3800–7900 Å with a spectral resolving power $R = 12,000$ at the 0.81 m telescope of the Three College Observatory operated by the University of North Carolina Greensboro with an échelle spectrograph manufactured by Shelyak Instruments<fnref n="13" /> were combined into one with a signal-to-noise ratio of ~300 near the DIB 5780 Å. Even with such a high signal, the DIB was hardly noticeable (EW $\le 0.02$ Å), which is consistent with its nondetection in the spectra of IRAS 07080+0605. Both the Gaia EDR3 distance ($788_{-20}^{+31}$ pc) and interstellar extinction ($A_V = 0.50 \pm 0.03$ mag) agree with the average interstellar extinction law shown in Figure 7.

<footnote n="13">¹³ https://www.shelyak.com/</footnote>
From the average object's color index $B-V=0.12 \pm 0.01$ mag (see Table 2) and the intrinsic color index $(B-V)_0 = -0.01 \pm 0.04$ mag for the derived $T_{\text{eff}}$ and log g (Bessell et al. 1998) one can get a color excess of $E(B-V) = 0.13 \pm 0.04$ mag due to a total (inter- and circumstellar) reddening. This value within the uncertainty coincides with the above-derived interstellar reddening and allows us to conclude that any circumstellar extinction around the object is wavelength independent.

### 3.6. Modeling of the Spectral Energy Distribution

A wide coverage of IR fluxes of IRAS 07080+0605 in all-sky surveys (e.g., IRAS, WISE, MSX, AKARI) allows us to attempt an SED modeling of IRAS 07080+0605 in order to derive some properties of its dusty environments. Following our findings in Section 3.1, we assume that the SED of IRAS 07080+0605 is composed of fluxes from an early A-type central star processed by a circumstellar dusty disk. Since the observed IR excess is larger than the stellar integrated flux in the optical spectral range, a significant amount of the stellar radiation has to be attenuated by the disk material.

The luminosity derived from the observed visual brightness is lower than expected even for a main-sequence star of the derived $T_{\text{eff}}$. It is inconsistent with the observed optical spectral features, such as the absorption Balmer line wings and strengths of some gravity-sensitive lines (e.g., Si II $\lambda$6347 and 6371). Using the maximum visual brightness recorded by the recent surveys ($V=12.0 \pm 0.1$ mag), the interstellar extinction derived above ($A_V=0.34 \pm 0.05$ mag), the Gaia EDR3 distance (542 $\pm$ 10 pc), and a bolometric correction for the adopted $T_{\text{eff}}$ ($BC_V = -0.01 \pm 0.07$ mag), one can derive an absolute bolometric magnitude of $M_{\text{bol}} = 3.0 \pm 0.2$ mag or log $L/L_\odot = 0.70 \pm 0.08$.

A minimum reasonable luminosity of the object follows from equating the total flux emitted by the underlying star and measured in the IR excess. Since the energy in the IR excess is ~10 times larger, the star's luminosity has to be at least 10 times larger as well (log $L/L_\odot \ge 1.7$). However, as the entire star's energy is not absorbed and reemitted by the circumstellar dust, the star should be even more luminous.

For the stellar flux we took an ATLAS9 model atmosphere provided by F. Castelli$^{14}$ ([M/H] = 0.0, $v_{\text{turb}} = 2.0$ km s$^{-1}$) for $T_{\text{eff}} = 8500$ K and log g = 2.0. We used the same disk-like model as in our previous studies of the emission-line objects IRAS 22150+6109 (Zakhozhay et al. 2018) and 3Pup (Miroshnichenko et al. 2020). We note that one should consider it with caution, as there is no detailed information about the spatial distribution of the circumstellar dust in this system (e.g., IR and submillimetric images). Therefore, our main goal in the SED modeling was to estimate how much dust emits at various wavelengths. To calculate the disk opacity, we used the code OpacityTool available through Project Diana$^{15}$ introduced by Woitke et al. (2016). The code calculates optical properties of various mixtures of spherical silicate and carbonaceous grains.

The dusty grain sizes were assumed to be distributed between 0.1 and 100 µm to make the circumstellar extinction wavelength independent. We varied the following two dust

<figure><img src="image_11.png" /><figcaption>**Figure 8.** The best-fit theoretical SED (solid line) to the photometric data of IRAS 07080+0605. Red filled circles: optical ground-based photometry from Table 2 and Miroshnichenko et al. (2007); open circles: near-IR JHK data from Table 3; red filled squares: WISE data from Cutri et al. (2021); black filled squares: MSX data from Egan et al. (2003); upward-pointing triangles: IRAS data from Helou & Walker (1988); downward-pointing triangles: AKARI data from Ishihara et al. (2010). The SED of the star without the disk attenuation is shown with the dashed line. The observed fluxes are normalized to that in the V band; the wavelengths are shown in microns.</figcaption></figure>

parameters used in the OpacityTool: the mass fraction of carbonaceous material (from 0 to 1, with an increment of 0.1) and the power-law index of the size distribution (from -3.5 to -3.1, with an increment of 0.05). The chosen grain size distribution allowed us to reproduce the observed fluxes in the visible range, where the observed SED and the spectral line content indicated the presence of a wavelength-independent circumstellar extinction. We also tried to reproduce the overall shape of the IR excess and some local features (e.g., the continuum variations near the 10 and 18 µm silicate features) by varying relative contributions of the silicate and carbonaceous grains. We did not attempt to model the polycyclic aromatic hydrocarbon (PAH) emission features, as this requires more information about the circumstellar environments, which is very limited.

The geometry of a disk tilted with respect to the line of sight is accounted for by the approach described in Zakhozhay et al. (2015). It assumes that the disk edges at the inner ($R_{\text{in}}$) and outer ($R_{\text{out}}$) radii have a cutoff flat geometry and emit as blackbodies with constant temperatures derived from the radiative equilibrium condition and a radial dust temperature dependence, $T_r = T_{\text{in}}(r/R_{\text{in}})^{-\bar{q}}$, where $T_{\text{in}}$ is the dust sublimation temperature at $R_{\text{in}}$. The value of $T_{\text{in}}$, sensitive to the near-IR fluxes, was found to be 1650 K, which is an intermediate temperature between the dust sublimation temperatures of pure silicate (~1200 K) and pure carbon (~2000 K) particles.

We used the dust surface density radial power-law index $p = -2.0$, which results from a spherical expansion with a constant velocity and is slightly lower than $p = -1.5$ assumed for collisionally dominated disks, while $R_{\text{in}}$, $R_{\text{out}}$, the dust mass $M_d$, the disk height at the inner radius $H_{\text{in}}$, the radial dust temperature index $\bar{q}$, and the disk inclination $i_{\text{disk}}$ were treated as free parameters. We also assumed that the disk has a gas-to-dust mass ratio of 100.

We have found that the best fit to the observed SED shown in Figure 8 is achieved with a disk model, which has the following parameters: $R_{\text{in}} = 0.42$ au, $R_{\text{out}} = 33$ au,

<footnote>$^{14}$ https://wwwuser.oats.inaf.it/castelli/grids/gridp00k2odfnew/fp00k2tab.html</footnote>

<footnote>$^{15}$ https://diana.iwf.oeaw.ac.at/data-results-downloads/fortran-package/</footnote>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 9.** Top panel: comparison of blue parts of the spectra of IRAS 07080+0605 (black line) and Red Rectangle (red line) taken with the ESPaDoNs spectropolarimeter at CFHT. Bottom panel: SEDs of IRAS 07080+0605 (black filled circles: photometric data; black solid line: IR spectrum taken at the Spitzer Space Observatory) and Red Rectangle (red open squares: photometric data; red solid line: IR spectrum taken at the Infrared Space Observatory) corrected for the interstellar reddening. The dashed line shows a theoretical SED for the model atmosphere for T<sub>eff</sub> = 8500 K, log g = 2.0. The same model atmosphere was chosen to show the IR excess, although the optically brighter component of Red Rectangle was found to be ~800 K cooler (Waelkens et al. 1992). The units of fluxes and wavelengths are the same as in Figure 8.</figcaption></figure>

$M_d = 8 \times 10^{-7} M_\odot$, $H_{in} = 0.1 R_{in}$, $i_{disk} = 79^\circ$ (nearly edge-on), $\bar{q} = 0.585$, mass fraction of the carbonaceous grains of 0.1–0.2, and a power law of the size distribution of –3.5. The luminosity of the central star in this model was 216 $L_\odot$, which corresponds to a radius of 6.8 $R_\odot$. Therefore, the stellar flux is attenuated by the dusty disk by a factor of ~43 at the effective wavelength of the V band.

The best fit is mostly sensitive to the disk inner and outer boundaries, which constrain the flux rise in the near-IR region and the flux slope at the longest wavelengths with existing data. The fraction of carbonaceous grains controls the mid-IR region features. Increasing it over 20% makes the SED featureless in the 10–20 µm region.

There is a degeneracy between the disk tilt angle and the amount of visual attenuation (a higher tilt results in a lower attenuation with nearly the same fit to the SED); the derived attenuation value implies that the star's intrinsic luminosity is consistent with our spectroscopic estimates of its fundamental

parameters ($T_{\text{eff}}$ and log g). A larger tilt angle ($i_{\text{disk}} \sim 90^\circ$) would result in a higher intrinsic luminosity, a lower surface gravity, and a larger mass of the star.

Although our SED model described above is not perfect, the calculated SED closely resembles the observed one (Figure 8). The model can be improved by adjusting the dust size distribution and composition, as well as expanding the parameter space, but it would require more information about the object's circumstellar environment (e.g., IR and submilli-metric fluxes or images), which is unavailable yet. Therefore, we will leave this work to the future.

## 4. Discussion

Our spectroscopic data, obtained, although sparsely, over a period of almost 17 yr, show no noticeable changes in the spectral line content. The emission-line profiles vary, exhibiting typically two or three emission peaks (see Figure 4). The presence of an additional, central peak in the profiles of these lines may indicate the presence of a variable stellar wind, as we suspected in some FS CMa-type objects (e.g., MWC 728; Miroshnichenko et al. 2015).

The IR excess of IRAS 07080+0605 is unusually strong compared to the observed flux in the visual spectral region. Assuming that the object's brightness is only affected by the low interstellar extinction, which is calculated from the color excess ($E(B-V) = 0.13 \pm 0.04$ mag) and corroborated by the absence of DIBs in the object's spectrum (see Section 3.5), and using the effective temperature derived above, the bolometric flux from the star turns out to be ~10 times lower than the integrated IR excess. This result implies that the star may be attenuated by an almost wavelength-independent circumstellar extinction that is supported by our SED modeling (Section 3.6). The emission features detected in the IR spectrum of IRAS 07080+0605 (see Figure 8 and bottom panel of Figure 9) belong to carbonaceous molecules, which are typically observed in very evolved stars (e.g., protoplanetary nebulae). Therefore, it is worth looking for similar objects at advanced evolutionary stages.

### 4.1. Comparison with Red Rectangle

Among many objects with IR excesses, there is one with a number of properties similar to those of IRAS 07080+0605. This is HD 44179, a binary system with a brighter A/F-type post-asymptotic giant branch (AGB) star and a much fainter hotter component. The system is surrounded by a prominent visual nebula known as Red Rectangle. Both the absorption-line optical spectrum and the IR spectrum resemble those of IRAS 07080+0605 (top panel of Figure 9).

The integrated flux in the IR excess of Red Rectangle is ~18 times larger than the bolometric flux of the brighter stellar component (Oomen et al. 2019). Men'schikov et al. (2002) modeled the SED of Red Rectangle and found that large carbonaceous grains in the circumstellar disk/torus attenuate the visible system component by a factor of ~30. As we showed in Section 3.6, a similar solution is applicable to IRAS 07080+0605, but the limited information about the dust properties in this system prevented us from conducting a more thorough SED modeling.

The largest difference between the two objects is that IRAS 07080+0605 shows no detectable nebula around it, although both are located at about the same distance from the
Sun (~0.5 kpc for IRAS 07080+0605 and ~0.7 kpc for Red Rectangle). Furthermore, IRAS 07080+0605 shows wider absorption profiles of the Balmer lines (Figure 8, top panel), which implies a higher surface gravity than that of Red Rectangle (log *g* = 1.5; Men'schikov et al. 2002). This is consistent with our estimate of log *g* ~ 2.0 for IRAS 07080+0605 mentioned above (see Section 3.1 and Figure 3, right panel).

Visible components of most post-AGB binaries with dusty envelopes are cooler (FGK spectral type) than IRAS 07080+0605 and demonstrate weaker absorption lines compared to those in the spectra of the same spectral types with the solar elemental abundances. The latter effect has been explained by depletion of refractory elements from the star's atmosphere (Oomen et al. 2019). We found that the absorption-line spectrum of IRAS 07080+0605 contains some lines highly sensitive to the luminosity, such as the Si II λλ6347 and 6371 lines and the O I triplet at 7772–7775 Å mentioned above, whose strengths are closer to those of giants and supergiants. This suggests that the visual component of IRAS 07080+0605 is a low-mass star that mimics a massive evolved star similarly to the dusty post-AGB binaries.

The ASAS SN light curve of Red Rectangle looks similar to that of IRAS 07080+0605, showing cyclic variations with an average amplitude of 0.2 mag. Our periodogram analysis shows the strongest peak in the Fourier power spectrum at a period of 309.5 days. Similar periods of 298 and 318 days, along with an RV amplitude of ~26 km s⁻¹, are listed in Van Winckel et al. (1995) and Waelkens et al. (1996). The latter authors suggested that the photometric variability of Red Rectangle is due to variations of the scattering angle toward the observer as the visible component moves along the orbit. Spectral lines of Red Rectangle exhibit periodic positional variations following the photometric period. Oomen et al. (2019) list a spectroscopic period of 317.6 ± 1.1 days and an orbital eccentricity of 0.27 ± 0.03.

Men'schikov et al. (2002) proposed an evolutionary scenario for Red Rectangle as a binary system with the component's initial masses of 2.3 and 1.9 $M_\odot$ and current masses of 0.57 $M_\odot$ (post-AGB) and 0.35 $M_\odot$ (white dwarf). Our data showing that IRAS 07080+0605 exhibits a similar RV amplitude (~25 km s⁻¹; see Section 3.3) and photometric cycles of ~190 days may imply its similar nature.

The absence of a visible nebula around IRAS 07080+0605, which is supported by our SED modeling results for the total dust mass in its disk of ~10⁻⁷ $M_\odot$ and an outer disk radius of 33 au, both of which are much smaller than those of Red Rectangle (0.01 $M_\odot$ and 43,000 au, respectively; Men'schikov et al. 2002), suggests that IRAS 07080+0605 is undergoing an earlier evolutionary phase. The possible secondary component of IRAS 07080+0605 should be much hotter than the visual one because there is no indication of its contribution to the object's spectrum longward of the Balmer jump.

## 4.2. Nature and Evolutionary Status

The emission component of the Hα line is very strong (EW ~ 50–60 Å; see Table 5) for a star with such a low $T_{eff}$. The lowest-temperature Be stars (spectral types A1–A2) and most Herbig Ae stars typically exhibit Hα EWs ≤ 30 Å (see, e.g., Coté & van Kerkwijk 1993; Miroshnichenko et al. 1999), except for the youngest ones that are still embedded in

<figure><img src="image_14.png" /><figcaption>**Figure 10.** H-R diagram with positions of FS CMa objects with known fundamental parameters. Several recently recognized binary systems are marked with their names. The positions of IRAS 07080+0605 and HD 44179 uncorrected for the dust attenuation are shown by the square and the diamond, respectively, while the corrected positions coincide with the end of the vertical arrowhead lines. Evolutionary tracks for single rotating stars are taken from Ekström et al. (2012) and shown by solid lines along with the main-sequence location. Numbers by the tracks indicate initial masses in solar units.</figcaption></figure>

protostellar nebulae (e.g., AB Aur, HK Ori, and KK Oph; Manoj et al. 2006).

The pre-main-sequence status for IRAS 07080+0605 was proposed in both of the above-mentioned papers (Arias et al. 2018; Condori et al. 2019). This hypothesis is based on the presence of an emission-line spectrum and a strong IR excess. However, no signs of a star-forming region or a nebulosity are seen around the object, whose distance is not large enough to miss them. The low surface gravity determined here is inconsistent with an isolated Herbig Ae star status near main sequence. Furthermore, the PAH emission features are typically very weak in the IR spectra of Herbig Ae stars (e.g., Meeus et al. 2001), which is in a striking contrast with those detected in the SSO spectrum of IRAS 07080+0605 (see bottom panel of Figure 9).

Finally, the slope of the IR excess at wavelengths longer than its peak at ~10 µm is too steep for a young star as was pointed out by Miroshnichenko (2007), who ruled out the young nature of the FS CMa group. This conclusion has not been challenged yet. Therefore, we do not see sufficient evidence for a young age of the object.

The positions of IRAS 07080+0605 and Red Rectangle in the H-R diagram with and without correction for the attenuation by the circumstellar dust are shown in Figure 10. Compared to the positions of other FS CMa group members also shown in this figure, IRAS 07080+0605 is among the least massive and least luminous. If our visual attenuation estimate is correct, then its current mass can be ~3 $M_\odot$. Its location beyond the main sequence is supported by the presence of carbonaceous dust grains (IR emission features; see bottom panel of Figure 9), which is considered a sign of an advanced evolutionary stage. Waelkens et al. (1996) explain the dust composition in Red Rectangle by dredge-ups, which occur at the red giant stage.

The entire set of properties of IRAS 07080+0605 is hard to explain in the framework of the evolution of a single star. Given the ruled-out pre-main-sequence status, the circumstellar dust, whose presence is evident from the IR excess, is unlikely
to survive from the system formation time. The underlying star of such a low $T_{\text{eff}}$ would not provide enough ionizing radiation to account for the strong $H\alpha$ line, while a low mass (if the luminosity is not corrected for the circumstellar extinction) would not provide a stellar wind, which is strong enough to account for dust formation in the material ejected by the star. Additionally, the presence of a central peak in the line profiles (e.g., $H\alpha$) probably forms either in a bipolar outflow or in a region around the first Lagrangian point of the binary. In any case, it manifests the existence of a circumstellar structure, which is more complicated than just an axisymmetric disk.

Another sign that IRAS 07080+0605 is undergoing an early phase of the post-AGB dusty binary evolution (e.g., Oomen et al. 2019) is that it shows no obvious signs of the atmospheric volatile element depletion, which is common in this group of objects and in its member Red Rectangle. Van Winckel (2014) pointed out the uniqueness of Red Rectangle, as it exhibits a range of phenomena that include the Extended Red Emission, an optical nebula that can be seen even on images taken through red filters, depletion of refractory elements that can get condensed to dusty grains, etc. However, it is unclear why Red Rectangle stands out in a large group of objects with very similar properties. Our study of IRAS 07080+0605, which currently lacks some of these properties, suggests that it can develop into an analog of Red Rectangle.

## 5. Conclusions

We presented the results of the high-resolution spectroscopic and multicolor photometric observations of the emission-line-type object IRAS 07080+0605. By studying a large range of the optical spectrum, we showed that the visible component has a lower effective temperature ($T_{\text{eff}} = 8500 \pm 500$ K) and a lower surface gravity ($\log g = 2.0 \pm 0.5$) than previously published values: $T_{\text{eff}} = 13,000$ K, $\log g = 4.5$ (Arias et al. 2018), and $T_{\text{eff}} = 10,100 \pm 700$ K (Condori et al. 2019).

IRAS 07080+0605 exhibits an unusually strong emission-line spectrum for such a low temperature of the star and is both photometrically and spectroscopically variable. We revealed the cyclic nature of the optical brightness variations but have not detected any stable periodicity. The length of the cycles (~190 days) suggests that the variations are due to processes in the circumstellar dusty disk.

Based on the determined fundamental parameters, the absence of a visible nebula and a star-forming region near the object's location, and the properties of the IR excess (a steep SED slope toward longer wavelengths and strong PAH emissions), we rule out the young (pre-main-sequence) status of IRAS 07080+0605.

A number of properties of IRAS 07080+0605 are similar to those of Red Rectangle. These include a strong IR excess, which turned out to be ~10 times larger than the bolometric flux from the visible star; a set of narrow emission lines in the IR spectrum most likely due to carbonaceous (PAH) dusty particles; a heavy obscuration of the underlying star by the dusty disk that implies a gray circumstellar extinction; and cyclic photometric variations.

At the same time, there are big differences in the behavior of the two objects. In particular, IRAS 07080+0605 exhibits no optical nebula around the star, and its optical spectrum shows no signs of refractory element depletion (which is common in dusty protoplanetary systems). Overall, IRAS 07080+0605 satisfies the observational and physical criteria for the FS CMA

group (Miroshnichenko 2007), a leading hypothesis of whose nature remains binary evolution with nonconservative mass transfer. The entire set of observed properties of IRAS 07080+0605 led us to a suggestion that it can be an analog of Red Rectangle at an early evolutionary stage.

It is definitely worth monitoring the object both spectroscopically and photometrically to search for a possible orbital period. Interferometric and other observations with a high spatial resolution may be successful in further revealing its nature, as the angular size of its dusty and even gaseous envelope/disk can be a fraction of a minute of arc at its distance ($542 \pm 10$ pc) measured by Gaia EDR3.

We thank the anonymous referee for a very careful reading of the manuscript and useful critique, which allowed us to present our findings in a clearer and more comprehensive way. This paper is partly based on observations obtained at the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique de France, and the University of Hawaii, as well as on observations obtained at the 2.7 m Harlan J. Smith telescope of the McDonald Observatory (Texas, USA) and the 2.1 m of the Observatorio Astronómico Nacional San Pedro Martir (Baja California, Mexico). This research was funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (grant No. AP08856419). S.K. acknowledges Al-Farabi Kazakhstan National University for a postdoc position. S.Z. acknowledges support from DGAPA/PAPIIT Project IN 102120. The observations at the Canada-France-Hawaii Telescope were performed with care and respect from the summit of Maunakea, which is a significant cultural and historic site. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.

## ORCID iDs

S. A. Khokhlov https://orcid.org/0000-0001-5163-508X

A. S. Miroshnichenko https://orcid.org/0000-0002-3833-1038

S. V. Zharikov https://orcid.org/0000-0003-2526-2683

K. N. Grankin https://orcid.org/0000-0001-5707-8448

O. V. Zakhozhay https://orcid.org/0000-0002-4199-6356

N. Manset https://orcid.org/0000-0001-7548-0190

A. A. Arkharov https://orcid.org/0000-0001-6730-9641

V. M. Larionov https://orcid.org/0000-0002-4640-4356

A. A. Khokhlov https://orcid.org/0000-0001-6987-9058

A. T. Agishev https://orcid.org/0000-0001-9788-7485

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