2,016 research outputs found
Disentangling the spatial substructure of Cygnus OB2 from Gaia DR2
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical SocietyFor the first time, we have explored the spatial substructure of the Cygnus OB2 association using parallaxes from the recent second Gaia data release. We find significant line-of-sight substructure within the association, which we quantify using a parametrized model that reproduces the observed parallax distribution. This inference approach is necessary due to the non-linearity of the parallax distance transformation and the asymmetry of the resulting probability distribution. Using a Markov Chain Monte Carlo ensemble sampler and an unbinned maximum likelihood test, we identify two different stellar groups superposed on the association. We find the main Cygnus OB2 group at ∼1760 pc, further away than recent estimates have envisaged, and a foreground group at ∼1350 pc. We also calculate individual membership probabilities and identify outliers as possible non-members of the association.Peer reviewe
Terminal velocities of luminous, early-type SMC stars
Ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) are
used to determine terminal velocities for 11 O and B-type giants and
supergiants in the Small Magellanic Cloud (SMC) from the Si IV and C IV
resonance lines. Using archival data from observations with the Goddard
High-Resolution Spectrograph and the International Ultraviolet Explorer
telescope, terminal velocities are obtained for a further five B-type
supergiants. We discuss the metallicity dependence of stellar terminal
velocities, finding no evidence for a significant scaling between Galactic and
SMC metallicities for Teff < 30,000 K, consistent with the predictions of
radiation driven wind theory for supergiant stars. A comparison of the
ratio between the SMC and Galactic samples, while
consistent with the above statement, emphasizes that the uncertainties in the
distances to galactic O-stars are a serious obstacle to a detailed comparison
with theory. For the SMC sample there is considerable scatter in this ratio at
a given effective temperature, perhaps indicative of uncertainties in stellar
masses.Comment: 28 pages, 8 figures, accepted by ApJ; minor revisions prior to
acceptanc
Structural behaviour of copper chloride catalysts during the chlorination of CO to phosgene
The interaction of CO with an attapulgite-supported Cu(II)Cl2 catalyst has been examined in a micro-reactor arrangement. CO exposure to the dried, as-received catalyst at elevated temperatures leads to the formation of CO2 as the only identifiable product. However, phosgene production can be induced by a catalyst pre-treatment where the supported Cu(II)Cl2 sample is exposed to a diluted stream of chlorine. Subsequent CO exposure at ~ 370°C then leads to phosgene production. In order to investigate the origins of this atypical set of reaction characteristics, a series of x-ray absorption experiments were performed that were supplemented by DFT calculations. XANES measurements establish that at the elevated temperatures connected with phosgene formation, the catalyst is comprised of Cu+ and a small amount of Cu2+. Moreover, the data show that unique to the chlorine pre-treated sample, CO exposure at elevated temperature results in a short-lived oxidation of the copper. On the basis of calculated CO adsorption energies, DFT calculations indicate that a mixed Cu+/Cu2+ catalyst is required to support CO chemisorption
HST astrometry in the 30 Doradus region: II. Runaway stars from new proper motions in the Large Magellanic Cloud
We present a catalog of relative proper motions for 368,787 stars in the 30
Doradus region of the Large Magellanic Cloud (LMC), based on a dedicated
two-epoch survey with the Hubble Space Telescope (HST) and supplemented with
proper motions from our pilot archival study. We demonstrate that a relatively
short epoch difference of 3 years is sufficient to reach a 0.1 mas
yr level of precision or better. A number of stars have relative proper
motions exceeding a 3-sigma error threshold, representing a mixture of Milky
Way denizens and 17 potential LMC runaway stars. Based upon 183 VFTS OB-stars
with the best proper motions, we conclude that none of them move faster than
0.3 mas yr in each coordinate -- equivalent to 70 km
s. Among the remaining 351 VFTS stars with less accurate proper motions,
only one candidate OB runaway can be identified. We rule out any OB star in our
sample moving at a tangential velocity exceeding 120 km s. The
most significant result of this study is finding 10 stars over wide range of
masses, which appear to be ejected from the massive star cluster R136 in the
tangential plane to angular distances from out to
, equivalent to 8-98 pc. The tangential velocities of these
runaways appear to be correlated with apparent magnitude, indicating a possible
dependence on the stellar mass.Comment: 45 pages (in referee format), 12 figures, 3 tables. Submitted to AJ.
Comments are welcom
Sodium Density Associates with Nighttime Systolic Blood Pressure in Young Healthy Adults
poste
Hubble Tarantula Treasury Project: Unraveling Tarantula's Web. II. Optical and Near Infrared Star Formation History of the Starburst Cluster NGC 2070 in 30 Doradus
We present a study of the recent star formation of 30 Doradus in the Large
Magellanic Cloud (LMC) using the panchromatic imaging survey Hubble Tarantula
Treasury Project (HTTP). In this paper we focus on the stars within 20 pc of
the center of the massive ionizing cluster of 30 Doradus, NGC 2070. We
recovered the star formation history by comparing deep optical and NIR
color-magnitude diagrams (CMDs) with state-of-the-art synthetic CMDs generated
with the latest PARSEC models, which include all stellar phases from pre-main
sequence to post- main sequence. For the first time in this region we are able
to measure the star formation using intermediate and low mass stars
simultaneously. Our results suggest that NGC2070 experienced a prolonged
activity. In particular, we find that the star formation in the region: i)
exceeded the average LMC rate ~ 20 Myr ago; ii) accelerated dramatically ~ 7
Myr ago; and iii) reached a peak value 1-3 Myr ago. We did not find significant
deviations from a Kroupa initial mass function down to 0.5 Msun. The average
internal reddening E(B-V) is found to be between 0.3 and 0.4 mag.Comment: Submitted to Ap
A Period and a Prediction for the Of?p Spectrum Alternator HD 191612
The observational picture of the enigmatic O-type spectrum variable HD191612
has been sharpened substantially. A symmetrical, low-amplitude light curve with
a period near 540 d has recently been reported from Hipparcos photometry. This
period satisfies all of the spectroscopy since at least 1982, including
extensive new observations during 2003 and 2004, and it has predicted the next
transition during September--October 2004. Measurements of the H alpha
equivalent width reveal a sharp emission peak in the phase diagram, in contrast
to the apparently sinusoidal light curve. The He II absorption-line strength is
essentially constant, while He I varies strongly, possibly filled in by
emission in the O6 state, thus producing the apparent spectral-type variations.
The O8 state appears to be the "normal" one. Two intermediate O7 observations
have been obtained, which fall at the expected phases, but these are the only
modern observations of the transitions so far. The period is too long for
rotation or pulsation; although there is no direct evidence as yet for a
companion, a model in which tidally induced oscillations drive an enhanced wind
near periastron of an eccentric orbit appears promising. Further observations
during the now predictable transitions may provide a critical test. Ultraviolet
and X-ray observations during both states will likely also prove illuminating.Comment: 7 pages, 3 figures, 1 table; scheduled for the 2004 December 10 issue
of ApJL, Vol. 617, No. 1. ApJ
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