PHENOMENON: TOWARDS SOLVING A LONG

ISSN 1845–8319

FS CMA TYPE BINARIES WITH THE B[e] PHENOMENON: TOWARDS SOLVING A LONG-STANDING PUZZLE A. S. MIROSHNICHENKO1 , S. V. ZHARIKOV2 , N. MANSET3 , C. ROSSI4 and V. F. POLCARO5 1

University of North Carolina at Greensboro, Department of Physics and Astronomy, Greensboro, NC, USA 2 Instituto de Astronomía, Universidad Nacional Autónoma de Mexico, Apdo. Postal 877, Ensenada, 22800, Baja California, Mexico 3 CFHT Corporation, 65–1238 Mamalahoa Hwy, Kamuela, HI 96743, USA 4 Universitá La Sapienza Roma - Pza A Moro 5, I–00162 Roma, Italy 5 Istituto di Astrofisica Spaziale e Fisica Cosmica, INAF, Via del Fosso del Cavaliere 100, 00133, Rome, Italy Abstract. The B[e] phenomenon is defined as the simultaneous presence of low-excitation forbidden line emission and strong infrared excess in the spectra of early-type stars. It was discovered in our galaxy nearly 40 years ago and is associated with objects at different evolutionary stages, ranging from the pre-main-sequence to the planetary nebula stage. Nearly half of the originally identified group members remained unclassified until recently. Our studies of the unclassified objects resulted in constraining their properties and fundamental parameters, expansion of the group, discovering binary features in many of its members, and suggesting a new name for the group (FS CMa type objects). Nevertheless, the nature of the secondary components and the evolutionary stage of the systems are still unclear. We review the group properties, present the best studied members, and discuss current understanding of the objects nature. Key words: emission-line stars - circumstellar matter - binary systems

1.

Introduction

Stars are always surrounded by a certain amount of circumstellar (CS) matter which reprocess the starlight. When this amount is large, the reprocessed light can be detected in the form of spectral lines in emission and/or altered photospheric spectral energy distribution (SED). Stars and stellar systems of all masses can be surrounded by CS envelopes at various evolutionary stages. The envelope may be a result of star formation, mass loss from a single star, or mass exchange between stellar companions or their winds in Cent. Eur. Astrophys. Bull. 37 (2013) 1, 57–66

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binary/multiple systems. They may include only gas or provide favorable conditions for dust formation. Studying CS matter and processes that lead to its formation provide insights into understanding of important issues, such as evolution of galaxies and planet formation, refine our knowledge of adjacent quiet evolutionary stages and explain the causes for evolutionary transitions. Many CS phenomena have been successfully explained by the current theory of stellar evolution. This includes the presence of large CS disks around pre-main-sequence stars, debris disks around main-sequence Vegatype stars, and optically-thick dusty envelopes around AGB stars. Nevertheless, some phenomena remain unexplained even with the currently available wealth of data. One of the long-standing challenges of modern astrophysics is to explain the B[e] phenomenon. The B[e] phenomenon was discovered by Allen and Swings (1976) in Btype stars that exhibit forbidden (e.g., [O i], [Fe ii], and [N ii]) and permitted emission lines (e.g., H i and Fe ii) along with large IR excesses due to CS dust. Lamers et al. (1998) recognized four subgroups of objects with the B[e] phenomenon and known evolutionary status, namely: pre-main-sequence Herbig Ae/Be stars, symbiotic binaries (a cool giant and a white dwarf or a neutron star), compact Planetary Nebulae, and supergiants. They confirmed the discoverers’ suggestion that the B[e] phenomenon is found in objects at very different evolutionary stages but with similar conditions in their CS envelopes. However, they were unable to classify ∼50% of the originally selected 65 Galactic objects. Our studies of the unclassified objects with the B[e] phenomenon led us to excluding several possibilities for their nature and evolutionary status and defining a new group of FS CMa objects (named after the prototype object of the group, Swings, 2006). In particular, we showed that the FS CMa objects are not pre-main-sequence stars and most likely not single stars (Miroshnichenko, 2007; Miroshnichenko et al., 2007; 2009). Below we describe properties of the group objects, introduce some new findings from our research program, and highlight some mysteries that need to be solved. 2.

Properties of the FS CMa Group Objects

The FS CMa objects show the following observed properties. The first one is a steep decrease of the IR flux at λ ≥ 10µm (Figure 1), unusual in hot 58

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stars that emit enough high-energy photons to warm even distant CS dust. This suggests that their dusty envelopes are compact. The second property is an extremely strong line emission with typical equivalent widths of the Hα line from 200 Å to 700 Å (Figure 2). It is accompanied by free-free and free-bound radiation that produces a large veiling of the stellar spectra. In addition, the CS distortion of the optical brightness can be up to ±1mag, depending on the system’s geometry and the tilt to the line of sight (Miroshnichenko et al., 2005; Carciofi et al., 2006).

Figure 1: Comparison of the spectral energy distribution of two early A–type objects: the pre-main-sequence Herbig star AB Aur and the FS CMa object IRAS 07080+0605. Photometric data for AB Aur are shown by filled circles, while those for IRAS 07080+0605 by open squares. The solid line shows a Spitzer spectrum of IRAS 07080+0605. The data for IRAS 07080+0605 are shifted by 0.5dex down from those of AB Aur.

The third property is a wide luminosity range, 2.5 ≤ log L/L⊙ ≤ 4.5 (Figure 3). We derived the luminosity from kinematic distances with corrections for the CS veiling only for a few objects (e.g., IRAS 00470+6429, Carciofi, Miroshnichenko, and Bjorkman 2010). Even taking into account the above mentioned effect of CS veiling, it becomes clear they are intrinsically much fainter than B[e] supergiants (see Figure 4). If the FS CMa Cent. Eur. Astrophys. Bull. 37 (2013) 1, 57–66

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Figure 2: Hα line profiles of four FS CMa objects. Intensity is normalized to the nearby continuum, radial velocities are shown in km s−1 .

objects do produce CS dust, then their luminosity range suggests that CS dust near hot stars is much more common than previously thought. The region of the Hertzsprung–Russell diagram occupied by FS CMa objects suggests some ideas about their evolutionary status. There are only two classes of dusty objects in this region that contain a B–type star: premain-sequence Herbig Ae/Be stars and post-AGB Proto-Planetary nebulae. Unlike FS CMa objects, pre-main-sequence stars possess larger dusty disks/envelopes, exhibit stronger far-IR excesses and retain them longer than the near-IR excesses (Miroshnichenko et al., 1996; Malfait et al., 1998). Only exposure to UV radiation from nearby stars in star-forming regions seems to destroy cold dust first (Hollenbach and Adams, 2004), but FS CMa objects are not found in such regions. Similarly, post-AGB stars do not match well with the observed prop60



Figure 3: A Hertzsprung-Russell diagram for Galactic dust-forming objects with the B[e] phenomenon (Miroshnichenko, 2007). B[e] supergiants (sgB[e]) are shown by filled circles, FS CMa objects by open circles. Solid lines: the zero-age main-sequence (ZAMS) and evolutionary tracks for rotating single stars (Ekström et al., 2012) with initial masses indicated.

erties of FS CMa objects. Post-AGB stars with initial masses of ≥ 5 M⊙ evolve so fast that spectral changes due to increasing Teff can be detected within a decade (Blöcker, 1995). Additionally, their IR spectral energy distributions (SEDs) typically peak at λ ≥ 30 µm. Neither of these features is observed in FS CMa objects (Miroshnichenko, 2007). Low-mass (≤1 M⊙ ) post-AGB RV Tau stars evolve very slowly and pass through the Teff range of B–type stars still retaining hot dust. They even have IRAS colors very similar to those of the FS CMa objects. However, RV Tau stars exhibit much weaker line emission, and the groups can be separated using near-IR colors (Miroshnichenko et al., 2007). Nevertheless, some FS CMa objects are somewhat similar to intermediate-mass post-AGB stars (Miroshnichenko et al., 2009). We address this subject in the next section. Cent. Eur. Astrophys. Bull. 37 (2013) 1, 57–66

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We believe that the most likely cause for the large amount of CS matter near FS CMa objects is mass transfer in binary systems. It is currently not clear that this is the case, but our observations have already detected a number of companion stars. Ten objects show signatures of a cool companion (Figure 4) or a degenerate one (e.g., the FS CMa object CI Cam seems to have a neutron star or a white dwarf companion). Several objects exhibit radial velocity variations or spectral features that may be attributed to orbital motion. Companions in a few more were detected through spectroastrometry (Baines et al., 2006) or interferometry (Millour et al., 2009). In all cases but one (MWC 623, Zickgraf, 2001), the B–type primaries are more luminous (∆V ≥ 2 mag) than the secondaries. I/Ic

IRAS03421+2935

1.1

IRAS07455-3143

IRAS08307-3748 1.0

Li I

Ca I

0.9

6710

Å

6720

Figure 4: Spectra of some FS CMa objects with a cool companion. The Li i 6708 Å and Ca i 6717 Å lines were found in our CFHT spectra. Intensities are normalized to the continuum and wavelengths are shown in Angströms.

3.

Unsolved Problems and Mysteries

There are several problems in current understanding of the nature and evolutionary status of the FS CMa group. Here we list three of them. 62



First, it is still unclear whether at least some FS CMa objects are single stars. Perhaps, they are unevolved ∼3–20 M⊙ stars that undergo episodes of strong mass loss, the causes of which are not predicted by the current theory ˙ of stellar evolution. Miroshnichenko (2008) found that mass loss rates of M −6 −1 ≥ 10 M⊙ yr are required to explain the Balmer line strengths in most FS CMa objects. A similar large mass loss rate was found for the FS CMa object IRAS 00470+6429 (Carciofi et al., 2010). Theory predicts such rates only for single supergiants with L ≥ 105 L⊙ (Vink et al., 2001). Second problem concerns the presence of Li i lines in the spectra of virtually all FS CMa objects with cool companions. We think that the cool companions are typically coll giants (most likely of K–type, as no prominent molecular features are observed), although it is only clearly seen in the case of MWC 623 (Zickgraf, 2001) in whose spectrum numerous lines of the cool companion are very pronounced. Strong Li i lines are typical in young T Tau stars, but unusual in cool giants (e.g., Kumar, Reddy, and Lambert, 2011). No explanation (e.g., enhanced mixing due to rapid rotation, accreting a big planet) has been found for the Li–excess in these stars. However, nearly 50% of rapidly rotating G/K–giants (v sin i ≥ 8 km s−1 ) are Li–rich (Drake et al., 2002). Binarity may explain the unusually abundant CS matter as well as the Li–excess and the more rapid rotation of the cool giants by mass transfer in the past (no Roche lobe filling effects are observed). Therefore, these two groups are worthwhile studying together. Finally, there is still a possibility that FS CMa objects are proto-planetary nebulae. There are 41 B– and A–type objects with IRAS data in the latest catalog of Proto-Planetary Nebulae that contains over 400 objects (Szczerba et al., 2007). Seven of them satisfy the photometric criteria for FS CMa objects (Miroshnichenko et al., 2007), but only one (MWC 939, which is a candidate to the FS CMa group) exhibits a strong line emission. Additionally, intermediate-mass Proto-Planetary Nebulae clearly exhibit extended optical nebulae, while even the closest FS CMa objects located at 0.3–0.5 kpc (e.g., FS CMa and HD 50138) from the Sun appear point-like. However, there is a group of cooler (A– and F–type) post-AGB binaries that show IR excesses similar to those of the FS CMa objects and no noticeable nebulae (e.g., van Winckel, 2007). FS CMa objects can be descendants of these postAGB binaries that exchange momentum with their CS disks which causes a delay in the CS matter ejection into interstellar space (Dermine et al., 2012). Cent. Eur. Astrophys. Bull. 37 (2013) 1, 57–66

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4.

Conclusions

We presented the current state of our research on a group of Galactic objects with the B[e] phenomenon called FS CMa objects. This group was established on the basis of formerly known unclassified objects with the B[e] phenomenon (Lamers et al., 1998). Twenty three of the thirty original unclassified objects (from Allen and Swings, 1976) are unlikely to be B[e] supergiants, nine more objects were introduced by Miroshnichenko (2007), another ten by Miroshnichenko et al. (2007). The group, which is already the largest among those containing a hot star and CS dust, keeps growing. We found six more objects with properties similar to those of FS CMa objects (Miroshnichenko et al., 2011) cross-correlating the Hamburg emissionline star survey (Kohoutek and Wehmeyer, 1999) and the 2MASS catalog (Scrutskie et al., 2006). More candidates to the group have been found in the NOMAD catalog (Zacharias et al., 2004). Currently we take low- and high-resolution optical spectra of the recently found objects and continue a long-term spectroscopic and photometric monitoring of the well-established group members. Our next goal is to model the spectral line profiles and SEDs of the objects with the largest number of observations obtained and their study spectral and photometric variability. We will then determine parameters of stars and CS envelopes and compare the results with models of binary evolution (e.g., van Rensbergen et al., 2008). Acknowledgements We acknowledge support from a PAPIIT grant IN103912. A.M. acknowledges support from the American Astronomical Society International Travel Grant program and from the Department of Physics and Astronomy of the University of North Carolina at Greensboro. This paper is party based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l′ Univers of the Centre National de la Recherche Scientifique de France, and the University of Hawaii. Partially based on data obtained at the 2.1–m telescope of the San Pedro Martir Observatory. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France, as well as data products from the Two Micron All Sky Survey. 64



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