522 research outputs found
On the optical--infra-red continuum emission from equatorial discs of supergiant B[e] stars
Two models of the circumstellar disc around supergiant B[e] stars are
discussed: an equatorial wind model produced by wind bi-stability, and a
Keplerian viscous disc model. Both models are successful in providing a site
for dust formation once they have cooled sufficiently. However, the
optical--infra-red continuum is calculated and it is found that both models
have significant trouble in accounting for observations. In particular the
optical--near-IR emission is accounted for, but the dust emission is
underestimated by at least an order of magnitude. Variations in the structure
of the models (the temperature variation with radius, the density structure and
the dust opacity) are investigated to assess how (in)appropriate the standard
models are for supergiant B[e] star discs. Changing the temperature structure,
and making simple dust opacity changes within the disc has little effect on the
resultant continuum emission. By altering the density structure of the discs,
the continuum may be accounted for by both models: the equatorial wind model
requires a very flat density profile which is impossible to explain with any
accelerating wind, and the viscous disc model's density structure is required
to fall off less steeply with radius than would have been expected, although
this may be explained from consideration of viscous processes in the disc. It
is recognised that both theoretical interpretations have difficulties and
unsolved problems.Comment: 9 pages, 6 figures, accepted by Astronomy and Astrophysic
Ionization structure in the winds of B[e] supergiants: I. Ionization equilibrium calculations in a H plus He wind
The non-spherically symmetric winds of B[e] supergiants are investigated. An
empirical density distribution is chosen that accounts for the density
concentrations and ratios derived from observations, and our model winds are
assumed to contain only hydrogen and helium. We first calculate the approximate
ionization radii for H and He and compare the results with the ionization
fractions calculated from the more accurate ionization balance equations. We
find that winds with a r^-2 density distribution turn out to reach a constant
ionization fraction as long as the wind density is low, i.e. in polar
direction. For the high density equatorial regions, however, we find that the
winds become neutral just above the stellar surface of the hot and massive B[e]
supergiants forming a disk-like neutral region. In such a disk molecules and
dust can form even very near the hot central star.Comment: 11 pages, 8 figures, accepted for publication in A&
Disk winds of B[e] supergiants
The class of B[e] supergiants is characterized by a two-component stellar
wind consisting of a normal hot star wind in the polar zone and a slow and
dense disk-like wind in the equatorial region. The properties of the disk wind
are discussed using satellite UV spectra of stars seen edge-on, i.e. through
the equatorial disk. These observations show that the disk winds are extremely
slow, 50-90 km/s, i.e. a factor of about 10 slower than expected from the
spectral types. Optical emission lines provide a further means to study the
disk wind. This is discussed for line profiles of forbidden lines formed in the
disk.Comment: 7 pages, LaTeX, 3 ps figures, uses lamuphys.sty from Springer-Verlag,
to be published in the proceedings of IAU Coll. 169 "Variable and
Non-spherical Stellar Winds in Luminous Hot Stars" held in Heidelberg 199
Kinematical structure of the circumstellar environments of galactic B[e]-type stars
High resolution line profiles are presented for selected forbidden and
permitted emission lines of a sample of galactic B[e]-type stars. The spectral
resolution corresponds to 5-7 km/s with the exception of some line profiles
which were observed with a resolution of 9-13 km/s. All Ha profiles are
characterized by a narrow split or single emission component with a width of
about 150-250 km/s (FWHM) and broad wings with a full width of ~1000-2000 km/s.
The Ha profiles can be classified into three groups: double-peaked profiles
representing the majority, single-peaked emission-line profiles, and normal P
Cygni-type profiles. The forbidden lines exhibit in most cases double-peaked
profiles. The split forbidden line profiles have peak separations of as little
as 10 km/s. The ratio of violet to red emission peak intensities, V/R, is
predominantly smaller or equal to 1. Theoretical profiles were calculated for
the optically thin case. A latitude-dependent stellar wind with a radial
expansion and a velocity decreasing from the pole to the equator was adopted.
In addition an equatorial dust ring with various optical depths was assumed.
This model can explain split lines and line asymmetries observed in some stars.
Moreover, the V/R ratios can be understood in terms of this model. The
comparison of the observed line profiles with the models thus confirms the
assumption of disk-like line-formation regions as commonly adopted for
B[e]-type stars.Comment: Astronomy & Astrophysics, in pres
Discovery of New, Dust-Poor B[e] Supergiants in the Small Magellanic Cloud
We present the discovery of three new B[e] supergiants (sgB[e] stars) in the
Small Magellanic Cloud (SMC). All three stars (R15, R38, and R48) were
identified in the course of our Runaways and Isolated O Type Star Spectroscopic
Survey of the SMC (RIOTS4). The stars show optical spectra that closely
resemble those of previously known B[e] stars, presenting numerous
low-ionization forbidden and permitted emission lines such as [Fe II] and Fe
II. Furthermore, our stars have luminosities of log(L/L_sun) > 4, demonstrating
that they are supergiants. However, we find lower infrared excesses and weaker
forbidden emission lines than for previously identified B[e] supergiants. Thus
our stars appear to either have less material in their circumstellar disks than
other sgB[e] stars, or the circumstellar material has lower dust content. We
suggest that these may constitute a new subclass of dust-poor sgB[e] stars.Comment: 7 pages, 6 figures, accepted to Ap
New spectroscopic observations of the B[e]/K binary system MWC 623
The B[e]/K binary system MWC 623 was reinvestigated using new spectroscopic
observations. The absorption lines of the K and the B star do not exhibit any
significant radial velocity variations over a time interval of 14 years. The
spectral classification using a recent echelle spectrum yielded spectral types
of K2II-Ib and B4III. The luminosity class of the K star gives an estimate of
the distance towards MWC 623 of 2.4(+1.4/-0.9) kpc. This is consistent with the
kinematic distance of 2.0(+0.6/-0.3) kpc. The masses derived from the locations
of the binary components in the H-R diagram are 7+-1.5 Msun and 7.5+-2.5 Msun
for the B and K star, respectively, i.e. the mass ratio is close to 1. Both
stars are coeval with an age of 50+10/-20 Myr as shown by the comparison with
isochrones. The high luminosity of the K star excludes a pre-main sequence
evolutionary phase as explanation for the strong LiI lambda 6708 absorption
line observed in the late-type component. Rather, the high lithium abundance is
a consequence of the young age. Likewise, the B[e] star is a slightly evolved
object starting it post-main sequence evolution.Comment: 8 pages, 11 figures, 3 tables, accepted for publication in Astronomy
& Astrophysic
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 S 65 is quite
surprising in light of a previous non-detection by McGregor et al. 1989, as
well as a high resolution (R=50000) Gemini/Phoenix spectrum of this star taken
nine months earlier showing no emission. Based on analysis of the optical
spectrum by Kraus et al. 2010, we suspect that the sudden appearance of
molecular emission could be due to density build up in an outflowing viscous
disk, 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 LBVs.Comment: Letter accepted for publication in MNRAS. 5 pages, 2 figures, 1 tabl
X-ray and optical observations of 1RXS J154814.5-452845: a new intermediate polar with soft X-ray emission
We report the identification of the ROSAT all-sky survey source 1RXS
J154814.5-452845as new intermediate polar and present the results from
follow-up optical and X-ray observations. The source shows pulsations with a
period of 693 s both in the optical and X-ray light curves and the detection of
a synodic frequency strongly suggests that this is the rotation period of the
white dwarf. Although the one day aliasing and the sparse optical data coverage
does not allow to unambiguously identify the orbital period, the most likely
values of 9.37 h and 6.72 h add 1RXS J154814.5-452845 to the intermediate
polars with the longest orbital periods known. The optical spectrum displays
features from the late type secondary and shows the presence of broad
absorption lines at \Hbet and higher order Balmer lines which may be a
signature of the white dwarf atmosphere, very similar to V 709 Cas. The average
X-ray spectra as obtained by the EPIC instruments on board XMM-Newton show hard
emission typical for this class of objects but also the presence of soft
blackbody-like emission similar to that seen from soft intermediate polars and
thought to arise from the white dwarf surface heated by the hard X-rays. The
best fit model comprises thermal emission from multi-temperature plasma in
collisional ionization equilibrium with a continuous temperature distribution
up to a maximum of 60 keV, an Fe fluorescence line at 6.4 keV and with
equivalent width of 260 eV and a blackbody component with kT of 86 eV. The hard
X-ray emission is absorbed by matter covering 47% of the X-ray source with an
equivalent hydrogen density of \ohcm{23}. The remaining hard emission is
absorbed by a much reduced column density of 1.5\hcm{21} as is the soft
blackbody emission. (truncated)Comment: 14 pages, Latex, with 19 figures, accepted for publication in
Astronomy and Astrophysic
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