171 research outputs found
The spin rates and spin evolution of the O components in WR+O binaries
Despite 50 years of extensive binary research we have to conclude that the
Roche lobe overflow/mass transfer process that governs close binary evolution
is still poorly understood. It is the scope of the present paper to lift a tip
of the veil by studying the spin-up and spin-down processes of the O-type
components of WR+O binaries. We critically analyze the available observational
data of rotation speeds of the O-type components in WR+O binaries. By combining
a binary evolutionary code and a formalism that describes the effects of tides
in massive stars with an envelope in radiative equilibrium, we compute the
corresponding rotational velocities during the Roche lobe overflow of the
progenitor binaries. In all the studied WR+O binaries, we find that the O-type
stars were affected by accretion of matter during the RLOF of the progenitor.
This means that common envelope evolution which excludes any accretion onto the
secondary O-star, has not played an important role to explain the WR+O
binaries. Moreover, although it is very likely that the O-type star progenitors
were spun-up by the mass transfer, many ended the RLOF/mass transfer phase with
a rotational velocity that is significantly smaller than the critical rotation
speed. This may indicate that during the mass transfer phase there is a
spin-down process which is of the same order as, although significantly less
than that of the spin-up process. We propose a Spruit-Tayler type dynamo
spin-down suggested in the past to explain the rotation speeds of the mass
gainers in long-period Algols.Comment: 6 pages, accepted for publication in A&A; accepted versio
Symbiotic stars in the Local Group of Galaxies
Preliminary results of the ongoing search for symbiotic binary stars in the
Local Group of Galaxies are presented and discussed.Comment: Proceedings of Physics of Evolved Stars 2015: A Conference Dedicated
to the Memory of Olivier Chesneau, Nice, 8-12 June 201
Stellar Populations and Variable Stars in the Core of the Globular Cluster M5
We report the discovery of a variable blue straggler in the core of the
globular cluster M5, based on a 12-hour long series of images obtained with the
Planetary Camera aboard the Hubble Space Telescope. In addition, we present the
light curves of 28 previously unknown or poorly studied large-amplitude
variable stars (all but one are RR Lyrae) in the cluster core. A (V, U-I)
color-magnitude diagram shows 24 blue stragglers within 2 core radii of the
cluster center. The blue straggler population is significantly more centrally
concentrated than the horizontal branch and red giant stars.Comment: 22 pages, 18 postscript figures, 1 postscript table, AJ, in press
(Feb 1998
The diffusion-induced nova scenario. CK Vul and PB 8 as possible observational counterparts
We propose a scenario for the formation of DA white dwarfs with very thin
helium buffers. For these stars we explore the possible occurrence of
diffusion-induced CNO- flashes, during their early cooling stage. In order to
obtain very thin helium buffers, we simulate the formation of low mass remnants
through an AGB final/late thermal pulse (AFTP/LTP scenario). Then we calculate
the consequent white dwarf cooling evolution by means of a consistent treatment
of element diffusion and nuclear burning. Based on physically sounding white
dwarf models, we find that the range of helium buffer masses for these
diffusion-induced novas to occur is significantly smaller than that predicted
by the only previous study of this scenario. As a matter of fact, we find that
these flashes do occur only in some low-mass (M < 0.6M) and low metallicity
(Z_ZAMS <0.001) remnants about 10^6 - 10^7 yr after departing from the AGB. For
these objects, we expect the luminosity to increase by about 4 orders of
magnitude in less than a decade. We also show that diffusion-induced novas
should display a very typical eruption lightcurve, with an increase of about a
few magnitudes per year before reaching a maximum of M_V ~ -5 to -6. Our
simulations show that surface abundances after the outburst are characterized
by logNH/NHe ~ -0.15...0.6 and N>C>O by mass fractions. Contrary to previous
speculations we show that these events are not recurrent and do not change
substantially the final H-content of the cool (DA) white dwarf. (Abridged)Comment: 16 pages, 8 figures, 3 tables. Replaced to match the final version
published by MNRAS. The definitive version is available at
http://onlinelibrary.wiley.com/journal/10.1111/%28ISSN%291365-296
The Early Palomar Program (1950-1955) for the Discovery of Classical Novae in M81: Analysis of the Spatial Distribution, Magnitude Distribution, and Distance Suggestion
Data obtained in the 1950-1955 Palomar campaign for the discovery of
classical novae in M81 are set out in detail. Positions and apparent B
magnitudes are listed for the 23 novae that were found. There is modest
evidence that the spatial distribution of the novae does not track the B
brightness distribution of either the total light or the light beyond an
isophotal radius that is 70\arcsec from the center of M81. The nova
distribution is more extended than the aforementioned light, with a significant
fraction of the sample appearing in the outer disk/spiral arm region. We
suggest that many (perhaps a majority) of the M81 novae that are observed at
any given epoch (compared with say years ago) are daughters of
Population I interacting binaries. The conclusion that the present day novae
are drawn from two population groups, one from low mass white dwarf secondaries
of close binaries identified with the bulge/thick disk population, and the
other from massive white dwarf secondaries identified with the outer thin
disk/spiral arm population, is discussed. We conclude that the M81 data are
consistent with the two population division as argued previously from (1) the
observational studies on other grounds by Della Valle et al. (1992, 1994),
Della Valle & Livio (1998), and Shafter et al. (1996) of nearby galaxies, (2)
the Hatano et al. (1997a,b) Monte Carlo simulations of novae in M31 and in the
Galaxy, and (3) the Yungelson et al. (1997) population synthesis modeling of
nova binaries. Two different methods of using M81 novae as distance indicators
give a nova distance modulus for M81 as , consistent with the
Cepheid modulus that is the same value.Comment: 24 pages, 7 figures, accepted to PAS
A Far-Ultraviolet Survey of 47 Tucanae.II The Long-Period Cataclysmic Variable AKO 9
We present time-resolved, far-ultraviolet (FUV) spectroscopy and photometry
of the 1.1 day eclipsing binary system AKO 9 in the globular cluster 47
Tucanae. The FUV spectrum of AKO 9 is blue and exhibits prominent C IV and He
II emission lines. The spectrum broadly resembles that of long-period,
cataclysmic variables in the galactic field.
Combining our time-resolved FUV data with archival optical photometry of 47
Tuc, we refine the orbital period of AKO 9 and define an accurate ephemeris for
the system. We also place constraints on several other system parameters, using
a variety of observational constraints. We find that all of the empirical
evidence is consistent with AKO 9 being a long-period dwarf nova in which mass
transfer is driven by the nuclear expansion of a sub-giant donor star. We
therefore conclude that AKO 9 is the first spectroscopically confirmed
cataclysmic variable in 47 Tuc.
We also briefly consider AKO 9's likely formation and ultimate evolution.
Regarding the former, we find that the system was almost certainly formed
dynamically, either via tidal capture or in a 3-body encounter. Regarding the
latter, we show that AKO 9 will probably end its CV phase by becoming a
detached, double WD system or by exploding in a Type Ia supernova.Comment: 40 pages, 11 figures, to appear in the Dec 20 issue of ApJ; minor
changes to match final published versio
A dwarf nova in the globular cluster M13
Dwarf novae in globular clusters seem to be rare with only 13 detections in
the 157 known Galactic globular clusters. We report the identification of a new
dwarf nova in M13, the 14th dwarf nova identified in a globular cluster to
date. Using the 2m Faulkes Telescope North, we conducted a search for stars in
M13 that show variability over a year (2005-2006) on timescales of days and
months. This led to the detection of one dwarf nova showing several outbursts.
A Chandra X-ray source is coincident with this dwarf nova and shows both a
spectrum and variability consistent with that expected from a dwarf nova, thus
supporting the identification. We searched for a counterpart in Hubble Space
Telescope ACS/WFC archived images and found at least 11 candidates, of which we
could characterize only the 7 brightest, including one with a 3 sigma H-alpha
excess and a faint blue star. The detection of one dwarf nova when more could
have been expected likely indicates that our knowledge of the global Galactic
population of cataclysmic variables is too limited. The proportion of dwarf
novae may be lower than found in catalogs, or they may have a much smaller duty
cycle in general as proposed by some population synthesis models and recent
observations in the field.Comment: Accepted for publication in ApJ, 12 pages, 12 figures, 5 tables (v2
contains corrections of the proofs
The origin of intergalactic thermonuclear supernovae
The population synthesis method is used to study the possibility of
explaining the appreciable fraction (20^+12_15%) of the intergalactic (no-host)
type Ia supernovae observed in galaxy clusters (Gal-Yam ete al. 2003) by binary
whote dwarf merginngs in the cores of globular clusters. In a typical globular
cluster, the number of merging double white dwarfs is fount to be smaller than
10^{-13} per year per average cluster star during the entire evolution of the
cluster, which is a factor of 3 higher than in a Milky-Way-type galaxy. From 5
to 30% of the merging white dwarfs are dynamically expelled from the cluster
with barycenter velocities up to 150 km/s. SN Ia explosions during the mergers
of binary white dwarfs in dense star clusters may account for \sim 1% of the
total rate of SN Ia in the central parts of galaxy clusters if the baryon mass
fraction in such star clusters is \sim 0.3%.Comment: 8 pages, 3 figs. Astronomy Letters (in press
RetroSpective cohort stUdy of PD-L1 expression in REcurrent and/or MEtastatic squamous cell carcinoma of the head and neck (SUPREME-HN)
n/
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