The Astrophysical Journal Letters, 793:L22 (5pp), 2014 October 1

doi:10.1088/2041-8205/793/2/L22

C 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

A BLUE POINT SOURCE AT THE LOCATION OF SUPERNOVA 2011DH
Gastón Folatelli1 , Melina C. Bersten1 , Omar G. Benvenuto2,3 , Schuyler D. Van Dyk4 , Hanindyo Kuncarayakti5,6 ,Keiichi Maeda7 , Takaya Nozawa8 , Ken’ichi Nomoto1,9 , Mario Hamuy5,6 , and Robert M. Quimby1
1 Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, Kashiwa, Chiba 277-8583, Japan; gaston.folatelli@ipmu.jp
2 Facultad de Ciencias Astronómicas y Geofı́sicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, B1900FWA La Plata, Argentina
3 Instituto de Astrofı́sica de La Plata (IALP), CONICET, Argentina
4 Spitzer Science Center/Caltech, Mailcode 220-6, Pasadena, CA 91125, USA
5 Millennium Institute of Astrophysics (MAS), Casilla 36-D, Santiago, Chile
6 Departamento de Astronomı́a, Universidad de Chile, Casilla 36-D, Santiago, Chile
7 Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
8 National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan

Received 2014 August 15; accepted 2014 August 28; published 2014 September 10

ABSTRACT
We present Hubble Space Telescope (HST) observations of the field of the Type IIb supernova (SN) 2011dh in
M51 performed at ≈1161 rest-frame days after explosion using the Wide Field Camera 3 and near-UV filters
F 225W and F 336W . A star-like object is detected in both bands and the photometry indicates it has negative(F 225W − F 336W ) color. The observed object is compatible with the companion of the now-vanished yellow
supergiant progenitor predicted in interacting binary models. We consider it unlikely that the SN is undergoingstrong interaction and thus estimate that it makes a small contribution to the observed flux. The possibilities of
having detected an unresolved light echo or an unrelated object are briefly discussed and judged unlikely. Adopting
a possible range of extinction by dust, we constrain parameters of the proposed binary system. In particular, theefficiency of mass accretion onto the binary companion must be below 50%, if no significant extinction is producedby newly formed dust. Further multiband observations are required in order to confirm the identification of theobject as the companion star. If confirmed, the companion star would already be dominant in the UV–opticalregime, so it would readily provide a unique opportunity to perform a detailed study of its properties.
Key words: binaries: close – supernovae: general – supernovae: individual (SN 2011dh)
Online-only material: color figures

a compact progenitor, possibly a Wolf–Rayet (WR) star. However, through inspection of deep pre-explosion images from the
Hubble Space Telescope (HST), Maund et al. (2011) and Van
Dyk et al. (2011) independently found an object at the SN location that was compatible with an extended yellow supergiant
star (YSG), and not with a WR star. The question arose as towhether this object was the actual progenitor, a companion ina binary system, or an unrelated object in the line of sight.
While Van Dyk et al. (2011) speculated that the actual progenitor was an unseen hot compact star, Maund et al. (2011)
suggested the possibility of having a YSG progenitor in a close
binary system, similar to the case of SN 1993J. By increasing
the mass-loss rates in single stellar evolution models, Georgy(2012) was able to produce YSG progenitors of core-collapse
SNe, although without providing a physical motivation for themodified mass loss. Soon after, Soderberg et al. (2012) suggested the progenitor must have been a compact star basedon early radio observations of the SN. The latter was subsequently supported by Krauss et al. (2012) and Bietenholz et al.
(2012). However, Maeda (2012) and Horesh et al. (2013) laterquestioned the ability of radio data to indicate the actual progenitor size, thus arguing against the necessity of a compact
progenitor.
In the meantime, Bersten et al. (2012) performed a moredetailed, numerical hydrodynamical modeling of the SN light
curve and expansion velocity, and showed that the progenitor
could not have been a massive WR star and, very importantly,that it had a radius of R  200 R , compatible with a YSG
star. Bersten et al. (and later Benvenuto et al. 2013, in greater
detail) presented evolutionary models of close binary systems

1. INTRODUCTION
After decades of sustained progress in stellar evolution theory, the connection between different types of core-collapse supernovae (SNe) and their progenitors remains partly unknown.
Hydrogen-rich, Type II SNe, have been found to arise from redsupergiant stars, as expected from evolutionary models (Smartt
2009; Van Dyk 2012). Conversely, the origin of hydrogen-poor
SNe (Types Ib, Ic, and IIb) is unclear and one important questionis how their progenitors expel the hydrogen envelope. This process may be regulated by wind, which is expected to be stronger
for larger progenitor mass. However, there is compelling evidence that binarity and, in particular, mass-transfer processes ininteracting binaries, must play an important role in the evolutionof most massive stars (Sana et al. 2012).
In this context, characterizing the progenitors of Type IIb
SNe, which retain a small fraction of their hydrogen envelopes,
becomes particularly relevant. One of the best-studied objectsof this type is SN 1993J, whose progenitor has been identified
as a massive binary system composed of a K-type supergiant
progenitor plus a B-type supergiant companion (Maund et al.
2004; Maund & Smartt 2009; Fox et al. 2014). More recently,
the discovery of the Type IIb SN 2011dh in M51 provided theopportunity—with improved observational capability—of identifying yet another progenitor of this relatively uncommon typeof core-collapse SNe. Very early light curves and spectra wereinterpreted by Arcavi et al. (2011), using an analytical representation of the post-shock-breakout cooling, to be indicative of
9Hamamatsu Professor.

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Folatelli et al.

(a) F225W

N

N

WFC3 2014 Aug

E(b) F336W
WFC3 2014 Aug

E

N

1"

1"

(c) F555W(d) F336W

WFC3 2013 MarchWFPC2 2005 Nov

N

E

E

1"

1"

Figure 1. (a) and (b) Portions of the deep F 225W - and F 336W -band images obtained with the HST and WFC3/UVIS on 2014 August 7.2 UT. (c) 2013 March
image obtained with WFC3/UVIS and filter F 555W showing the fading SN ejecta (Van Dyk et al. 2013). (d) Pre-explosion image obtained with the WFPC2 on
2005 November showing the progenitor detection (Maund et al. 2011; Van Dyk et al. 2011). Images in all four panels are centered at the SN location and scale andorientation are indicated. A point source is detected in both near-UV images from 2014 August, as indicated with tick marks. The position of the source is coincident
with that of the SN in the 2013 March image to a projected distance of 0.15 pc.

companion star of the YSG progenitor and to characterize theproperties of the progenitor system.

that naturally provided a complete picture of the progenitorof SN 2011dh. Their calculations were able to simultaneously
explain the explosion of a YSG star with a final mass compatible
with the result of the hydrodynamical modeling, with a small
amount of hydrogen in the envelope, suitable for producing a
SN IIb, and with a companion star that would not be detected in
the pre-explosion images.
The debate was finally settled by Van Dyk et al. (2013) with
new HST optical imaging of the SN site obtained 21 months
after explosion (this was confirmed by Ergon et al. 2014a)
using ground-based images. The new images showed the preexplosion object had disappeared, thus proving that the YSG star
had exploded. At the moment of those observations the SN ejecta
were too bright to determine the presence of the companion star
proposed by Bersten et al. (2012) and Benvenuto et al. (2013).
It is only recently that it is possible to find that last piece ofthe puzzle that can confirm the binary nature of the progenitor.
Observations should be carried out in the UV range to improvethe chances of detecting the hot companion.
This Letter presents deep near-UV imaging of the site of
SN 2011dh obtained with HST when the SN had an age ofabout 1161 days. The observations were used to search for the

2. OBSERVATIONS AND PHOTOMETRY
The site of SN 2011dh was re-observed on 2014 August
7.2 UT10 with HST and the Wide Field Camera 3 (WFC3)
using the UVIS channel and filters F 225W and F 336W .
The observations were obtained through Cycle 21 program
GO-13426 (PI: J. Maund) with which our program GO-13433(PI: G. Folatelli) was merged once it was realized that bothprograms shared the same target, similar science, and similar
instrumental setup. The exposure times were 3772 s in F 225W ,and 1784 s in F 336W . In Figure 1 we show the images aroundthe location of the SN. For comparison, we show a previous
image obtained with WFC3/UVIS and filter F 555W on 2013
March 2 through program GO-13029 (PI: A. V. Filippenko).
This 2013 image clearly shows the fading SN, which was
also detected in a F 814W -band image obtained simultaneously(Van Dyk et al. 2013). Figure 1 also shows the pre-explosion
10 Dates are given in UT time throughout the paper.

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Folatelli et al.

F 336W -image obtained with the Wide Field Planetary Camera
2 (WFPC2) on 2005 November 13 (GO-10501; PI: Chandar),where the progenitor was detected with low signal-to-noise ratio(Van Dyk et al. 2011; Maund et al. 2011).
A point source is identified at the location of the SN in both2014 August images (Folatelli et al. 2014). We performed image
registration of our 2014 F 336W image relative to the F 555W
image from 2013, using 35 common stars. The formal rms
uncertainty in the geometric transformation was 0.08 pixel ineach direction. The transformed location of the SN was offset
from the blue object by only 0.05 pixel in each axis. Adding
both sources of uncertainty in quadrature, the new object iscoincident with the SN to 0.1 pixel, or 4 mas. At an assumed
distance to M51 of 7.8 Mpc (Ergon et al. 2014a, from an averageof several measurements), the new object is within ≈0.15 pc
from the SN position. We performed point-spread function(PSF) photometry on the HST images using the Dolphot v2.0
package (Dolphin 2000). The resulting VEGAMAG magnitudes
for the detected object are mF225W = 24.57 ± 0.11 mag, andmF336W = 24.94 ± 0.11 mag.
3. ANALYSIS
Figure 2. Estimated brightness of the fading SN. Squares indicate the U- andB-band light curves of SN 2011dh from Ergon et al. (2014b). Filled triangles
show the light curves of SN 1987A (Hamuy & Suntzeff 1990; Walker & Suntzeff
1991) shifted to match the evolution of SN 2011dh (see the text). Open triangles
show the synthetic photometry in the HST bands computed from spectra of
SN 1987A (Pun et al. 1995) and shifted to the scale of SN 2011dh. The solid
curve is a fit to the late-time decline of the U-band light curve of SN 1987A.
Dashed curves are the same fit, shifted vertically to match the synthetic light
curves in F 225W and F 336W bands. The vertical dotted line indicates theepoch of our HST observations where the SN brightness was estimated from theshifted decline fits.
(A color version of this figure is available in the online journal.)

3.1. Nature of the Detected Object
The object detected in our new HST images coincides
with the SN location, as shown in Section 2. We thus tested
whether the detected flux could be explained by the fading
SN alone or if it required an additional source. We estimatedthe contribution from the SN ejecta at an age of 1161 days
based on previous observations of SN 2011dh, and assuming
the light-curve decline followed that of SN 1987A at a similarage. SN 1987A provides the richest available data set of latetime UV observations. Even if its early-time evolution was
different from that of SN 2011dh, at the phase considered
here, it is fair to assume the light curves of both SNe areregulated by radioactive decay processes and thus show asimilar shape. We thus assumed there is no strong interactionof the ejecta with the circumstellar medium (CSM). In thefollowing we conservatively assumed only Galactic extinctionof AV = 0.10 mag for SN 2011dh (Schlafly & Finkbeiner 2011),
and a total extinction of AV = 0.53 ± 0.06 mag for SN 1987A(see Pun et al. 1995). A standard reddening law of Cardelli et al.
(1989) was adopted, with RV = 3.1.
The procedure for estimating the UV flux of the SN is depicted
in Figure 2. We matched the B-band light curve of SN 1987A
from Hamuy & Suntzeff (1990) and Walker & Suntzeff (1991)
to the latest photometry of SN 2011dh at ≈700 days from Ergon
et al. (2014b). From this we determined an offset of 11.5 mag
between both SNe. Assuming the difference in extinction given
above, the offset applied in the U band was of 10.8 mag. We
then calculated synthetic photometry in the U, B, F 225W , andF 336W bands using the spectra of SN 1987A published by Pun
et al. (1995), after correcting for the difference in reddening. The
synthetic photometry was roughly consistent with the observed
U- and B-band photometry, which provides confidence for its
use to derive F 225W and F 336W magnitudes. The spectra of
SN 1987A extended until about 800 days after explosion. Thereis a final spectrum obtained at about 1050 days, but it is too
noisy in the near-UV range to be used here.
We extrapolated the synthetic photometry in F 225W andF 336W , to the epoch of our HST observations. For this purpose,
we used a cubic polynomial fit to the U-band light curveof SN 1987A between 700 and 1500 days, and assumed theF 225W and F 336W bands followed the same decline rate.
With this, the extrapolated magnitudes of the SN ejecta in
the HST bands were mF225W (1161 days) = 27.3 ± 0.5 mag,and mF336W (1161 days) = 26.4 ± 0.5 mag. The uncertainties
were estimated based on the extinction uncertainty, and on thevariation in (U–near-UV) colors among the latest spectra of SN
1987A. This analysis suggests that in the absence of strong CSM
interaction, the SN was ≈2.7 mag and ≈1.5 mag fainter thanthe observed object in F 225W and F 336W , respectively.
The emission from the expanding SN ejecta may be revitalized if it encounters substantial circumstellar material and ashock is produced. This would cause a flattening of the light
curve and it would introduce signatures in the spectrum. No evidence of strong interaction was seen in the optical light curves
or spectra of SN 2011dh at 700 days after explosion (Shivvers et al. 2013; Ergon et al. 2014b; Jerkstrand et al. 2014),
in X-rays at ≈500 days (Maeda et al. 2014), or in radio at≈1000 days (A. Kamble 2014, private communication). Contrary to SN 1993J which showed signs of interaction in thiswavelength range as soon as 100 days after explosion (e.g., Filippenko et al. 1994; Kohmura et al. 1994), SN 2011dh appears
to be comparatively free of CSM (Maeda et al. 2014). Even if
CSM interaction strengthened after ≈1000 days, it would be
difficult to explain the observed blue UV color. Spectra of SN
1993J at similar ages (Fransson et al. 2005) show no strong lines
in the range of the F 225W filter that could be responsible forthe observed color.
An alternative explanation to the observed flux may be anunresolved echo of the SN light reflected in our direction. Any
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Folatelli et al.

and MF336W = −5.15 mag. The allowed range of absolutemagnitudes is roughly compatible with main-sequence, B0–B2
type stars.
There is the possibility that additional dust is formed in the SN
ejecta, which would introduce extra extinction. Nozawa et al.
(2010) predict that significant amounts of dust can form in theejecta of Type IIb SNe approximately between 300 and 700 days
after the explosion, but that the average grain size is smaller thanin Type II P SNe. In addition, UV emission from a putative hotstar present within the ejecta can destroy the small dust grains.
Dust formation has been suggested for SN 2011dh as early as
150 days (Sahu et al. 2013; Ergon et al. 2014b; Jerkstrand et al.
2014), however, it is difficult to quantify the amount of extinctionin the near-UV regime. If present, additional dust would imply
an intrinsically more luminous and bluer object than what wasderived above.
Figure 3 also shows the HST photometry compared withthe predictions from the close-binary evolutionary models of
Benvenuto et al. (2013). The suggested progenitor system hadzero-age masses of 16 M (the exploding star) and 10 M (theaccreting companion), and an initial orbital period of 125 days.
Depending on the value of the mass-accretion efficiency parameter, β, the model predicted different final masses, luminosities,
and temperatures for the companion. The companion star spectra were adopted from atmosphere models of Kurucz (1993)
with an effective temperature and surface gravity as given by
the binary models and were scaled to a distance of M51. To
this we added an SN contribution using the spectrum of SN
1987A at ≈800 days, dereddened, and scaled to the expected at1161 days as estimated in Section 3.1 (Figure 2). The resulting“SN + companion” spectra were reddened by Galactic extinction and synthetic photometry was computed in the F 225W
and F 336W bands. As shown in Figure 3, the observed object
is remarkably compatible with the SN + companion photometry
provided the accretion efficiency is low. For comparison, the diagram shows the estimated position of the SN ejecta. Assuming
additional extinction in the host galaxy of AV  0.3 mag, theproposed companion star is compatible with the observations forβ  0.5. This implies a mass range of 10  M  16 M , forthe secondary star at the time of the SN explosion. Note, however, that we expect a certain degree of degeneracy in the models
between β and the initial mass of the companion, which would
require more extensive observations and modeling to disentangle the degeneracy. If β is small, one can expect part of the mass
lost by the primary to be in the surroundings and to eventually beshocked by the ejecta. In the models of Benvenuto et al. (2013),
however, the mass-transfer rate is high (∼10−3 M yr−1 ) only at∼105 yr before the explosion, so we can assume the ejecta (expanding at ∼104 km s−1 ) will reach this material (if expelled at∼10 km s−1 ) not earlier than ∼102 yr.
The predicted SN + companion photometry for the optical
HST observations obtained on 2014 August 10 (GO-13426) are
25.8  mF435W  26.6 mag, 26.0  mF555W  26.7 mag, and26.0  mF814W  26.4 mag. If there is additional dust formedin the ejecta, the object would be intrinsically brighter, which
would imply a larger companion mass, presumably due to alarger accretion efficiency.

1.0
0.75
0.5

B0.5V
B1V

0.25

B1.5V
B2V

B2.5V

Figure 3. F 336W vs. (F 225W − F 336W ) color–magnitude diagram. The
observed object is shown with a red diamond, without correcting for extinction.
The black dot shows the expected location of the SN, as estimated in Figure 2.
Blue squares indicate the expected observations from the SN plus a binary
companion (SN + companion) according to the models of Benvenuto et al.
(2013), for varying mass-accretion efficiency (β; labeled next to each point). A
distance of 7.8 Mpc was adopted, and the SN + companion model photometry
was reddened by Galactic extinction of AV = 0.10 mag to reproduce theobservations. Green dots show the location of main sequence stars for thesame distance and reddening. The arrow indicates the reddening vector for an
additional extinction of AV = 0.3 mag that may be produced by dust in M51(see the text for details).
(A color version of this figure is available in the online journal.)

such echo should arise from within ∼1 pc from the SN. At that
distance, we only expect to find CSM created from the pre-SN
mass loss, which can be assumed to have too low a density
to produce a detectable echo. In addition, light from the SN
should be backward-scattered, which is relatively inefficient in
the near-UV (Sugerman 2003).
It is in principle possible that the observed emission is due toan unrelated source in the line of sight. However, we consider
this unlikely because such an object, presumably an OB typestar, should be projected at 0.15 pc from the SN, which leaves
a small volume across the disk of the face-on galaxy M51.
3.2. Putative Companion Star Properties
From the analysis in the previous section it is likely thatthe detected object is the companion of the YSG progenitorof SN 2011dh. If this is the case, its observed near-UV colorand magnitude can be used to characterize the companion
star, as shown in Figure 3. Adopting a Galactic extinction
only and an average distance of 7.8 ± 0.9 Mpc, the absolutemagnitudes of the object are MF225W = −5.11 ± 0.29 mag,and MF336W = −4.66 ± 0.29 mag. Extinction by dust in M51 is
uncertain. Arcavi et al. (2011) and Ritchey & Wallerstein (2012)used the strength of Na I D lines in the SN spectra to suggest thatextinction was low. However, from color analysis of the stellar
population in the vicinity of the SN, Murphy et al. (2011) foundextinction of AV ≈ 0.3 mag. Assuming the latter value, thecorrected absolute magnitudes would be MF225W = −5.88 mag

4. CONCLUSIONS
We have shown the detection in deep near-UV images
obtained with HST of a blue point source at the location of theType IIb SN 2011dh. The source’s photometry is compatible
with it being the progenitor companion predicted by Bersten
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We thank Armin Rest for his light-echo calculations anddiscussion. This research is supported by the World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan, and by Grants-in-Aid for Scientific Research (23224004, 23540262, 23740141, 23740175, 26400222,
26400223, 26800100). M.H. and H.K. acknowledge support
from the Millennium Institute of Astrophysics (MAS; Programa
Iniciativa Cientı́fica Milenio del Ministerio de Economı́a, Fomento y Turismo de Chile, grant IC120009). H.K. is supported
by FONDECYT (grant 3140563). O.G.B. is member of the Carrera del Investigador Cientı́fico of CIC, Argentina.

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