A&A 575, A10 (2015) Astronomy DOI: 10.1051/0004-6361/201425371 & c ESO 2015 Astrophysics 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. BasedonISAAC/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 fea- tures of Mg ii,Feii], 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 – techniques: spectroscopic – open clusters and associations: individual: Mercer 70 1. Introduction binaries (SymB[e]), and unclassified B[e]-type stars (un- clB[e]). The latter was a miscellaneous group of objects that In the 1970s, several authors (Geisel 1970; Ciatti et al. 1974; these authors could not include in any of the four remaining Allen & Swings 1972, 1976) noticed that a peculiar group subclasses. In some cases, objects were incorporated into the un- of Be stars showed forbidden emission lines together with a clB[e] group simply when the lack of conclusive observational strong infrared excess. Members of this group were later re- evidence made the classification unclear (e.g. CD−42◦11721; ferred as “B[e] stars” (see e.g. Klutz & Swings 1977; Zickgraf Borges Fernandes et al. 2006) or controversial (a typical case is & Schulte-Ladbeck 1989; Bergner et al. 1995). After more than MWC 349A; see Strelnitski et al. 2013, and references therein). two decades of research, it became evident that the B[e] class As expected, when new analyses of such objects were carried actually formed a very heterogeneous group of objects in di- out, part of them were moved to another B[e] subclass (see e.g. verse evolutionary phases, ranging from pre-main sequence stars Borges Fernandes et al. 2003). to early stages of planetary nebulae. Consequently, Lamers et al. (1998) proposed to drop the term “B[e] stars” and coined However, other unclB[e] stars had well-determined obser- the expression “B[e] phenomenon” to describe the common vational parameters that undoubtedly prevented them from be- ff observational features of these objects. ing allocated in a di erent B[e] subclass. Among these, the Lamers et al. (1998) established five subclasses of ob- most extensively studied cases are FS CMa (see e.g. Swings jects showing the B[e] phenomenon, namely: B[e] super- et al. 1980; Sorrell 1989; Israelian et al. 1996; de Winter & giants (sgB[e]), pre-main sequence B[e]-type stars (HAeB[e]), van den Ancker 1997; Muratorio et al. 2006) and HD 50138 (e.g. compact planetary nebulae (cPNB[e]), symbiotic B[e] Hutsemekers 1985; Pogodin 1997; Jaschek & Andrillat 1998; Borges Fernandes et al. 2009). Briefly, each one of these objects Based on observations collected at the European Organisation is composed of a star undergoing photometric and spectroscopic for Astronomical Research in the Southern Hemisphere, Chile, under irregular variations, a dense circumstellar dusty disk, and a high- program IDs 083.D-0765 and 087.D-0957. velocity polar wind. Since the Hipparcos satellite has allowed Article published by EDP Sciences A10, page 1 of 19 A&A 575, A10 (2015) to calculate their distances accurately, FS CMa and HD 50138 low masses and luminosities of the companions as well as small have precise luminosity measurements (van den Ancker et al. separations between components. 1998), which placed them in the main-sequence band or slightly Also, none of the secured FS CMa stars has been confirmed above. Neither of them showed phenomena that are related to as a member of a coeval population, which hampers the in- other B[e] subtypes, specifically: Roche lobe overflow, high- quiry of their evolutionary state. Although the FS CMa star excitation forbidden lines, or association to nebulae or star- IRAS 00470+6429 is projected towards the Cas OB7 association forming regions. On the other hand, both stars, as well as other (Miroshnichenko et al. 2007), membership is unlikely due to the similar objects, presented a distinctive steep decrease in the mid- absence of a interstellar component of a Na i line that is present infrared flux (Sheikina et al. 2000; Miroshnichenko et al. 2002, in the majority of the Cas OB7 members (Miroshnichenko et al. 2006), implying that the dusty disk is compact and warm. 2009). In light of the above explained evidence, Miroshnichenko In this context, discovery of FS CMa objects as part of co- (2007) grouped these low-luminosity B[e] objects with warm eval, codistant star groups is crucial for understanding the na- dust together in a new class, taking FS CMa as the prototype. ture and evolutionary state of FS CMa stars, linking the observa- According to the definition established by this author, FS CMa tional properties of these objects with the characteristics of the stars are those that fulfill the following criteria: host populations, especially regarding ages. In this paper, we re- port the first detections of FS CMa stars in two clusters, specif- – Spectral types between O9 and A2. ically Mercer 20 and Mercer 70 (hereafter Mc20 and Mc70). – Presence of the B[e] phenomenon, i.e. forbidden emission We also study these objects through the new approach of a joint lines along with large infrared excess. analysis with the cluster populations. Interestingly, the discov- – Sharp decrease in the mid-infrared excess for λ 20 μm. ery we present below is carried out through observations in the – Luminosity range: 2.5 log (L/L) 4.5. Paschen-α line that were initially aimed at finding hot mas- – Location outside of star-forming regions. sive evolved cluster members. Pα observations have been al- ready proven to be very effective to find such stars in clusters Subsequent observations (Miroshnichenko et al. 2007, 2009, (Davies et al. 2012a,b) or in the central region of the Milky Way 2011a,b; Borges Fernandes et al. 2009, 2012; Rodríguez et al. (Mauerhan et al. 2010a; Dong et al. 2011), however this method 2012; Polster et al. 2012; Liermann et al. 2014) and modeling has never been used to locate lower luminosity emission-line (Carciofi et al. 2010; Borges Fernandes et al. 2011)ofFSCMa objects like FS CMa stars. stars have not been capable of unraveling the enigmatic na- ture 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 emis- sion of the FS CMa star AS 78 to obtain a very crude estimate Clusters were observed on 2008 June 30 (Mc20) and 2008 −6 of M˙ ≈ 1.5 × 10 M/yr. Later, Carciofi et al. (2010) calcu- July 12 (Mc70) with the NICMOS camera onboard the Hubble −7 lated a more accurate value of M˙ ≈ 2.7 × 10 M/yr for the Space Telescope (HST), as part of observing program #11545 model of IRAS 00470+6429, taking into account the geometri- (PI: Davies). Near-infrared images were taken through filters cal effects through a latitude-dependent mass-loss rate per solid F160W and F222M, as well as the narrow-band filters whose angle, together with constraints in the latitudinal density profile wavelengths correspond to Pα (F187N) and its adjacent contin- that allowed dust formation in a disk. In contrast, the mass-loss uum (F190N). Each frame has a field of view of 51.2 × 51.2 theoretical recipe of Vink et al.