3,261 research outputs found
Rotation and Macroturbulence in Metal-poor Field Red Giant and Red Horizontal Branch Stars
We report the results for rotational velocities, Vrot sin i, and
macroturbulence dispersion, zeta(RT), for 12 metal-poor field red giant branch
stars and 7 metal-poor field red horizontal branch stars. The results are based
on Fourier transform analyses of absorption line profiles from high-resolution
(R ~ 120,000), high-S/N (~ 215 per pixel) spectra obtained with the Gecko
spectrograph at CFHT. We find that the zeta(RT) values for the metal-poor RGB
stars are very similar to those for metal-rich disk giants studied earlier by
Gray and his collaborators. Six of the RGB stars have small rotational values,
less than 2.0 km/sec, while five show significant rotation, over 3 km/sec. The
fraction of rapidly rotating RHB stars is somewhat lower than found among BHB
stars. We devise two empirical methods to translate the line-broadening results
obtained by Carney et al. (2003, 2008) into Vrot sin i for all the RGB and RHB
stars they studied. Binning the RGB stars by luminosity, we find that most
metal-poor field RGB stars show no detectable sign, on average, of rotation.
However, the most luminous stars, with M(V) <= -1.5, do show net rotation, with
mean values of 2 to 4 km/sec, depending on the algorithm employed, and these
stars also show signs of radial velocity jitter and mass loss.Comment: accepted for publication in the Astronomical Journa
Particle Dark Energy
We explore the physics of a gas of particles interacting with a condensate
that spontaneously breaks Lorentz invariance. The equation of state of this gas
varies from 1/3 to less than -1 and can lead to the observed cosmic
acceleration. The particles are always stable. In our particular class of
models these particles are fermions with a chiral coupling to the condensate.
They may behave as relativistic matter at early times, produce a brief period
where they dominate the expansion with w<0 today, and behave as matter at late
time. There are no small parameters in our models, which generically lead to
dark energy clustering and, depending on the choice of parameters, smoothing of
small scale power.Comment: 8 pages, 5 figures; minor update with added refs; version appearing
in Phys. Rev.
Blue Horizontal Branch Stars in Old, Metal-Rich Stellar Systems
Twenty years ago, Burstein et al. (1984)suggested that strong CN and Hbeta absorption meant younger ages among globular clusters in the Andromeda galaxy (M31), unless blue stars above the main-sequence turnoff or on the horizontal branch were uncommonly prominent. Here we test these suggestions by fitting the detailed mid-ultraviolet (2280-3120A) and optical (3850-4750A) spectra of one moderately metal-rich M31 globular cluster, G1. We explore the effects of a wide range of non-solar temperatures and abundance ratios, by combining a small set of theoretical stellar spectra like those of Peterson et al. (2001) that were calculated using extensively updated atomic-line constants. To match the mid-UV fluxes of G1, we find that hot components with Teff >= 8000K must be included. We obtain a very good fit with cool and hot blue horizontal branch (BHB) stars, but less satisfactory fits for blue straggler stars, those hotter than the main-sequence turnoff. The G1 color-magnitude diagram does show cool BHB stars, and the color of its giant branch supports the metallicity of one-sixth the solar value that we deduce. The turnoff temperature of the best-fit model is consistent with that of turnoff stars in galactic globular clusters and the field halo, indicating G1 is comparably old. Because metal-rich cool BHB and extremely blue HB stars have now been found within our own Galaxy, we suggest that these hot horizontal-branch stars be considered in fitting spectra of metal-rich populations such as the Andromeda globular clusters, to avoid possible underestimates of their ages. We plan to make the relevant spectral calculations available as part of our Hubble Treasury Program
BVRIJK light curves and radial velocity curves for selected Magellanic Cloud Cepheids
We present high precision and well sampled BVRIJK light curves and radial
velocity curves for a sample of five Cepheids in the SMC. In addition we
present radial velocity curves for three Cepheids in the LMC. The low
metallicity (Fe/H ~ -0.7) SMC stars have been selected for use in a
Baade-Wesselink type analysis to constrain the metallicity effect on the
Cepheid Period-Luminosity relation. The stars have periods of around 15 days so
they are similar to the Cepheids observed by the Extragalactic Distance Scale
Key Project on the Hubble Space Telescope. We show that the stars are
representative of the SMC Cepheid population at that period and thus will
provide a good sample for the proposed analysis. The actual Baade-Wesselink
analysis are presented in a companion paper.Comment: Accepted for publication in A&A, 23 pages, 10 figures, data tables
will be made available electronically from the CD
A Keck HIRES Doppler Search for Planets Orbiting Metal-Poor Dwarfs. II. On the Frequency of Giant Planets in the Metal-Poor Regime
We present an analysis of three years of precision radial velocity
measurements of 160 metal-poor stars observed with HIRES on the Keck 1
telescope. We report on variability and long-term velocity trends for each star
in our sample. We identify several long-term, low-amplitude radial-velocity
variables worthy of follow-up with direct imaging techniques. We place lower
limits on the detectable companion mass as a function of orbital period. Our
survey would have detected, with a 99.5% confidence level, over 95% of all
companions on low-eccentricity orbits with velocity semi-amplitude K > 100 m/s,
or M_p*sin(i) > 3.0 M_JUP*(P/yr)^(1/3), for orbital periods P< 3 yr. None of
the stars in our sample exhibits radial-velocity variations compatible with the
presence of Jovian planets with periods shorter than the survey duration. The
resulting average frequency of gas giants orbiting metal-poor dwarfs with -2.0
< [Fe/H] < -0.6 is f_p<0.67% (at the 1-sigma confidence level). We examine the
implications of this null result in the context of the observed correlation
between the rate of occurrence of giant planets and the metallicity of their
main-sequence solar-type stellar hosts. By combining our dataset with the
Fischer & Valenti (2005) uniform sample, we confirm that the likelihood of a
star to harbor a planet more massive than Jupiter within 2 AU is a steeply
rising function of the host's metallicity. However, the data for stars with
-1.0 < [Fe/H] < 0.0 are compatible, in a statistical sense, with a constant
occurrence rate f_p~1%. Our results can usefully inform theoretical studies of
the process of giant planet formation across two orders of magnitude in
metallicity.Comment: 59 pages, 7 tables, 8 figures. Accepted for publication in the
Astrophysical Journa
A Keck/HIRES Doppler Search for Planets Orbiting Metal-Poor Dwarfs. I. Testing Giant Planet Formation and Migration Scenarios
We describe a high-precision Doppler search for giant planets orbiting a
well-defined sample of metal-poor dwarfs in the field. This experiment
constitutes a fundamental test of theoretical predictions which will help
discriminate between proposed giant planet formation and migration models. We
present here details on the survey as well as an overall assessment of the
quality of our measurements, making use of the results for the stars that show
no significant velocity variation.Comment: 25 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Substance use and depressive and anxiety symptoms among out-of-school adolescent girls and young women in Cape Town, South Africa
Background. There is a high prevalence of substance use among youth in South Africa (SA), and adolescent girls and young women (AGYW) experience high rates of depression and anxiety. Substance use behaviours and mental health are associated with other public health problems among AGYW such as HIV and unintended pregnancy. Therefore, understanding the relationship between substance use and mental health is imperative to improve AGYW’s health.Objectives. To examine the association between heavy drinking, marijuana, methamphetamine and methaqualone (Mandrax) use and depressive and anxiety symptoms among AGYW aged 16 - 19 years who have dropped out of school in Cape Town, SA.Methods. Data for this report come from the baseline data of 500 participants of an ongoing cluster-randomised trial assessing the efficacy of a young woman-focused intervention to reduce substance use and HIV risk. After AGYW consented/assented to participate, they completed a urine drug screen and a baseline questionnaire.Results. Logistic and negative binomial regressions, controlling for clustering at the neighbourhood level, revealed that frequency of depressive symptoms was significantly and positively related to a positive drug screen for Mandrax (β=0.07; p=0.03). All other associations between the frequency of depressive symptoms and substance use were not statistically significant (ps>0.05). The associations between frequency of anxiety symptoms and substance use were not statistically significant (ps>0.05).Conclusions. Our findings highlight the need to address substance use, especially Mandrax use and its associated risk, and depression in an integrated, youth-friendly setting
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