393 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
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
Stagnation zone during the turning of Duplex SAF 2205 stainless steels alloy
Duplex stainless alloys are extremely sensitive to cutting speed for strain hardening during machining. Tool wear for these materials is dominated by the adhesion wear because of formation of built-up edge (BUE) that upsurges the flank wear considerably. In addition, flute damage is a significant problem during drilling of those alloys. To address this issue, this paper investigates the mechanism of BUE creation in stagnation region of duplex SAF 2205 alloys during material removal by turning process. The investigation of chip root through SEM and electron backscatter diffraction (EBSD) revealed build-up of ferritic bands at the stagnation zone. Higher capacity of austenite phase to deform plastically is accountable for the ferrite build-up. This was detected as a possible activating mechanism of built-up edge. The flow pattern of austenite phase designates faster deforming compare to that of ferrite phases
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
Using Gaussian Processes to detect AGN flares
A key feature of active galactic nuclei (AGN) is their variability across all
wavelengths. Typically, AGN vary by a few tenths of a magnitude or more over
periods lasting from hours to years. By contrast, extreme variability of AGN --
large luminosity changes that are a significant departure from the baseline
variability -- are known as AGN flares. These events are rare and their
timescales poorly constrained, with most of the literature focusing on
individual events. It has been suggested that extreme AGN variability including
flares can provide insights into the accretion processes in the disk. With
surveys such as the Legacy Survey of Space and Time (LSST) promising millions
of transient detections per night in the coming decade, there is a need for
fast and efficient classification of AGN flares. The problem with the
systematic detection of AGN flares is the requirement to detect them against a
stochastically variable baseline; the ability to define a signal as a
significant departure from the ever-present variability is a statistical
challenge. Recently, Gaussian Processes (GPs) have revolutionised the analysis
of time-series data in many areas of astronomical research. They have, however,
seen limited uptake within the field of transient detection and classification.
Here we investigate the efficacy of Gaussian Processes to detect AGN flares in
both simulated and real optical light curves. We show that GP analysis can
successfully detect AGN flares with a false-positive rate of less than seven
per cent, and we present examples of AGN light curves that show extreme
variability.Comment: 14 pages, 25 figures, accepted for publication in MNRA
Evidence for high accretion-rates in Weak-Line T Tauri stars?
We have discovered T Tauri stars which show startling spectral variability
between observations seperated by 20 years. In spectra published by Bouvier &
Appenzeller (1992) these objects showed very weak H-alpha emission, broad CaII
absorption and so called ``composite spectra'', where the spectral type
inferred from the blue region is earlier than that inferred from the red. We
present here new spectroscopy which shows that all four stars now exhibit
strong H-alpha emission, narrow CaII emission and a spectral type which is
consistent at all wavelengths.
We propose a scheme to understand these changes whereby the composite spectra
of these stars can be explained by a period of active accretion onto the
central, young star. In this scheme the composite spectrum consists of a
contribution from the stellar photosphere and a contribution from a hot,
optically thick, accretion component. The optically thick nature of the
accretion flow explains the weakness of the H-alpha emission during this phase.
Within this scheme, the change to a single spectral type at all wavelengths and
emergence of strong H-alpha emission are consistent with the accretion columns
becoming optically thin, as the accretion rate drops. There is a strong analogy
here with the dwarf novae class of interacting binaries, which show similar
behaviour during the decline from outbursts of high mass-transfer rate.
The most important consequence of this interpretation is that these objects
bring into question the association of Weak-Line T Tauri stars (WTTs) with
non-accreting or discless objects. In light of this result we consider the
justification for this paradigm.Comment: 6 pages, 3 figures. Accepted for publication in MNRA
Discovery of dipolar chromospheres in two white dwarfs
This paper reports the ULTRACAM discovery of dipolar surface spots in two cool magnetic white dwarfs with Balmer emission lines, while a third system exhibits a single spot, similar to the prototype GD 356. The light curves are modelled with simple, circular, isothermal dark spots, yielding relatively large regions with minimum angular radii of 20°. For those stars with two light-curve minima, the dual spots are likely observed at high inclination (or colatitude); however, identical and antipodal spots cannot simultaneously reproduce both the distinct minima depths and the phases of the light-curve maxima. The amplitudes of the multiband photometric variability reported here are all several times larger than that observed in the prototype GD 356; nevertheless, all DAHe stars with available data appear to have light-curve amplitudes that increase towards the blue in correlated ratios. This behaviour is consistent with cool spots that produce higher contrasts at shorter wavelengths, with remarkably similar spectral properties given the diversity of magnetic field strengths and rotation rates. These findings support the interpretation that some magnetic white dwarfs generate intrinsic chromospheres as they cool, and that no external source is responsible for the observed temperature inversion. Spectroscopic time-series data for DAHe stars is paramount for further characterization, where it is important to obtain well-sampled data, and consider wavelength shifts, equivalent widths, and spectropolarimetry
The not-so-massive black hole in the microquasar GRS1915+105
We present a new dynamical study of the black hole X-ray transient GRS1915+105 making use of near-infrared spectroscopy obtained with X-shooter at the VLT. We detect a large number of donor star absorption features across a wide range of wavelengths spanning the H and K bands. Our 24 epochs covering a baseline of over 1 year permit us to determine a new binary ephemeris including a refined orbital period of P=33.85 +/- 0.16 d. The donor star radial velocity curves deliver a significantly improved determination of the donor semi-amplitude which is both accurate (K_2=126 +/- 1 km/s) and robust against choice of donor star template and spectral features used. We furthermore constrain the donor star's rotational broadening to vsini=21 +/-4 km/s, delivering a binary mass ratio of q=0.042 +/- 0.024. If we combine these new constraints with distance and inclination estimates derived from modelling the radio emission, a black hole mass of M_BH=10.1 +/- 0.6 M_sun is inferred, paired with an evolved mass donor of M_2=0.47 +/- 0.27 M_sun. Our analysis suggests a more typical black hole mass for GRS1915+105 rather than the unusually high values derived in the pioneering dynamical study by Greiner et al. (2001). Our data demonstrate that high-resolution infrared spectroscopy of obscured accreting binaries can deliver dynamical mass determinations with a precision on par with optical studies
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