90 research outputs found
Rotation in the ZAMS: Be and Bn stars
We show that Be stars belong to a high velocity tail of a single B-type star
rotational velocity distribution in the MS. This implies that: 1) the number
fraction N(Be)/N(Be+B) is independent of the mass; 2) Bn stars having ZAMS
rotational velocities higher than a given limit might become Be stars.Comment: 3 pages ; to appear in the proceedings of the Sapporo meeting on
active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A.
Okazak
Open clusters: III. Fundamental parameters of B stars in NGC 6087, NGC 6250, NGC 6383 and NGC 6530. B type stars with circumstellar envelopes
Context. Stellar physical properties of star clusters are poorly known. Aims.
Our goals are to perform a spectrophotometric study of the B star population in
open clusters to derive accurate stellar parameters, search for the presence of
circumstellar envelopes, and discuss the characteristics of these stars.
Methods. The BCD spectrophotometric system is a powerful tool to obtain
fundamental parameters and infer the main properties of open clusters: distance
modulus, color excess, and age. We inspected the Balmer discontinuity to seek
circumstellar disks and identify Be-star candidates. High resolution spectra in
the H region are used to confirm the Be nature. Results. We provide
Teff , log g, Mv , Mbol and spectral types for a sample of 68 stars in the
field of the open clusters NGC 6087, NGC 6250, NGC 6383, and NGC 6530, as well
as the cluster distances, ages and reddening. Then, based on a sample of 230 B
stars in the direction of the 11 open clusters studied along this series of
three papers, we report 6 new Be stars, 4 blue straggler candidates, and 15
B-type stars (called Bdd) with a double Balmer discontinuity. We also find that
the majority of the Be stars are dwarfs and present a maximum at the spectral
type B2-B4 in young and intermediate-age open clusters. Another maximum of Be
stars is observed at the spectral type B6-B8 in open clusters older than 40
Myr, where the population of Bdd stars also becomes relevant. Conclusions. Our
results support previous statements that the Be phenomenon is present along the
whole main sequence band and occurs in very different evolutionary states. We
find clear evidence of an increase of stars with circumstellar envelopes with
cluster age. The Be phenomenon reaches its maximum in clusters of intermediate
age and the number of B stars with circumstellar envelopes (Be+Bdd stars) is
also high for the older clusters
A K-band spectral mini-survey of Galactic B[e] stars
We present a mini-survey of Galactic B[e] stars mainly undertaken with the
Large Binocular Telescope (LBT). B[e] stars show morphological features with
hydrogen emission lines and an infrared excess, attributed to warm
circumstellar dust. In general, these features are assumed to arise from dense,
non-spherical, disk-forming circumstellar material in which molecules and dust
can condensate. Due to the lack of reliable luminosities, the class of Galactic
B[e] stars contains stars at very different stellar evolutionary phases like
Herbig AeBe, supergiants or planetary nebulae. We took near-infrared long-slit
K-band spectra for a sample of Galactic B[e] stars with the LBT-Luci I.
Prominent spectral features, such as the Brackett gamma line and CO band heads
are identified in the spectra. The analysis shows that the stars can be
characterized as evolved objects. Among others we find one LBV candidate
(MWC314), one supergiant B[e] candidate with 13CO (MWC137) and in two cases
(MWC623 and AS 381) indications for the existence of a late-type binary
companion, complementary to previous studies. For MWC84, IR spectra were taken
at different epochs with LBT-Luci I and the GNIRS spectrograph at the Gemini
North telescope. The new data show the disappearance of the circumstellar CO
emission around this star, previously detectable over decades. Also no signs of
a recent prominent eruption leading to the formation of new CO disk emission
are found during 2010 and 2013.Comment: 10 pages, 7 figures, 4 tables, accepted for publication in MNRAS (in
press
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_solar = 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.Comment: 20 pages, 13 figures, 7 tables. Accepted for publication in A&
Slow Radiation-Driven Wind Solutions of A-Type Supergiants
The theory of radiation-driven winds succeeded in describing terminal
velocities and mass loss rates of massive stars. However, for A-type
supergiants the standard m-CAK solution predicts values of mass loss and
terminal velocity higher than the observed values. Based on the existence of a
slow wind solution in fast rotating massive stars, we explore numerically the
parameter space of radiation-driven flows to search for new wind solutions in
slowly rotating stars, that could explain the origin of these discrepancies. We
solve the 1-D hydrodynamical equation of rotating radiation-driven winds at
different stellar latitudes and explore the influence of ionization's changes
throughout the wind in the velocity profile. We have found that for particular
sets of stellar and line-force parameters, a new slow solution exists over the
entire star when the rotational speed is slow or even zero. In the case of slow
rotating A-type supergiant stars the presence of this novel slow solution at
all latitudes leads to mass losses and wind terminal velocities which are in
agreement with the observed values. The theoretical Wind Momentum-Luminosity
Relationship derived with these slow solutions shows very good agreement with
the empirical relationship. In addition, the ratio between the terminal and
escape velocities, which provides a simple way to predict stellar wind energy
and momentum input into the interstellar medium, is also properly traced.Comment: 7 Pages, 3 figures, Astrophysical Journal, Accepte
K- and L-band spectroscopy of Be stars
We describe the behaviour of IR hydrogen emission lines of a sample of Be stars and discuss the physical properties of the circumstellar envelopes of Be stars classified in Groups I and II (Mennickent et al. 2009). We find that while Humphreys and Pfund lines of Group I stars form in an optically thick envelope/disk, Group II stars show Pfund lines that form in an optically thick medium and Humphreys lines originating in optically thinner regions. The transition between Groups I and II could be understood in terms of the evolution of the circumstellar disk of the star and might bring clues on the mechanism originating the Be phenomeno
Simultaneous J-, H-, K- and L-band spectroscopic observations of galactic Be stars. I. IR atlas
It is already accepted that Be stars are surrounded by circumstellar
envelopes, which are mostly compatible with a disc geometry in Keplerian
rotation. We aim to obtain a more complete characterisation of the properties
of the circumstellar environment of Be stars that helps to constrain the
theoretical models of the Be phenomenon. We present near-infrared,
medium-resolution spectra of a sample of galactic Be stars with different
spectral subtypes and luminosity classes. We measure different parameters of
the hydrogen recombination lines from the Paschen, Brackett, Pfund, and
Humphreys series, and use them to diagnose physical conditions in the
circumstellar environment. We analysed the equivalent-width (EW) ratio between
Br and Br lines and different diagrams of flux ratios. We also
identify lines from He I, C I, N I, O I, Na I, Mg I, Mg II, Si I, Fe I, and Fe
II. Analysing the EW measurements of particular He I, Mg II, Fe I, Fe II and O
I lines, we find that for some lines they correlate with the spectral type of
the star. Particularly, the emission of the O I~m line
decreases towards the later spectral types. We present an atlas of 22 Be stars,
that covers a wide infrared (IR) spectral range with quasi-simultaneous
observations. From a detailed analysis, we define new complementary criteria to
Mennickent's classification of Be stars according to their disc opacity. Some
objects in our sample present compact thick envelopes, while in others the
envelope is extended and optically thin. The correlation between the full
widths at half maximum (FWHM) and the peak separation ()
versus for the Br10, Br, and Hu14 lines reveals that the
broadening mechanism is rotational. The Ly fluorescence is a key
mechanism to explain the intensity of the emission of Mg II and O I lines.Comment: 27 pages, 14 figures. Accepted for publication in A&
Inhomogeneous molecular ring around the B[e] supergiant LHA 120-S 73
We aim to improve our knowledge on the structure and dynamics of the
circumstellar disk of the LMC B[e] supergiant LHA 120-S 73. High-resolution
optical and near-IR spectroscopic data were obtained over a period of 16 and 7
years, respectively. The spectra cover the diagnostic emission lines from
[CaII] and [OI], as well as the CO bands. These features trace the disk at
different distances from the star. We analyzed the kinematics of the individual
emission regions by modeling their emission profiles. A low-resolution
mid-infrared spectrum was obtained as well, which provides information on the
composition of the dusty disk. All diagnostic emission features display
double-peaked line profiles, which we interpret as due to Keplerian rotation.
We find that LHA 120-S 73 is surrounded by at least four individual rings of
material with alternating densities (or by a disk with strongly non-monotonic
radial density distribution). Moreover, we find that the molecular ring must
have gaps or at least strong density inhomogeneities, or in other words, a
clumpy structure. The mid-infrared spectrum displays features of oxygen- and
carbon-rich grain species, which indicates a long-lived, stable dusty disk. We
cannot confirm the previously reported high value for the stellar rotation
velocity. The line profile of HeI 5876 A is strongly variable in both width and
shape and resembles of those seen in non-radially pulsating stars. A proper
determination of the real underlying stellar rotation velocity is hence not
possible. The existence of multiple stable and clumpy rings of alternating
density recalls ring structures around planets. Although there is currently
insufficient observational evidence, it is tempting to propose a scenario with
one (or more) minor bodies or planets revolving around LHA 120-S 73 and
stabilizing the ring system, in analogy to the shepherd moons in planetary
systems.Comment: 14 pages, 13 figure, accepted for pulication in A&
Critical study of the distribution of rotational velocities of Be stars; II: Differential rotation and some hidden effects interfering with the interpretation of the Vsin i parameter
We assume that stars may undergo surface differential rotation to study its
impact on the interpretation of and on the observed distribution
of ratios of true rotational velocities u=V/V_\rm c (V_\rm c is
the equatorial critical velocity). We discuss some phenomena affecting the
formation of spectral lines and their broadening, which can obliterate the
information carried by concerning the actual stellar rotation. We
studied the line broadening produced by several differential rotational laws,
but adopted Maunder's expression
as an attempt to account for
all of these laws with the lowest possible number of free parameters. We
studied the effect of the differential rotation parameter on the
measured parameter and on the distribution of ratios
u=V/V_\rm c. We conclude that the inferred is smaller than
implied by the actual equatorial linear rotation velocity V_\rm eq if the
stars rotate with . For a
given the deviations of are larger when . If
the studied Be stars have on average , the number of rotators with
V_\rm eq\simeq0.9V_\rm c is larger than expected from the observed
distribution ; if these stars have on average , this number
is lower than expected. We discuss seven phenomena that contribute either to
narrow or broaden spectral lines, which blur the information on the rotation
carried by and, in particular, to decide whether the Be phenomenon
mostly rely on the critical rotation. We show that two-dimensional radiation
transfer calculations are needed in rapid rotators to diagnose the stellar
rotation more reliably.Comment: To appear in A&
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