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An Analogue of the Galactic Yellow Hypergiant IRC +10420?

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An Analogue of the Galactic Yellow Hypergiant IRC +10420? Open Research Online The Open University’s repository of research publications and other research outputs IRAS 18357-0604 – an analogue of the galactic yellow hypergiant IRC +10420? Journal Item How to cite: Clark, J. S.; Negueruela, I. and González-Fernández, C. (2014). IRAS 18357-0604 – an analogue of the galactic yellow hypergiant IRC +10420? Astronomy & Astrophysics, 561, article no. A15. For guidance on citations see FAQs. c [not recorded] https://creativecommons.org/licenses/by-nc-nd/4.0/ Version: Version of Record Link(s) to article on publisher’s website: http://dx.doi.org/doi:10.1051/0004-6361/201322772 Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online’s data policy on reuse of materials please consult the policies page. oro.open.ac.uk A&A 561, A15 (2014) Astronomy DOI: 10.1051/0004-6361/201322772 & c ESO 2013 Astrophysics IRAS 18357-0604 – an analogue of the galactic yellow hypergiant IRC +10420? J. S. Clark1, I. Negueruela2, and C. González-Fernández2,3 1 Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK e-mail: [email protected] 2 Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain 3 Institute of Astronomy, University of Cambridge, Madingly Road, Cambridge, CB3 0HA, UK Received 30 September 2013 / Accepted 10 November 2013 ABSTRACT Context. Yellow hypergiants represent a short-lived evolutionary episode experienced by massive stars as they transit to and from a red supergiant phase. As such, their properties provide a critical test of stellar evolutionary theory, while recent observations unex- pectedly suggest that a subset may explode as Type II supernovae. Aims. The galactic yellow hypergiant IRC +10420 is a cornerstone system for understanding this phase since it is the strongest post- RSG candidate known, has demonstrated real-time evolution across the Hertzsprung-Russell diagram and been subject to extensive mass loss. In this paper we report on the discovery of a twin of IRC +10420 - IRAS 18357-0604. Methods. Optical and near-IR spectroscopy are used to investigate the physical properties of IRAS 18357-0604 and also provide an estimate of its systemic velocity, while near- to mid-IR photometry probes the nature of its circumstellar environment. Results. These observations reveal pronounced spectral similarities between IRAS 18357-0604 and IRC +10420, suggesting compa- rable temperatures and wind geometries. IR photometric data reveals a similarly dusty circumstellar environment, although historical mass loss appears to have been heavier in IRC +10420. The systemic velocity implies a distance compatible with the red supergiant- dominated complex at the base of the Scutum Crux arm; the resultant luminosity determination is consistent with a physical association but suggests a lower initial mass than inferred for IRC +10420 (20 M versus ∼40 M). Evolutionary predictions for the physical properties of supernova progenitors derived from ∼18–20 M stars – or ∼12–15 M stars that have experienced enhanced mass loss as red supergiants – compare favourably with those of IRAS 18357-0604, which in turn appears to be similar to the the progenitor of SN2011dh; it may therefore provide an important insight into the nature of the apparently H-depleted yellow hypergiant progenitors of some Type IIb SNe. Key words. stars: emission-line, Be – circumstellar matter – stars: evolution 1. Introduction HR diagram; unfortunately observationally identifying such ob- jects has proved difficult. Oudmaijer et al. (2009) list three post- Yellow hypergiants (YHGs) are thought to represent a short RSG stars – IRC +10420 (see also Jones et al. 1993), HD 179821 lived episode of the post-main sequence (MS) evolution of mas- and RSGC1-F15 (cf. Davies et al. 2008), with a further three sive stars; first encountered on a redwards passage across the examples found in the literature – ρ Cas (Lobel et al. 2003), Hertzsprung-Russell (HR) diagram and, for more massive stars, HD 8752 (Nieuwenhuijzen et al. 2012) and IRAS 17163-3907 on a subsequent post-red supergiant (RSG) loop to higher tem- (Lagadec et al. 2011). Of these, the most compelling case is peratures. The precise mass range of stars that encounter this provided by IRC +10420 due to a N-enriched chemistry, pres- phase is a sensitive function of a number of stellar parame- ence of a massive ejection nebula and apparent real time blue- ters such as initial mass and mass loss rate, rotational velocity wards evolution across the HR diagram (Klochkova et al. 1997; and metalicity (e.g. Ekström et al. 2012), making their proper- Castro-Carrizo et al. 2007; Oudmaijer 1998). Currently demon- ties and population statistics critical tests of current stellar evo- strating the earliest and richest emission line spectrum of any lutionary theory. Moreover, with observational indications that > −4 −1 (candidate) galactic YHG as a result of its high luminosity, tem- YHGs drive extensive mass loss (transient rates 10 M yr ; perature and mass loss rate, it represents a cornerstone sys- Castro-Carrizo et al. 2007; Lobel et al. 2003) they may also play tem for understanding the transition of stars away from the an important role in mediating the formation of Wolf-Rayet stars RSG phase to luminous blue variable (LBV), Wolf-Rayet (WR) by stripping away the H-rich mantle of their progenitors. or supernova (SN). Given this, it is unfortunate that YHGs are amongst the rarest As part of an ongoing I-band spectroscopic survey of the en- sub-types of massive stars known, apparently reflecting the rel- virons of the RSG-dominated cluster complex/association at the ative brevity of this evolutionary phase. De Jager (1998)and base of the Scutum-Crux arm (RSG1-5; Figer et al. 2006;Davies de Jager & Nieuwenhuijzen (1997) suggested that the most lu- et al. 2007a, 2008; Clark et al. 2009; Negueruela et al. 2010, minous YHGs are post-RSG stars on a blue loop across the 2011, 2012) we observed the bright but poorly studied IR source IRAS 18357-0604 (henceforth IRAS 1835−06; =2MASS Partially based on service observations made with the William Herschel Telescope operated on the island of La Palma by the J18382341-0601269) located 14 from RSGC2/Stephenson 2, Isaac Newton Group in the Spanish Observatorio del Roque de los revealing an unusually rich emission line spectrum. With the Muchachos of the Instituto de Astrofísica de Canarias. optical region inaccessible due to high interstellar reddening, Article published by EDP Sciences A15, page 1 of 7 A&A 561, A15 (2013) IRAS18357 (black) IRC +10 420 (red) HD 223385 (A3; blue) Fig. 1. I band spectrum of IRAS 1835−06 (black, solid lines) plotted against IRC +10420 (red, dotted) and HD 223385 (A3 Ia; blue, solid). Spectra have been shifted in wavelength to account for their individual systemic radial velocities as measured from the N iλλ 8703.24, 8711.69, 8718.82 photospheric lines. Prominent emission and photospheric lines are indicated. follow-up near-IR observations were made, which confirmed a by averaging the spectra of 30 fibres located at known blank striking similarity to IRC +10420; in the remainder of this paper locations. The sky lines in each spectrum are evaluated and used we present and discuss these data and their implications. to scale the mean sky spectrum prior to subtraction. Near-IR spectra for IRAS 1835−06 were obtained on 2013 August 13 using LIRIS at the WHT telescope, as part of its 2. Data acquisition and reduction service programme. As the source is very bright in this wave- length regime, we chose a narrow slit (0.65) that, coupled with The I-band spectrum was obtained on the night of 2012 July 7 medium resolution grisms, yields an average resolving power with the fibre-fed dual-beam AAOmega spectrograph on around 3000 for the J, H and K bands. The data were taken us- the 3.9 m Anglo-Australian Telescope (AAT) at the Australian ing the standard scheme for near-IR observations, nodding the Astronomical Observatory. The instrument was operated with source along the slit in an ABBA pattern so to make it easy the Two Degree Field (“2dF”) multi-object system as front- to remove sky emission when reducing the spectra. Said reduc- end. Light is collected through an optical fibre with a projected tion was carried out in the standard fashion using the lirisdr diameter of 2. 1ontheskyandfedintothetwoarmsviaa package1. To correct for telluric absorption, a B9 star was ob- dichroic beam-splitter with crossover at 5700 Å. Each arm of the served alongside IRAS 18357-0604. Its spectrum was compared AAOmega system is equipped with a 2k × 4k E2V CCD detec- with an ATLAS model of the same spectral type, making sure tor (the red arm CCD is a low-fringing type) and an AAO2 CCD that the differences around the prominent hydrogen absorption controller. Due to the high reddening to the cluster and short ex- lines were low. The normalised ratio of the observed standard posure times, only the red arm registered usable spectra. The red and the model was used then to correct our spectra. The selected arm was equipped with the 1700D grating, blazed at 10 000 Å. B9 star, HIP091705, was chosen for its proximity to the target, This grating provides a resolving power R = 10 000 over slightly yet it turned out to be an unresolved binary with a late type com- more than 400 Å. The central wavelength was set at 8600 Å.The panion. This leaves some residuals when correcting for telluric exact wavelength range observed for each spectrum depends on absorption that show up as small CO emission bands, particu- the position of the target in the 2dF field.
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