Estrellas con fenómeno B[e]: modelo de envoltura de gas y polvo

Entre las estrellas O-B con lı́neas en emisión existe un grupo de objetos peculiares, las estrellas con fenómeno B[e], que se caracteriza por presentar lı́neas prohibidas en emisión originadas en una envoltura circunestelar gaseosa y fuertes excesos de radiación infrarroja producida por polvo circunestelar. Esta fenomenologı́a puede presentarse en objetos que se encuentran en estados evolutivos muy diferentes pe- ro cuyas envolturas extendidas presentan condiciones fı́sicas similares. Lamers et al. (1998) clasificaron a estos objetos en cinco subgrupos: nebulosas planetarias compactas, estrellas simbióticas, estrellas supergigantes, estrellas de pre-secuencia HAeB[e] y un subgrupo de objetos aún sin clasificar, denominado UnclB[e], que presenta si- multáneamente caracterı́sticas comunes a dos o más de los subgrupos mencionados. Las envolturas extendidas de gas y polvo que rodean a las estrellas B[e] dificultan la estimación de los parámetros fundamentales de la estrella subyacente, tales como la temperatura efectiva y la luminosidad, dado que dichas envolturas “borran” en mayor o menor grado las caracterı́sticas espectrales fotosféricas. Más aún, en muchos casos, no resulta sencillo separar el efecto producido por la extinción circunestelar del correspondiente a la extinción interestelar, lo cual conduce a estimaciones im- precisas de la luminosidad y la distancia. Estos parámetros resultan de fundamental importancia para discutir sobre el origen y el estado evolutivo de los objetos. En particular, los objetos que manifiestan el fenómeno B[e] son generalmente muy distantes y sólo un número reducido de ellos tiene paralajes medidas con el satélite HIPPARCOS . Sólo en los casos en que estos objetos están vinculados a sistemas estelares o asociaciones es posible obtener una estimación confiable de sus distancias y luminosidades. Generalmente, los métodos indirectos tradicionales utilizados, tales como las relaciones estadı́sticas entre distancias, intensidades de lı́neas interestelares y excesos de color, y la distribución de la absorción interestelar (Zorec 1998), son inviables debido a las anomalı́as en los colores producidas por la extinción circunestelar e interestelar. Particularmente, la radiación ultravioleta es fuertemente absorbida y utilizada para calentar el polvo circunestelar, reapareciendo como exceso de radiación infrarroja (Savage et al. 1978). Nuestro objetivo es modelar la distribución de energı́a de las estrellas que pre- sentan el fenómeno B[e]. Con este fin estudiamos como se modifica el flujo intrı́nseco 1de una estrella de tipo B normal por los distintos componentes del medio: envoltura circunestelar gaseosa, envoltura circunestelar de polvo y medio interestelar. Para ello implementamos un modelo simple de envoltura de gas y polvo que nos permite reproducir diferentes distribuciones de energı́a del sistema estrella+envoltura a partir de la variación de los parámetros libres elegidos. Mediante la confrontación de las distribuciones de energı́a teóricas con las observadas, este estudio permitirá estimar propiedades fı́sicas de la envoltura circunestelar, ası́ como también parámetros fundamentales del objeto, tales como temperatura y gravedad superficial. Esta información servirá de base para la determinación de distancias y luminosidades que contribuirán a definir el estado evolutivo de estos objetos.Facultad de Ciencias Astronómicas y Geofísica

e

The disc formation mechanism of B[e] supergiants is one of the puzzling phenomena in massive star evolution. Rapid stellar rotation seems to play an important role for the non-spherically symmetric mass-loss leading to a high-density disc- or ring-like structure of neutral material around these massive and luminous objects. The radial density and temperature structure as well as the kinematics within this high-density material are, however, not well studied. Based on the high-resolution optical spectra of a sample of B[e] supergiants in the Magellanic Clouds we especially searched for tracers of the kinematics within their discs. Besides the well-known [Oi] lines, we discovered the [Caii] λλ7291, 7324 lines which can be used as a complementary set of disc tracers. We find that these lines originate from very high density regions, located closer to the star than the [Oi] λ5577 line-forming region. The line profiles of both the [Oi] and the [Caii] lines indicate that the discs or rings of high-density material are in Keplerian rotation. We estimate plausible ranges of disc inclination angles for the sample of B[e] supergiants and suggest that the star LHA120-S22 might have a spiral arm rather than a disc.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

e

The disc formation mechanism of B[e] supergiants is one of the puzzling phenomena in massive star evolution. Rapid stellar rotation seems to play an important role for the non-spherically symmetric mass-loss leading to a high-density disc- or ring-like structure of neutral material around these massive and luminous objects. The radial density and temperature structure as well as the kinematics within this high-density material are, however, not well studied. Based on the high-resolution optical spectra of a sample of B[e] supergiants in the Magellanic Clouds we especially searched for tracers of the kinematics within their discs. Besides the well-known [Oi] lines, we discovered the [Caii] λλ7291, 7324 lines which can be used as a complementary set of disc tracers. We find that these lines originate from very high density regions, located closer to the star than the [Oi] λ5577 line-forming region. The line profiles of both the [Oi] and the [Caii] lines indicate that the discs or rings of high-density material are in Keplerian rotation. We estimate plausible ranges of disc inclination angles for the sample of B[e] supergiants and suggest that the star LHA120-S22 might have a spiral arm rather than a disc.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Determination of fundamental parameters and circumstellar properties for a sample of B[e] stars

We develop a simple model to derive theoretical continuum energy distributions for B[e] stars, consisting of a B star surrounded by an envelope made of gas and dust. We select a sample of B[e] objects for which we construct the observed energy distributions, from 0.1 to 100 μm, using available photometric and spectroscopic data. We present some preliminary fittings.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Determination of fundamental parameters and circumstellar properties for a sample of B[e] stars

We develop a simple model to derive theoretical continuum energy distributions for B[e] stars, consisting of a B star surrounded by an envelope made of gas and dust. We select a sample of B[e] objects for which we construct the observed energy distributions, from 0.1 to 100 μm, using available photometric and spectroscopic data. We present some preliminary fittings.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Probing the ejecta of evolved massive stars in transition : A VLT/SINFONI K-band survey

Massive evolved stars in transition phases, such as luminous blue variables (LBVs), B[e] supergiants (B[e]SGs), and yellow hypergiants (YHGs), are not well understood, and yet crucial steps in determining accurate stellar and galactic evolution models. The circumstellar environments of these stars reveal their mass-loss history, identifying clues to both their individual evolutionary status and the connection between objects of different phases. Here we present a survey of 25 such evolved massive stars (16 B[e]SGs, 6 LBVs, 2 YHGs, and 1 Peculiar Oe star), observed in the K-band with the Spectrograph for INtegral Field Observation in the Near-Infrared (SINFONI; R = 4500) on the ESO VLT UT4 8 m telescope. The sample can be split into two categories based on spectral morphology: one group includes all of the B[e]SGs, the Peculiar Oe star, and two of the LBVs, while the other includes the YHGs and the rest of the LBVs. The difference in LBV spectral appearance is due to some objects being in a quiescent phase and some objects being in an active or outburst phase. CO emission features are found in 13 of our targets, with first time detections for MWC 137, LHA 120-S 35, and LHA 115-S 65. From model fits to the CO band heads, the emitting regions appear to be detached from the stellar surface. Each star with 12CO features also shows 13CO emission, signaling an evolved nature. Based on the level of 13C enrichment, we conclude that many of the B[e]SGs are likely in a pre-Red Supergiant phase of their evolution. There appears to be a lower luminosity limit of log L/L⊙ = 5.0 below which CO is not detected. The lack of CO features in several high luminosity B[e]SGs and variability in others suggests that they may in fact be LBV candidates, strengthening the connection between these two very similar transition phases.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

e

The disc formation mechanism of B[e] supergiants is one of the puzzling phenomena in massive star evolution. Rapid stellar rotation seems to play an important role for the non-spherically symmetric mass-loss leading to a high-density disc- or ring-like structure of neutral material around these massive and luminous objects. The radial density and temperature structure as well as the kinematics within this high-density material are, however, not well studied. Based on the high-resolution optical spectra of a sample of B[e] supergiants in the Magellanic Clouds we especially searched for tracers of the kinematics within their discs. Besides the well-known [Oi] lines, we discovered the [Caii] λλ7291, 7324 lines which can be used as a complementary set of disc tracers. We find that these lines originate from very high density regions, located closer to the star than the [Oi] λ5577 line-forming region. The line profiles of both the [Oi] and the [Caii] lines indicate that the discs or rings of high-density material are in Keplerian rotation. We estimate plausible ranges of disc inclination angles for the sample of B[e] supergiants and suggest that the star LHA120-S22 might have a spiral arm rather than a disc.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

The sudden appearance of CO emission in LHA 115-S 65

Molecular emission has been detected in several Magellanic Cloud B[e] supergiants. In this Letter, we report on the detection of CO band head emission in the B[e] supergiant LHA 115-S 65, and present a K-band near-infrared spectrum obtained with the Spectrograph for INtegral Field Observation in the Near-Infrared (SINFONI; R = 4500) on the ESO VLT UT4 telescope. The observed molecular band head emission in S65 is quite surprising in the light of a previous non-detection by McGregor, Hyland & McGinn, as well as a high-resolution (R = 50 000) Gemini/Phoenix spectrum of this star taken nine months earlier showing no emission. Based on analysis of the optical spectrum by Kraus, Borges Fernandes & de Aráujo, we suspect that the sudden appearance of molecular emission could be due to density buildup in an outflowing viscous disc, as seen for Be stars. This new discovery, combined with variability in two other similar evolved massive stars, indicates an evolutionary link between B[e] supergiants and luminous blue variables.Facultad de Ciencias Astronómicas y Geofísica

Evidence of the evolved nature of the B[e] star MWC 137

The evolutionary phase of B[e] stars is difficult to establish due to the uncertainties in their fundamental parameters. For instance, possible classifications for the Galactic B[e] star MWC 137 include pre-main-sequence and post-main-sequence phases, with a large range in luminosity. Our goal is to clarify the evolutionary stage of this peculiar object, and to study the CO molecular component of its circumstellar medium. To this purpose, we modeled the CO molecular bands using high-resolution K-band spectra. We find that MWC 137 is surrounded by a detached cool (T = 1900 ± 100 K) and dense (N = (3 ± 1) × 1021 cm-2) ring of CO gas orbiting the star with a rotational velocity, projected to the line of sight, of 84 ± 2 km s-1. We also find that the molecular gas is enriched in the isotope 13C, excluding the classification of the star as a Herbig Be. The observed isotopic abundance ratio (12C/13C = 25 ± 2) derived from our modeling is compatible with a proto-planetary nebula, main-sequence, or supergiant evolutionary phase. However, based on some observable characteristics of MWC 137, we propose that the supergiant scenario seems to be the most plausible. Hence, we suggest that MWC 137 could be in an extremely short-lived phase, evolving from a B[e] supergiant to a blue supergiant with a bipolar ring nebula.Facultad de Ciencias Astronómicas y Geofísica

The spectroscopic orbits and physical parameters of GG Carinae

Aims. GG Car is an eclipsing binary classified as a B[e] supergiant star. The aims of our study are to improve the orbital elements of the binary system in order to obtain the actual orbital period of this system. We also compare the spectral energy distribution of the observed fluxes over a wide wavelength range with a model of a circumstellar envelope composed of gas and dust. This fitting allows us to derive the physical parameters of the system and its environment, as well as to obtain an estimation of the distance to GG Car. Methods. We analyzed about 55 optical and near infrared spectrograms taken during 1996-2010. The spectroscopic orbits were obtained by measuring the radial velocities of the blueshifted absorptions of the He I P-Cygni profiles, which are very representative of the orbital motion of both stars. On the other hand, we modeled the spectral energy distribution of GG Car, proposing a simple model of a spherical envelope consisting of a layer close to the central star composed of ionized gas and other outermost layers composed of dust. Its effect on the spectral energy distribution considering a central B-type star is presented. Comparing the model with the observed continuum energy distribution of GG Car, we can derive fundamental parameters of the system, as well as global physical properties of the gas and dust envelope. It is also possible to estimate the distance taking the spectral regions into account where the theoretical data fit the observational data very well and using the set of parameters obtained and the value of the observed flux for different wavelengths. Results. For the first time, we have determined the orbits for both components of the binary through a detailed study of the He I lines, at λλ4471, 5875, 6678, and 7065 Å, thereby obtaining an orbital period of 31.033 days. An eccentric orbit with e = 0.28 and a mass ratio q = 2.2 ± 0.9 were calculated. Comparing the model with the observed continuum energy distribution of GG Car, we obtain T eff = 23 000 K and log g = 3. The central star is surrounded by a spherical envelope consisting of a layer of 3.5 stellar radii composed of ionized gas and other outermost dust layers with E B-V = 0.39. These calculations are not strongly modified if we consider two similar B-type stars instead of a central star, provided our model suggests that the second star might contribute less than 10% of the primary flux. The calculated effective temperature is consistent with an spectral type B0-B2 and a distance to the object of 5 ± 1 kpc was determined.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat