53 research outputs found
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&
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&
Interplay between pulsations and mass loss in the blue supergiant 55 Cygnus = HD 198478
Blue supergiant stars are known to display photometric and spectroscopic
variability that is suggested to be linked to stellar pulsations. Pulsational
activity in massive stars strongly depends on the star's evolutionary stage and
is assumed to be connected with mass-loss episodes, the appearance of
macroturbulent line broadening, and the formation of clumps in the wind. To
investigate a possible interplay between pulsations and mass-loss, we carried
out an observational campaign of the supergiant 55 Cyg over a period of five
years to search for photospheric activity and cyclic mass-loss variability in
the stellar wind. We modeled the H, He I, Si II and Si III lines using the
nonlocal thermal equilibrium atmosphere code FASTWIND and derived the
photospheric and wind parameters. In addition, we searched for variability in
the intensity and radial velocity of photospheric lines and performed a moment
analysis of the line profiles to derive frequencies and amplitudes of the
variations. The Halpha line varies with time in both intensity and shape,
displaying various types of profiles: P Cygni, pure emission, almost complete
absence, and double or multiple peaked. The star undergoes episodes of variable
mass-loss rates that change by a factor of 1.7-2 on different timescales. We
also observe changes in the ionization rate of Si II and determine a
multiperiodic oscillation in the He I absorption lines, with periods ranging
from a few hours to 22.5 days. We interpret the photospheric line variations in
terms of oscillations in p-, g-, and strange modes. We suggest that these
pulsations can lead to phases of enhanced mass loss. Furthermore, they can
mislead the determination of the stellar rotation. We classify the star as a
post-red supergiant, belonging to the group of alpha Cyg variables.Comment: 20 pages, 18 figures, 3 tables, accepted to Astronomy & Astrophysic
Fundamental Parameters of He-Weak and He-Strong Stars
We carried out low resolution spectroscopic observations in the wavelength
range 3400-4700 A of 20 He-weak and 8 He-strong stars to determine their
fundamental parameters by means of the Divan-Chalonge-Barbier (BCD)
spectrophotometric system. For a few He-weak stars we also estimate the
effective temperatures and the angular diameters by integrating absolute fluxes
observed over a wide spectral range. Non-LTE model calculations are carried out
to study the influence of the He/H abundance ratio on the emergent radiation of
He-strong stars and on their Teff determination. We find that the effective
temperatures, surface gravities and bolometric absolute magnitudes of He-weak
stars estimated with the BCD system and the integrated flux method are in good
agreement between each other, and they also agree with previous determinations
based on several different methods. The mean discrepancy between the visual
absolute magnitudes derived using the Hipparcos parallaxes and the BCD values
is on average 0.3 mag for He-weak stars, while it is 0.5 mag for He-strong
stars. For He-strong stars, we note that the BCD calibration, based on stars in
the solar environment, leads to overestimated values of Teff. By means of model
atmosphere calculations with enhanced He/H abundance ratios we show that larger
He/H ratios produce smaller BD which naturally explains the Teff
overestimation. We take advantage of these calculations to introduce a method
to estimate the He/H abundance ratio in He-strong stars. The BD of HD 37479
suggests that the Teff of this star remains fairly constant as the star
spectrum undergoes changes in the intensity of H and He absorption lines. Data
for the He-strong star HD 66765 are reported for the first time.Comment: Accepted for publication in A&
Discovery of Raman-scattered lines in the massive luminous emission-line star LHA 115-S 18
LHA 115-S 18 is a very peculiar emission-line star exhibiting the B[e]
phenomenon. Located in the Small Magellanic Cloud, its spectrum shows features
of an extremely wide range of excitation and ionization stages, extending from
highly ionized atomic lines (Si IV, C IV, He II) in the UV and optical regions
to molecular emission bands of CO and TiO in the optical and IR regions. The
most distinguishing spectral characteristic of LHA 115-S 18 is the high
variability detected in the He II {\lambda}4686 emission line, which can be a
very conspicuous or completely invisible feature.
In this work, we report on another peculiarity of LHA 115-S 18. From
high-resolution optical spectra taken between 2000 and 2008, we discovered the
appearance and strengthening of two emission features at {\lambda}6825 \AA, and
{\lambda}7082 \AA,, which we identified as Raman-scattered lines. This is the
first time these lines have been detected in the spectrum of a massive luminous
B[e] star. As the classification of LHA 115-S 18 is highly controversial, we
discuss how the discovery of the appearance of Raman-scattered lines in this
peculiar star might help us to solve this puzzle.Comment: Letter accepted for publication in MNRAS. 5 pages, 3 figure
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