448 research outputs found
Radial and rotational velocities of young brown dwarfs and very low-mass stars in the Upper Scorpius OB association and the rho Ophiuchi cloud core
We present the results of a radial velocity (RV) survey of 14 brown dwarfs
(BDs) and very low-mass (VLM) stars in the Upper Scorpius OB association
(UScoOB) and 3 BD candidates in the rho Ophiuchi dark cloud core. We obtained
high-resolution echelle spectra at the Very Large Telescope using Ultraviolet
and Visual Echelle Spectrograph (UVES) at two different epochs for each object,
and measured the shifts in their RVs to identify candidates for binary/multiple
systems in the sample. The average time separation of the RV measurements is
21.6d, and our survey is sensitive to the binaries with separation < 0.1 au. We
found that 4 out of 17 objects (or 24^{+16}_{-13} per cent by fraction) show a
significant RV change in 4-33d time scale, and are considered as
binary/multiple `candidates.' We found no double-lined spectroscopic binaries
in our sample, based on the shape of cross-correlation curves. The RV
dispersion of the objects in UScoOB is found to be very similar to that of the
BD and VLM stars in Chamaeleon I (Cha I). We also found the distribution of the
mean rotational velocities (v sin i) of the UScoOB objects is similar to that
of the Cha I, but the dispersion of v sin i is much larger than that of the Cha
I objects.Comment: 10 pages, 5 figures, accepted for publication in MNRA
On the evidence for brown-dwarf secondary stars in cataclysmic variables
We present the K-band spectrum of the cataclysmic variable LL And, obtained
using NIRSPEC on Keck-II. The spectrum shows no evidence for the absorption
features observed by Howell & Ciardi (2001), which these authors used to claim
a detection of a brown-dwarf secondary star in LL And. In light of our new
data, we review the evidence for brown-dwarf secondary stars in this and other
cataclysmic variables.Comment: 6 pages, to appear in Monthly Notices, accepte
The K-band spectrum of the Cataclysmic Variable RXJ 0502.8+1624 (Tau 4)
We present the K-band spectrum of the cataclysmic variable RXJ 0502.8+1624
(Tau 4). The spectrum shows a broad, smooth hump, with no absorption lines from
the secondary star visible. This result indicates that the infrared light of
this system is dominated by cyclotron emission, and, in combination with the
optical spectrum and X-ray properties, suggests that Tau 4 is a polar-type
cataclysmic variable (CV).
The system was chosen for study because the broadband JHK colours of Tau 4
are consistent with an L-type dwarf, suggesting that this system might harbour
an elusive sub-stellar secondary star. The result presented here, along with
the recent discovery of cyclotron emission in the cataclysmic variable EF Eri,
suggests that care must be taken when using the broadband JHK colours of CVs
when targeting searches for sub-stellar secondary starsComment: 4 pages, to appear as research note in A&
Pre-main-sequence isochrones -- II. Revising star and planet formation timescales
We have derived ages for 13 young (<30 Myr) star-forming regions and find
they are up to a factor two older than the ages typically adopted in the
literature. This result has wide-ranging implications, including that
circumstellar discs survive longer (~10-12 Myr) and that the average Class I
lifetime is greater (~1 Myr) than currently believed.
For each star-forming region we derived two ages from colour-magnitude
diagrams. First we fitted models of the evolution between the zero-age
main-sequence and terminal-age main-sequence to derive a homogeneous set of
main-sequence ages, distances and reddenings with statistically meaningful
uncertainties. Our second age for each star-forming region was derived by
fitting pre-main-sequence stars to new semi-empirical model isochrones. For the
first time (for a set of clusters younger than 50 Myr) we find broad agreement
between these two ages, and since these are derived from two distinct mass
regimes that rely on different aspects of stellar physics, it gives us
confidence in the new age scale. This agreement is largely due to our adoption
of empirical colour-Teff relations and bolometric corrections for
pre-main-sequence stars cooler than 4000 K.
The revised ages for the star-forming regions in our sample are: ~2 Myr for
NGC 6611 (Eagle Nebula; M 16), IC 5146 (Cocoon Nebula), NGC 6530 (Lagoon
Nebula; M 8), and NGC 2244 (Rosette Nebula); ~6 Myr for {\sigma} Ori, Cep OB3b,
and IC 348; ~10 Myr for {\lambda} Ori (Collinder 69); ~11 Myr for NGC 2169; ~12
Myr for NGC 2362; ~13 Myr for NGC 7160; ~14 Myr for {\chi} Per (NGC 884); and
~20 Myr for NGC 1960 (M 36).Comment: 28 pages, 18 figures, 34 tables, accepted for publication in MNRAS.
All photometric catalogues presented in this paper are available online at
the Cluster Collaboration homepage
http://www.astro.ex.ac.uk/people/timn/Catalogues
A lithium depletion boundary age of 22 Myr for NGC 1960
We present a deep Cousins RI photometric survey of the open cluster NGC 1960,
complete to R_C \simeq 22, I_C \simeq 21, that is used to select a sample of
very low-mass cluster candidates. Gemini spectroscopy of a subset of these is
used to confirm membership and locate the age-dependent "lithium depletion
boundary" (LDB) --the luminosity at which lithium remains unburned in its
low-mass stars. The LDB implies a cluster age of 22 +/-4 Myr and is quite
insensitive to choice of evolutionary model. NGC 1960 is the youngest cluster
for which a LDB age has been estimated and possesses a well populated upper
main sequence and a rich low-mass pre-main sequence. The LDB age determined
here agrees well with precise age estimates made for the same cluster based on
isochrone fits to its high- and low-mass populations. The concordance between
these three age estimation techniques, that rely on different facets of stellar
astrophysics at very different masses, is an important step towards calibrating
the absolute ages of young open clusters and lends confidence to ages
determined using any one of them.Comment: Accepted for publication in MNRA
Estimating the masses of extra-solar planets
All extra-solar planet masses that have been derived spectroscopically are
lower limits since the inclination of the orbit to our line-of-sight is unknown
except for transiting systems. It is, however, possible to determine the
inclination angle, i, between the rotation axis of a star and an observer's
line-of-sight from measurements of the projected equatorial velocity (v sin i),
the stellar rotation period (P_rot) and the stellar radius (R_star). This
allows the removal of the sin i dependency of spectroscopically derived
extra-solar planet masses under the assumption that the planetary orbits lie
perpendicular to the stellar rotation axis. We have carried out an extensive
literature search and present a catalogue of v sin i, P_rot, and R_star
estimates for exoplanet host stars. In addition, we have used Hipparcos
parallaxes and the Barnes-Evans relationship to further supplement the R_star
estimates obtained from the literature. Using this catalogue, we have obtained
sin i estimates using a Markov-chain Monte Carlo analysis. This allows proper
1-sigma two-tailed confidence limits to be placed on the derived sin i's along
with the transit probability for each planet to be determined. While a small
proportion of systems yield sin i's significantly greater than 1, most likely
due to poor P_rot estimations, the large majority are acceptable. We are
further encouraged by the cases where we have data on transiting systems, as
the technique indicates inclinations of ~90 degrees and high transit
probabilities. In total, we estimate the true masses of 133 extra-solar
planets. Of these, only 6 have revised masses that place them above the 13
Jupiter mass deuterium burning limit. Our work reveals a population of
high-mass planets with low eccentricities and we speculate that these may
represent the signature of different planetary formation mechanisms at work.Comment: 40 pages, 6 tables, 2 figures. Accepted for publication in the
Monthly Notices of the Royal Astronomical Society after editing of Tables 1 &
6 for electronic publication. Html abstract shortened for astro-ph submissio
The London Heat Island – surface and air temperature measurements in a park and street gorges
This paper reports results from short-term tests carried
out as part of a project to characterize the urban heat island
in London. The investigations looked at air temperatures
upstream and downstream of a park and the surface and air
temperatures within street gorges. It was found that the air in
the park was associated with lower mean (0.6°C [1.1°F] less)
and peak temperatures (1.1°C [2.0°F] less) compared to residential
or shopping streets on either side. The apparent cooling
influence of the park extended downstream between 200 and
400 meters (200 to 400 yards). Measurements in four street
gorges showed a wide variation in surface temperatures—up
to 22°C (40°F)—although 5ºC to 10°C (9ºF to 18°F) was more
typical. For a given façade, lighter surfaces were associated
with lower temperatures, between 6ºC and 10°C (11ºF and
18°F) cooler. A strong relationship was found between mean
gorge surface temperature and the gorge air temperature
measured at 6 m (20 ft) (half-gorge height). This was true for
both a sunny day and a cloudy day. The results suggest that
significant reductions in air temperature may be possible by
adjusting the albedo of urban surfaces
High-speed, multi-colour optical photometry of the anomalous X-ray pulsar 4U 0142+61 with ULTRACAM
We present high-speed, multi-colour optical photometry of the anomalous X-ray
pulsar 4U 0142+61, obtained with ULTRACAM on the 4.2-m William Herschel
Telescope. We detect 4U 0142+61 at magnitudes of i'=23.7+-0.1, g'=27.2+-0.2 and
u'>25.8, consistent with the magnitudes found by Hulleman et al.(2004) and
hence confirming their discovery of both a spectral break in the optical and a
lack of long-term optical variability. We also confirm the discovery of Kern &
Martin (2002) that 4U 0142+61 shows optical pulsations with an identical period
(~8.7 s) to the X-ray pulsations. The rms pulsed fraction in our data is
29+-8%, 5-7 times greater than the 0.2-8 keV X-ray rms pulsed fraction. The
optical and X-ray pulse profiles show similar morphologies and appear to be
approximately in phase with each other, the former lagging the latter by only
0.04+-0.02 cycles. In conjunction with the constraints imposed by X-ray
observations, the results presented here favour a magnetar interpretation for
the anomalous X-ray pulsars.Comment: 6 pages, 4 figures, accepted for publication in MNRA
Infrared spectroscopy of cataclysmic variables: III. Dwarf novae below the period gap and novalike variables
We present K-band spectra of the short-period dwarf novae YZ Cnc, LY Hya, BK
Lyn, T Leo, SW UMa and WZ Sge, the novalike variables DW UMa, V1315 Aql, RW
Tri, VY Scl, UU Aqr and GP Com, and a series of field dwarf stars with spectral
types ranging from K2-M6.
The spectra of the dwarf novae are dominated by emission lines of HI and HeI.
The large velocity and equivalent widths of these lines, in conjunction with
the fact that the lines are double-peaked in the highest inclination systems,
indicate an accretion disc origin. In the case of YZ Cnc and T Leo, for which
we obtained time-resolved data covering a complete orbital cycle, the emission
lines show modulations in their equivalent widths which are most probably
associated with the bright spot (the region where the gas stream collides with
the accretion disc). There are no clear detections of the secondary star in any
of the dwarf novae below the period gap, yielding upper limits of 10-30% for
the contribution of the secondary star to the observed K-band flux. In
conjunction with the K-band magnitudes of the dwarf novae, we use the derived
secondary star contributions to calculate lower limits to the distances to
these systems.
The spectra of the novalike variables are dominated by broad, single-peaked
emission lines of HI and HeI - even the eclipsing systems we observed do not
show the double-peaked profiles predicted by standard accretion disc theory.
With the exception of RW Tri, which exhibits NaI, CaI and 12CO absorption
features consistent with a M0V secondary contributing 65% of the observed
K-band flux, we find no evidence for the secondary star in any of the novalike
variables. The implications of this result are discussed.Comment: 13 pages, 5 figures, to appear in MNRA
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