First Detections of FS Canis Majoris Stars in Clusters. Evolutionary State

Astronomy & Astrophysics manuscript no. paper1_v7 c ESO 2018 August 20, 2018 First detections of FS Canis Majoris stars in clusters Evolutionary state as constrained by coeval massive stars⋆ D. de la Fuente1, F. Najarro1, C. Trombley2, B. Davies3, and D. F. Figer2 1 Centro de Astrobiología (CSIC/INTA), ctra. de Ajalvir km. 4, 28850 Torrejón de Ardoz, Madrid, Spain e-mail: [email protected] 2 Center for Detectors, Rochester Institute of Technology, 74 Lomb Memorial Drive, Rochester, NY 14623, USA 3 Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK Received 19 November 2014 / Accepted 23 december 2014 ABSTRACT Context. FS CMa stars are low-luminosity objects showing the B[e] phenomenon whose evolutionary state remains a puzzle. These stars are surrounded by compact disks of warm dust of unknown origin. Hitherto, membership of FS CMa stars to coeval populations has never been confirmed. Aims. The discovery of low-luminosity line emitters in the young massive clusters Mercer 20 and Mercer 70 prompts us to investigate the nature of such objects. We intend to confirm membership to coeval populations in order to characterize these emission-line stars through the cluster properties. Methods. Based on ISAAC/VLT medium-resolution spectroscopy and NICMOS/HST photometry of massive cluster members, new characterizations of Mercer 20 and Mercer 70 are performed. Coevality of each cluster and membership of the newly-discovered B[e] objects are investigated using our observations as well as literature data of the surroundings. Infrared excess and narrow-band photometric properties of the B[e] stars are also studied. Results. We confirm and classify 22 new cluster members, including Wolf-Rayet stars and blue hypergiants. Spectral types (O9-B1.5 V) and radial velocities of B[e] objects are compatible with the remaining cluster members, while emission features of Mg ii, Fe ii], and [Fe ii] are identified in their spectra. The ages of these stars are 4.5 and 6 Myr, and they show mild infrared excesses. Conclusions. We confirm the presence of FS CMa stars in the coeval populations of Mercer 20 and Mercer 70. We discuss the nature and evolutionary state of FS CMa stars, discarding a post-AGB nature and introducing a new hypothesis about mergers. A new search method for FS CMa candidates in young massive clusters based on narrow-band Paschen-α photometry is proposed and tested in photometric data of other clusters, yielding three new candidates. Key words. Stars: emission-line, Be – Stars: massive – Circumstellar matter – Open clusters and associations: individual: Mercer 20 – Open clusters and associations: individual: Mercer 70 – Techniques: spectroscopic 1. Introduction latter was a miscellaneous group of objects that these authors could not include in any of the four remaining sub- In the 1970s, several authors (Geisel 1970; Ciatti et al. 1974; classes. In some cases, objects were incorporated into the un- Allen & Swings 1972, 1976) noticed that a peculiar group clB[e] group simply when the lack of conclusive observational of Be stars showed forbidden emission lines together with evidence made the classification unclear (e.g. CD−42o11721; a strong infrared excess. Members of this group were later Borges Fernandes et al. 2006) or controversial (a typical case is referred as “B[e] stars” (see e.g. Klutz&Swings 1977; MWC 349A; see Strelnitski et al. 2013, and references therein). Zickgraf & Schulte-Ladbeck 1989; Bergner et al. 1995). After As expected, when new analyses of such objects were carried more than two decades of research, it became evident that the out, part of them were moved to another B[e] subclass (see e.g. B[e] class actually formed a very heterogeneous group of ob- arXiv:1412.7988v2 [astro-ph.SR] 13 Feb 2015 Borges Fernandes et al. 2003). jects in diverse evolutionary phases, ranging from pre-main sequence stars to early stages of planetary nebulae. Consequently, However, other unclB[e] stars had well-determined ob- Lamers et al. (1998) proposed to drop the term “B[e] stars” and servational parameters that undoubtedly prevented them coined the expression “B[e] phenomenon” to describe the com- from being allocated in a different B[e] subclass. Among mon observational features of these objects. these, the most extensively studied cases are FS CMa (see Lamerset al. (1998) established five subclasses of ob- e.g. Swings etal. 1980; Sorrell 1989; Israelian et al. 1996; jects showing the B[e] phenomenon, namely: B[e] supergiants deWinter&vandenAncker 1997; Muratorioetal. 2006) (sgB[e]), pre-main sequence B[e]-type stars (HAeB[e]), com- and HD 50138 (e.g. Hutsemekers 1985; Pogodin 1997; pact planetary nebulae (cPNB[e]), symbiotic B[e] binaries Jaschek & Andrillat 1998; BorgesFernandeset al. 2009). (SymB[e]), and unclassified B[e]-type stars (unclB[e]). The Briefly, each one of these objects is composed of a star un- dergoing photometric and spectroscopic irregular variations, ⋆ Based on observations collected at the European Organisation for a dense circumstellar dusty disk, and a high-velocity polar Astronomical Research in the Southern Hemisphere, Chile, under pro- wind. Since the HIPPARCOS satellite has allowed to calculate gram IDs 083.D-0765 & 087.D-0957. their distances accurately, FS CMa and HD 50138 have precise Article number, page 1 of 19 A&A proofs: manuscript no. paper1_v7 luminosity measurements (van den Ancker et al. 1998), which luminosities of the companions as well as small separations be- placed them in the main-sequence band or slightly above. Nei- tween components. ther of them showed phenomena that are related to other B[e] Also, none of the secured FS CMa stars has been confirmed subtypes, specifically: Roche lobe overflow, high-excitation for- as a member of a coeval population, which hampers the in- bidden lines, or association to nebulae or star-forming regions. quiry of their evolutionary state. Although the FS CMa star On the other hand, both stars, as well as other similar objects, IRAS 00470+6429 is projected towards the Cas OB7 association presented a distinctive steep decrease in the mid-infrared (Miroshnichenko et al. 2007), membership is unlikely due to the flux (Sheikina et al. 2000; Miroshnichenko et al. 2002, 2006), absence of a interstellar component of a Na i line that is present implying that the dusty disk is compact and warm. in the majority of the Cas OB7 members (Miroshnichenko et al. In light of the above explained evidence, Miroshnichenko 2009). (2007) grouped these low-luminosity B[e] objects with warm In this context, discovery of FS CMa objects as part of co- dust together in a new class, taking FS CMa as the prototype. eval, codistant star groups is crucial for understanding the nature According to the definition established by this author, FS CMa and evolutionary state of FS CMa stars, linking the observational stars are those that fulfill the following criteria: properties of these objects with the characteristics of the host populations, especially regarding ages. In this paper, we report – Spectral types between O9 and A2. the first detections of FS CMa stars in two clusters, specifically – Presence of the B[e] phenomenon, i.e. forbidden emission Mercer 20 and Mercer 70 (hereafter Mc20 and Mc70). We also lines along with large infrared excess. study these objects through the new approach of a joint analy- – Sharp decrease in the mid-infrared excess for λ & 20µm. sis with the cluster populations. Interestingly, the discovery we – Luminosity range: 2.5 . log(L/L⊙) . 4.5. present below is carried out through observations in the Paschen- – Location outside of star-forming regions. α line that were initially aimed at finding hot massive evolved cluster members. Pα observationshave been already proven to be Subsequent observations (Miroshnichenko et al. 2007, 2009, very effective to find such stars in clusters (Davies et al. 2012a,b) 2011a,b; Borges Fernandes et al. 2009, 2012; Rodríguez et al. or in the centralregionof the MilkyWay (Mauerhan et al. 2010a; 2012; Polster et al. 2012; Liermann et al. 2014) and modeling Dong et al. 2011), however this method has never been used to (Carciofi et al. 2010; Borges Fernandes et al. 2011) of FS CMa locate lower luminosity emission-line objects like FS CMa stars. stars have not been capable of unraveling the enigmatic nature of these objects. One of the most puzzling issues refers to the observationally inferred mass-loss rates, which are 2-3 dex 2. Observations and data reduction higher than predicted by wind theory for B-type main-sequence 2.1. Imaging stars. Miroshnichenko et al. (2000) used the optical H i emission of the FS CMa star AS 78 to obtain a very crude estimate of Clusters were observed on 2008 June 30 (Mc20) and 2008 July −6 M˙ ≈ 1.5 × 10 M⊙/yr. Later, Carciofi et al. (2010) calculated a 12 (Mc70) with the NICMOS camera onboard the Hubble Space −7 more accurate value of M˙ ≈ 2.7 × 10 M⊙/yr for the model of Telescope (HST), as part of observing program #11545 (PI: IRAS 00470+6429, taking into account the geometrical effects Davies). Near-infrared images were taken through filters F160W through a latitude-dependent mass-loss rate per solid angle, to- and F222M, as well as the narrow-band filters whose wave- gether with constraints in the latitudinal density profile that al- lengths correspond to Pα (P187N) and its adjacent continuum lowed dust formation in a disk. In contrast, the mass-loss the- (P190N). Each frame has a field of view of 51.2′′ × 51.2′′ and oretical recipe of Vink et al. (2000, 2001) yields 9 × 10−9 and a pixel scale of 0.203 arcsec/pix.

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