89 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 deep and wide-field view at the IC 2944 / 2948 complex in Centaurus
We employed the ESO MPI wide-field camera and obtained deep images in the VIc
pass-bands in the region of the IC 2944/2948 complex (l ~ 294; b ~ -1), and
complemented them with literature and archival data. We used this material to
derive the photometric, spectroscopic and kinematic properties of the brightest
(V < 16) stars in the region. The VI deep photometry on the other end, helped
us to unravel the lower main sequence of a few, possibly physical, star groups
in the area.
Our analysis confirmed previous suggestions that the extinction toward this
line of sight follows the normal law (Rv = 3.1). We could recognize B-type
stars spread in distance from a few hundred pc to at least 2 kpc. We found two
young groups (age ~ 3 Myr) located respectively at about 2.3 and 3.2 kpc from
the Sun. They are characterized by a significant variable extinction (E(B-V)
ranging from 0.28 to 0.45 mag), and host a significant pre-main sequence
population. We computed the initial mass functions for these groups and
obtained slopes Gamma from -0.94 to -1.02 (e_Gamma = 0.3), in a scale where the
classical Salpeter law is -1.35. We estimated the total mass of both main
stellar groups in ~ 1100 Mo, respectively. Our kinematic analysis
indicated that both groups of stars deviate from the standard rotation curve of
the Milky Way, in line with literature results for this specific Galactic
direction.
Finally, along the same line of sight we identified a third group of
early-type stars located at ~ 8 kpc from the Sun. This group might be located
in the far side of the Sagittarius-Carina spiral arm.Comment: 13 pages, 11 figures, accepted for publication in MNRA
Evolution of iron core white dwarfs
Recent measurements made by Hipparcos (Provencal et al. 1998) present
observational evidence supporting the existence of some white dwarf (WD) stars
with iron - rich, core composition. In this connection, the present paper is
aimed at exploring the structure and evolution of iron - core WDs by means of a
detailed and updated evolutionary code. In particular, we examine the evolution
of the central conditions, neutrino luminosity, surface gravity,
crystallization, internal luminosity profiles and ages. We find that the
evolution of iron - rich WDs is markedly different from that of their carbon -
oxygen counterparts. In particular, cooling is strongly accelerated as compared
with the standard case. Thus, if iron WDs were very numerous, some of them
would have had time enough to evolve at lower luminosities than that
corresponding to the fall - off in the observed WD luminosity function.Comment: 8 pages, 21 figures. Accepted for publication in MNRA
The embedded clusters DBS 77, 78, 102, and 160-161 and their link with the interstellar medium
Aims. We report a study of the global properties of some embedded clusters
placed in the fourth quadrant of the Milky Way to clarify some issues related
with their location into the Galaxy and their stellar formation processes.
Methods. We performed BVI photometric observations in the region of DBS 77, 78,
102, 160, and 161 clusters and infrared spectroscopy in DBS 77 region. They
were complemented with JHK data from VVV survey combined with 2MASS catalogue,
and used mid-infrared information from GLIMPSE catalogue. We also searched for
HI data from SGPS and PMN radio surveys, and previous spectroscopic stellar
classification. The spectroscopic and photometric information allowed us to
estimate the spectral classification of the brightest stars of each studied
region. On the other hand, we used the radio data to investigate the
interstellar material parameters and the continuum sources probably associated
with the respective stellar components. Results. We estimated the basic
physical parameters of the clusters (reddening, distance, age, and initial mass
function). We searched for HII regions located near to the studied clusters and
we analyzed the possible link between them. In the particular case of DBS
160-161 clusters, we identified the HI bubble B332.5-0.1-42 located around
them. We found that the mechanical energy injected to the interstellar medium
by the more massive stars of this couple of clusters was enough to generate the
bubble.Comment: 15 pages, 14 figures, 6 tables, accepted for publication in A&
White Dwarf Donors in Ultracompact Binaries: The Stellar Structure of Finite Entropy Objects
We discuss the mass-radius (M-R) relations for low-mass (M<0.1 Msun) white
dwarfs (WDs) of arbitrary degeneracy and evolved (He, C, O) composition. We do
so with both a simple analytical model and models calculated by integration of
hydrostatic balance using a modern equation of state valid for fully ionized
plasmas. The M-R plane is divided into three regions where either Coulomb
physics, degenerate electrons or a classical gas dominate the WD structure. For
a given M and central temperature, T_c, the M-R relation has two branches
differentiated by the model's entropy content. We present the M-R relations for
a sequence of constant entropy WDs of arbitrary degeneracy parameterized by M
and T_c for pure He, C, and O. We discuss the applications of these models to
the recently discovered accreting millisecond pulsars. We show the relationship
between the orbital inclination for these binaries and the donor's composition
and T_c. In particular we find from orbital inclination constraints that the
probability XTE J1807-294 can accommodate a He donor is approximately 15% while
for XTE J0929-304, it is approximately 35%. We argue that if the donors in
ultracompact systems evolve adiabatically, there should be 60-160 more systems
at orbital periods of 40 min than at orbital periods of 10 min, depending on
the donor's composition.Comment: emulateapj style, 11 pages, 12 figures. Accepted to the Astrophysical
Journal. Tables with interpolation routines of the M-R relations are
available at http://www.physics.ucsb.edu/~cjdeloye/research.htm
Dark matter burners
We show that a star orbiting close enough to an adiabatically grown
supermassive black hole (SMBH) can capture weakly interacting massive particles
(WIMPs) at an extremely high rate. The stellar luminosity due to annihilation
of captured WIMPs in the stellar core may be comparable to or even exceed the
luminosity of the star due to thermonuclear burning. The model thus predicts
the existence of unusual stars, essentially WIMP burners, in the vicinity of a
SMBH. We find that the most efficient WIMP burners are stars with degenerate
electron cores, e.g. white dwarfs (WDs); such WDs may have a very high surface
temperature. If found, such stars would provide evidence for the existence of
particle dark matter and can possibly be used to establish its density profile.
On the other hand, the lack of such unusual stars may provide constraints on
the WIMP density near the SMBH, as well as the WIMP-nucleus scattering and pair
annihilation cross-sections.Comment: 4 pages, 4 figures, emulateapj.cls; Astrophysical Journal Letters,
accepte
Far Ultraviolet Spectroscopic Explorer Spectroscopy of the Nova-like BB Doradus
We present an analysis of the Far Ultraviolet Spectroscopic Explorer
({\it{FUSE}}) spectra of the little-known southern nova-like BB Doradus. The
spectrum was obtained as part of our Cycle 8 {\it FUSE} survey of high
declination nova-like stars.
The FUSE spectrum of BB Dor, observed in a high state, is modeled with an
accretion disk with a very low inclination (possibly lower than 10deg).
Assuming an average WD mass of 0.8 solar leads to a mass accretion rate of
1.E-9 Solar mass/year and a distance of the order of 650 pc, consistent with
the extremely low galactic reddening in its direction. The spectrum presents
some broad and deep silicon and sulfur absorption lines, indicating that these
elements are over-abundant by 3 and 20 times solar, respectively
A Self-Occulting Accretion Disk in the SW Sex Star DW UMa
We present the ultraviolet spectrum of the SW Sex star and nova-like variable
DW UMa in an optical low state, as observed with the Space Telescope Imaging
Spectrograph on board the Hubble Space Telescope (HST). The data are well
described by a synthetic white dwarf (WD) spectrum with T_eff = 46,000 +/- 1000
K, log g = 7.60 +/- 0.15, v*sin(i) = 370 +/- 100 km/s and Z/Z_solar = 0.47 +/-
0.15. For this combination of T_eff and log g, WD models predict M_WD = 0.48
+/- 0.06 M_solar and R_WD = (1.27 +/- 0.18) * 10^9 cm. Combining the radius
estimate with the normalization of the spectral fit, we obtain a distance
estimate of d = 830 +/-150 pc.
During our observations, DW UMa was approximately 3 magnitudes fainter in V
than in the high state. A comparison of our low-state HST spectrum to a
high-state spectrum obtained with the International Ultraviolet Explorer shows
that the former is much bluer and has a higher continuum level shortward of
1450 A. Since DW UMa is an eclipsing system, this suggests that an optically
thick accretion disk rim blocks our view of the WD primary in the high state.
If self-occulting accretion disks are common among the SW Sex stars, we can
account for (i) the preference for high-inclination systems within the class
and (ii) their V-shaped continuum eclipses. Moreover, even though the emission
lines produced by a self-obscured disk are generally still double-peaked, they
are weaker and narrower than those produced by an unobscured disk. This may
allow a secondary line emission mechanism to dominate and produce the
single-peaked, optical lines that are a distinguishing characteristic of the SW
Sex stars.Comment: 9 pages, including 2 figures; accepted for publication in
Astrophysical Journal Letters; New version matches version in press (footnote
added to discussion section; figures now use color
The system parameters of DW Ursae Majoris
We present new constraints on the system parameters of the SW Sextantis star
DW Ursae Majoris, based on ultraviolet (UV) eclipse observations with the
Hubble Space Telescope. Our data were obtained during a low state of the
system, in which the UV light was dominated by the hot white dwarf (WD)
primary. Eclipse analysis, using the full Roche lobe geometry, allows us to set
firm limits on the masses and radii of the system components and the distance
between them: 0.67 \leq M_1/M_sun \leq 1.06, 0.008 \leq R_1/R_sun \leq 0.014,
M_2/M_sun > 0.16, R_2/R_sun > 0.28 and a/R_sun > 1.05. For q = M_2/M_1 < 1.5
the inclination must satisfy i > 71 degrees. Using Smith & Dhillon's
mass-period relation for CV secondaries, our estimates for the system
parameters become M_1/M_sun = 0.77 \pm 0.07, R_1/R_sun = 0.012 \pm 0.001,
M_2/M_sun = 0.30 \pm 0.10, R_2/R_sun = 0.34 \pm 0.04, q =0.39 \pm 0.12, i = 82
\pm 4 degrees and a/R_sun = 1.14 \pm 0.06. We have also estimated the spectral
type of the secondary, M3.5 \pm 1.0, and distance to the system, d =930 \pm 160
pc, from time-resolved I- and K-band photometry. Finally, we have repeated
Knigge et al.'s WD model atmosphere fit to the low-state UV spectrum of DW UMa
in order to account for the higher surface gravity indicated by our eclipse
analysis. In this way we obtained a second estimate for the distance, d = 590
\pm 100 pc, which allows us to obtain a second estimate for the spectral type
of the secondary, M7 \pm 2.0. We conclude that the true value for the distance
and spectral type will probably be in between the values obtained by the two
methods.Comment: 23 pages including 5 figures and 3 tables. Accepted for publication
in Ap
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