55 research outputs found
Follow-up Observations of the Second and Third Known Pulsating Hot DQ White Dwarfs
We present follow-up time-series photometric observations that confirm and
extend the results of the significant discovery made by Barlow et al.(2008)
that the Hot DQ white dwarfs SDSS J220029.08-074121.5 and SDSS
J234843.30-094245.3 are luminosity variable. These are the second and third
known members of a new class of pulsating white dwarfs, after the prototype
SDSS J142625.71+575218.3 (Montgomery et al. 2008). We find that the light curve
of SDSS J220029.08-074121.5 is dominated by an oscillation at 654.397+-0.056 s,
and that the light pulse folded on that period is highly nonlinear due to the
presence of the first and second harmonic of the main pulsation. We also
present evidence for the possible detection of two additional pulsation modes
with low amplitudes and periods of 577.576+-0.226 s and 254.732+-0.048 s in
that star. Likewise, we find that the light curve of SDSS J234843.30-094245.3
is dominated by a pulsation with a period of 1044.168+-0.012 s, but with no
sign of harmonic components. A new oscillation, with a low amplitude and a
period of 416.919+-0.004 s, is also probably detected in that second star. We
argue, on the basis of the very different folded pulse shapes, that SDSS
J220029.08-074121.5 is likely magnetic, while SDSS J234843.30-094245.3 is
probably not.Comment: 12 pages, 19 figures, accepted for publication in Ap
Follow-up Studies of the Pulsating Magnetic White Dwarf SDSS J142625.71+575218.3
We present a follow-up analysis of the unique magnetic luminosity-variable
carbon-atmosphere white dwarf SDSS J142625.71+575218.3. This includes the
results of some 106.4 h of integrated light photometry which have revealed,
among other things, the presence of a new periodicity at 319.720 s which is not
harmonically related to the dominant oscillation (417.707 s) previously known
in that star. Using our photometry and available spectroscopy, we consider the
suggestion made by Montgomery et al. (2008) that the luminosity variations in
SDSS J142625.71+575218.3 may not be caused by pulsational instabilities, but
rather by photometric activity in a carbon-transferring analog of AM CVn. This
includes a detailed search for possible radial velocity variations due to rapid
orbital motion on the basis of MMT spectroscopy. At the end of the exercise, we
unequivocally rule out the interacting binary hypothesis and conclude instead
that, indeed, the luminosity variations are caused by g-mode pulsations as in
other pulsating white dwarfs. This is in line with the preferred possibility
put forward by Montgomery et al. (2008).Comment: 11 pages in emulateApJ, 12 figures, accepted for publication in Ap
The young, wide and very low mass visual binary LOri167
We look for wide, faint companions around members of the 5 Myr Lambda Orionis
open cluster. We used optical, near-infrared, and Spitzer/IRAC photometry. We
report the discovery of a very wide very low mass visual binary, LOri167,
formed by a brown dwarf and a planetary-mass candidate located at 5 arcsec,
which seems to belong to the cluster. We derive Teff of 2125 and 1750 K. If
they are members, comparisons with theoretical models indicate masses of 17
(20-15) Mjup and 8 (13-7) Mjup, with a projected separation of 2000 AU. Such a
binary system would be difficult to explain in most models, particularly those
where substellar objects form in the disks surrounding higher mass stars.Comment: Astronomy & Astrophysics Letters, in pres
Hot subdwarf stars in close-up view - II. Rotational properties of single and wide binary subdwarf B stars
Subluminous B stars (sdBs) form the extremely hot end of the horizontal
branch and are therefore related to the blue horizontal branch (BHB) stars.
While the rotational properties of BHB stars have been investigated
extensively, studies of sdB stars have concentrated on close binaries that are
influenced by tidal interactions between their components. Here we present a
study of 105 sdB stars, which are either single stars or in wide binaries where
tidal effects become negligible. The projected rotational velocities have been
determined by measuring the broadening of metal lines using high-resolution
optical spectra. All stars in our sample are slow rotators (). Furthermore, the -distributions of single sdBs are similar to those of hot subdwarfs
in wide binaries with main-sequence companions as well as close binary systems
with unseen companions and periods exceeding . We show that
blue horizontal and extreme horizontal branch stars are also related in terms
of surface rotation and angular momentum. Hot blue horizontal branch stars
() with diffusion-dominated atmospheres are slow
rotators like the hot subdwarf stars located on the extreme horizontal branch,
which lost more envelope and therefore angular momentum in the red-giant phase.
The uniform rotation distributions of single and wide binary sdBs pose a
challenge to our understanding of hot subdwarf formation. Especially the high
fraction of helium white dwarf mergers predicted by theory seems to be
inconsistent with the results presented here.Comment: 12 pages, 7 figures, A&A, in pres
Binarity at the L/T brown dwarf transition
Current atmospheric models cannot reproduce some of the characteristics of
the transition between the L dwarfs with cloudy atmospheres and the T dwarfs
with dust-depleted photospheres. It has been proposed that a majority of the
L/T transition brown dwarfs could actually be a combinaison of a cloudy L dwarf
and a clear T dwarf. Indeed binarity seems to occur more frequently among L/T
transition brown dwarfs.
We aim to refine the statistical significance of the seemingly higher
frequency of binaries. Co-eval binaries would also be interesting test-beds for
evolutionary models. We obtained high-resolution imaging for six mid-L to
late-T dwarfs, with photometric distances between 8 and 33pc, using the
adaptive optics systems NACO at the VLT, and the Lick system, both with the
laser guide star.
We resolve none of our targets. Combining our data with published results, we
obtain a frequency of resolved L/T transition brown dwarfs of (31+21-15)%,
compared to (21+10-7)% and (14+14-7)% for mid-L and T dwarfs (90% of confidence
level). These fractions do not significantly support, nor contradict, the
hypothesis of a larger binary fraction in the L/T transition. None of our
targets has companions with effective temperatures as low as 360-1000K at
separations larger than 0.5".Comment: 6 pages, 4 figures, accepted by A&
Multiplicity of very low-mass objects in the Upper Scorpius OB association: a possible wide binary population
We report the initial results of a VLT/NACO high spatial resolution imaging
survey for multiple systems among 58 M-type members of the nearby Upper
Scorpius OB association. Nine pairs with separations below 100 have been
resolved. Their small angular separations and the similarity in the brightness
of the components (DMagK <1 for all of them), indicate there is a reasonable
likelihood several of them are true binaries rather than chance projections.
Follow-up imaging observations with WHT/LIRIS of the two widest binaries
confirm that their near-infrared colours are consistent with physical very low
mass binaries. For one of these two binaries, WHT/LIRIS spectra of each
component were obtained. We find that the two components have similar M6-M7
spectral types and signatures of low-gravity, as expected for a young brown
dwarf binary in this association. Our preliminary results indicate a possible
population of very low-mass binaries with semimajor axis in the range 100 AU
150 AU, which has not been seen in the Pleiades open cluster. If these
candidates are confirmed (one is confirmed by this work), these results would
indicate that the binary properties of very low-mass stars and brown dwarfs may
depend on the environment where they form.Comment: 11 pages, 1 table, 7 figures, request high resolution copies to
[email protected]
Resolved Hubble Space spectroscopy of ultracool binary systems
Using the low-resolution mode of the Space Telescope Imaging Spectrograph
(STIS) aboard the \emph{Hubble Space Telescope} (HST), we have obtained
spatially resolved spectra of 20 ultracool dwarfs. 18 of them belong to 9 known
very low-mass binary systems with angular separations in the range 0.37-0.098
arcseconds. We have derived spectral types in the range dM7.5 to dL6 from the
PC3 index, and by comparing our STIS spectra with ground-based spectra of
similar spectral resolution from Mart{\'\i}n et al. (1999). We have searched
for H emission in each object but it was clearly detected in only 2 of
them. We find that the distribution of H emission in our sample is
statistically different from that of single field dwarfs, suggesting an
intriguing anticorrelation between chromospheric activity and binarity for
M7--M9.5 dwarfs. We provide measuments of the strength of the main photospheric
features and the PC3 index, and we derive calibrations of spectral subclasses
versus F814W and K-band absolute magnitudes for a subset of 10 dwarfs in 5
binaries that have known trigonometric parallaxes.Comment: accepted for publication in A&
On the evolution of a star cluster and its multiple stellar systems following gas dispersal
We investigate the evolution, following gas dispersal, of a star cluster
produced from a hydrodynamical calculation. We find that when the gas,
initially comprising 60% of the mass, is removed, the system settles into a
bound cluster containing 30-40% of the stellar mass surrounding by an expanding
halo of ejected stars. The bound cluster expands from an initial radius of
<0.05 pc to 1-2 pc over 4-10 Myr, depending on how quickly the gas is removed,
implying that stellar clusters may begin with far higher stellar densities than
usually assumed. With rapid gas dispersal the most massive stars are found to
be mass segregated for the first ~1 Myr of evolution, but classical mass
segregation only develops for cases with long gas removal timescales.
Eventually, many of the most massive stars are expelled from the bound cluster.
Despite the high initial stellar density and the extensive dynamical evolution
of the system, we find that the stellar multiplicity is almost constant during
the 10 Myr of evolution. This is because the primordial multiple systems are
formed in a clustered environment and, thus, by their nature are already
resistant to further evolution. The majority of multiple system evolution is
confined to the decay of high-order systems and the formation of a significant
population of very wide (10^4-10^5 AU) multiple systems in the expanding halo.
This formation mechanism for wide binaries potentially solves the problem of
how most stars apparently form in clusters and yet a substantial population of
wide binaries exist in the field. Many of these wide binaries and the binaries
produced by the decay of high-order multiple systems have unequal mass
components, potentially solving the problem that hydrodynamical simulations of
star formation are found to under-produce unequal-mass solar-type binaries.Comment: Accepted by MNRAS, 18 pages, 13 figure
HS0702+6043: A star showing both short-period p-mode and long-period g-mode oscillations
[Context.] The hot subdwarf B star HS0702+6043 is known as a large-amplitude,
short-period p-mode pulsator of the EC14026 type. Its atmospheric parameters
place it at the common boundary between the empirical instability regions of
the EC14026 variables and the typically cooler long-period g-mode pulsators of
the PG1716 kind.
[Aims.] We analyse and interpret the photometric variability of HS0702+6043
in order to explore its asteroseismological potential.
[Methods.] We report on rapid wide band CCD photometric observations to
follow up on and confirm the serendipitous discovery of multiperiodic
long-period luminosity variations with typical time scales of ~1h in
HS0702+6043, in addition to the two previously known pulsations at 363s and
383s. In particular, we isolate a relatively low-amplitude (~4 mmag),
long-period (3538+-130s) light variation.
[Results.] We argue that the most likely origin for this luminosity variation
is the presence of an excited g-mode pulsation. If confirmed, HS0702+6043 would
constitute a rare addition to the very select class of pulsating stars showing
simultaneously parts of their pressure and gravity mode pulsation spectra. The
asteroseismological potential of such stars is immense, and HS0702+6043 thus
becomes a target of choice for future investigations. While our discovery
appears consistent with the location of HS0702+6043 at the common boundary
between the two families of pulsating sdB stars, it does challenge theory's
current description of stability and driving mechanisms in pulsating B
subdwarfs.Comment: 4 pages, 3 figures, accepted for publication in A&A Letter
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