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An Intermediate Luminosity Transient in Ngc 300: the Eruption of a Dust-Enshrouded Massive Star

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An Intermediate Luminosity Transient in Ngc 300: the Eruption of a Dust-Enshrouded Massive Star An Intermediate Luminosity Transient in Ngc 300: The Eruption of a Dust-Enshrouded Massive Star The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Berger, E., A. M. Soderberg, R. A. Chevalier, C. Fransson, R. J. Foley, D. C. Leonard, J. H. Debes, et al. 2009. An Intermediate Luminosity Transient in Ngc 300: The Eruption of a Dust-Enshrouded Massive Star. The Astrophysical Journal 699, no. 2: 1850–1865. Published Version doi:10.1088/0004-637x/699/2/1850 Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:42668748 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Open Access Policy Articles, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#OAP;This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Open Access Policy Articles, as set forth at http://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#OAP DRAFT VERSION FEBRUARY 21, 2013 Preprint typeset using LATEX style emulateapj v. 03/07/07 AN INTERMEDIATE LUMINOSITY TRANSIENT IN NGC300: THE ERUPTION OF A DUST-ENSHROUDED MASSIVE STAR E. BERGER1,A.M.SODERBERG1,2 ,R.A.CHEVALIER3,C.FRANSSON4,R.J.FOLEY1,5 ,D.C.LEONARD6,J.H.DEBES7, A. M. DIAMOND-STANIC8,A.K.DUPREE1,I.I.IVANS9,J.SIMMERER10,I.B.THOMPSON9,C.A.TREMONTI8 Draft version February 21, 2013 ABSTRACT We present multi-epoch high-resolution optical spectroscopy, UV/radio/X-ray imaging, and archival Hubble and Spitzer observations of an intermediate luminosity optical transient recently discoveredin the nearby galaxy NGC300. We find that the transient (NGC300OT2008-1)has a peak absolute magnitude of Mbol ≈ −11.8 mag, intermediate between novae and supernovae, and similar to the recent events M85OT2006-1 and SN2008S. Our high-resolution spectra, the first for this event, are dominated by intermediate velocity (∼ 200 − 1000 − km s 1) hydrogen Balmer lines and Ca II emission and absorption lines that point to a complex circumstellar environment, reminiscent of the yellow hypergiant IRC+10420. In particular, we detect broad Ca II H&K absorption with an asymmetric red wing extending to ∼ 103 km s−1, indicative of gas infall onto a massive and relatively compact star (blue supergiant or Wolf-Rayet star); an extended red supergiant progenitor is unlikely. The origin of the inflowing gas may be a previous ejection from the progenitor or the wind of a massive binary companion. The low luminosity, intermediate velocities, and overall similarity to a known eruptive star indicate that the event did not result in a complete disruption of the progenitor. We identify the progenitor in archival Spitzer observations, with deep upper limits from Hubble data. The spectral energy distribution points 4 to a dust-enshrouded star with a luminosity of about 6 × 10 L⊙, indicative of a ∼ 10 − 20 M⊙ progenitor (or binary system). This conclusion is in good agreement with our interpretation of the outburst and circumstellar properties. The lack of significant extinction in the transient spectrum indicates that the dust surrounding the progenitor was cleared by the outburst. We thus predict that the progenitor should be eventually visible with Hubble if the transient event marks an evolutionary transition to a dust-free state, or with Spitzer if the event marks a cyclical process of dust formation. Subject headings: stars:evolution — stars:mass loss — stars:circumstellar matter — stars:winds, outflows 1. INTRODUCTION and origins. Some have been classified as low luminosity 56 In recent years dedicated searches and serendipitous dis- core-collapse SNe, with inferred energies and Ni masses coveries have uncovered several optical transients with lu- that are at least an order of magnitudebelow typical SN events minosities intermediate between the well-studied classes of (e.g., Pastorello et al. 2004). Others, sometimes initially clas- nova eruptions (M ∼ −8 mag) and supernova (SN) explosions sified as type IIn SNe, have been subsequently tagged with the (M ∼ −17 mag). As can be expected from the wide gap in catch-all designation of “SN impostors” (e.g., Van Dyk et al. luminosity, these intermediate luminosity optical transients 2000), that includes events resembling luminous blue variable (hereafter, ILOTs11) appear to be diverse in their properties (LBV) eruptions. Finally, a small number of somewhat dim- mer events have been recently grouped under the proposed 1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cam- name “luminous red novae” (LRNe; Kulkarni et al. 2007), bridge, MA 02138 based on their red optical colors; the origin of LRNe and their 2 Hubble Fellow relation to each other remain unclear. Overall, the various 3 Department of Astronomy, University of Virginia, P.O. Box 400325, groupings have been subject to debate, and there is no clear Charlottesville, VA 22904-4325 4 Department of Astronomy, Stockholm University, AlbaNova, SE-106 91 agreement about the nature of individual objects or the degree arXiv:0901.0710v1 [astro-ph.HE] 7 Jan 2009 Stockholm, Sweden of overlap between the various designations. 5 Clay Fellow Equally important, the connection between the different 6 Department of Astronomy, San Diego State University, PA-210, 5500 types of events and different classes of progenitors remains Campanile Drive, San Diego, CA 92182-1221 7 Carnegie Institution of Washington, Department of Terrestrial Mag- unclear. For example, some LBV eruptions have been initially netism, 5241 Broad Branch Road, Washington, DC 20015 classified as SNe (e.g., SNe 1961V and 1954J; Goodrich et al. 8 Steward Observatory, University of Arizona, 933 North Cherry Avenue, 1989; Van Dyk et al. 2005). Similarly, the progenitor of Tucson, AZ 85721 the recent event SN2008S, which was detected in archival 9 Observatories of the Carnegie Institution of Washington, 813 Santa Bar- bara Street, Pasadena, CA 91101 Spitzer Space Telescope images, has been argued to be an 10 Lund Observatory, Box 43, SE-221 00 Lund, Sweden extreme AGB star (Prieto et al. 2008; Thompson et al. 2008) 11 We prefer the neutral designation intermediate luminosity optical tran- or an LBV (Smith etal. 2008). The nature of the event it- sient for this class of events since it does not impose a bias on the discovery self is also unclear, with claims of a low mass electron- method and initial mis-classification (as in the case of the “SN impostors” capture SN (Thompson et al. 2008) and an LBV-like outburst designation Van Dyk et al. 2000), it avoids various observational cuts on the optical properties (e.g., based on color as in the case of the “luminous red (Smith et al. 2008). The recent event M85OT2006-1, and the nova” designation Kulkarni et al. 2007) which may also be due to extrinsic possibly related eruptive object V838 Mon, have been spec- effects (e.g., extinction), it is less cumbersome and more intuitive than desig- ulated to possibly result from stellar mergers (Munari et al. nations based on prototype events (e.g., V838 Mon-like, η Car analogs), and perhaps most importantly, it makes no assumption about an eruptive versus 2002; Soker & Tylenda 2006; Kulkarni et al. 2007), but other explosive origin (as in the case of the “luminous red novae” or “SN impos- possibilities have been proposed such as a low-luminosity tors”). SN for M85OT2006-1 (Pastorello et al. 2007), and an AGB 2 pulse, nova eruption from an embedded common enve- troscopy. In addition, we obtained archival Hubble Space lope white dwarf, or planet capture for V838 Mon (e.g., Telescope and Spitzer Space Telescope observations at the po- Retter et al. 2006 and references therein). sition of the transient to search for a pre-discovery counter- Regardless of the exact interpretation, it is clear that the part. various proposed eruption/explosion scenarios mark impor- tant phases in the evolution of massive stars, possibly on 2.1. Astrometry the path to diverse types of SNe. Therefore, a mapping be- The original reported position of NGC300OT2008-1 was tween ILOTs and their progenitors will provide important α =00h54m34.16s, δ =−37◦38′28.6′′ (J2000), measured rel- constraints on the initial conditions (i.e., progenitors and cir- ative to the core of NGC300 (Monard 2008). As part of cumstellar properties) of SN explosions. The present uncer- our initial spectroscopic observations we obtained an r-band tainty in this mapping highlights the need for detailed obser- image of the field with the Low Dispersion Survey Spectro- vations that can uncover the conditions both prior to and dur- graph (LDSS-3) mounted on the Magellan/Clay 6.5-m tele- ing the ILOT events. These include the identification of pro- scope. Using 13 objects in common with the Naval Obser- genitors, a detailed study of the circumstellar environment, vatory Merged Astrometric Dataset (NOMAD), we measure and measurements of the event properties across the electro- the actual position of the transient to be α =00h54m34.52s, magnetic spectrum. Such observations will also allow us to δ =−37◦38′31.6′′ (J2000) with an uncertainty of about 0.4′′ determine the connection between the various proposed cate- in each coordinate. This position is 4.3′′ west and 3.0′′ south gories of ILOTs, and to map the overall diversity of this seem- of the IAUC position. The revised coordinates allowed us to ingly heterogeneous class. search for a progenitor in archival HST observations, and led In this paper we present such a combination of ob- Thompson et al. (2008) to identify a counterpart in archival servations for an ILOT discovered in the nearby galaxy Spitzer observations (see §6).
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