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Stanimir A. Metchev
Publication date
2003
fields
Physics
and research's language is
English
Authors
Stanimir A. Metchev Caltech
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(abridged) We present new 10-micron photometry of 21 nearby young stars obtained at the Palomar 5-meter and at the Keck I 10-meter telescopes as part of a program to search for dust in the habitable zone of young stars. Thirteen of the stars are in the F-K spectral type range (solar analogs), 4 have B or A spectral types, and 4 have spectral type M. We confirm existing IRAS 12-micron and ground-based 10-micron photometry for 10 of the stars, and present new insight into this spectral regime for the rest. Excess emission at 10 micron is not found in any of the young solar analogs, except for a possible 2.4-sigma detection in the G5V star HD 88638. The G2V star HD 107146, which does not display a 10-micron excess, is identified as a new Vega-like candidate, based on our 10-micron photospheric detection, combined with previously unidentified 60-micron and 100-micron IRAS excesses. Among the early-type stars, a 10-micron excess is detected only in HD 109573A (HR 4796A), confirming prior observations; among the M dwarfs, excesses are confirmed in AA Tau, CD -40 8434, and Hen 3-600A. A previously suggested N band excess in the M3 dwarf CD -33 7795 is shown to be consistent with photospheric emission.
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Stellar rotation is a crucial parameter driving stellar magnetism, activity and mixing of chemical elements. Furthermore, the evolution of stellar rotation is coupled to the evolution of circumstellar disks. Disk-braking mechanisms are believed to be responsible for rotational deceleration during the accretion phase, and rotational spin-up during the contraction phase after decoupling from the disk for fast rotators arriving at the ZAMS. We investigate the projected rotational velocities vsini of a sample of young stars with respect to the stellar mass and disk evolutionary state to search for possible indications of disk-braking mechanisms. We analyse the stellar spectra of 220 nearby (mostly <100pc) young (2-600 Myr) stars for their vsini, stellar age, Halpha emission, and accretion rates. The stars have been observed with FEROS and HARPS in La Silla, Chile. The spectra have been cross-correlated with appropriate theoretical templates. We build a new calibration to be able to derive vsini values from the cross-correlated spectra. Stellar ages are estimated from the LiI equivalent width at 6708 Ang. The equivalent width and width at 10% height of the Halpha emission are measured to identify accretors and used to estimate accretion rates. The vsini is then analysed with respect to the evolutionary state of the circumstellar disks to search for indications of disk-braking mechanisms in accretors. We find that the broad vsini distribution of our targets extends to rotation velocities of up to more than 100 km/s and peaks at a value of 7.8+-1.2 km/s, and that ~70% of our stars show vsini<30 km/s. Furthermore, we can find indications for disk-braking in accretors and rotational spin-up of stars which are decoupled from their disks. In addition, we show that a number of young stars are suitable for precise radial-velocity measurements for planet-search surveys.
The dependence of stellar molecular bands on the metallicity is studied using infrared L-band spectra of AGB stars (both carbon-rich and oxygen-rich) and M-type supergiants in the Large and Small Magellanic Clouds (LMC and SMC) and in the Sagittarius Dwarf Spheroidal Galaxy. The spectra cover SiO bands for oxygen-rich stars, and acetylene (C2H2), CH and HCN bands for carbon-rich AGB stars. The equivalent width of acetylene is found to be high even at low metallicity. The high C2H2 abundance can be explained with a high carbon-to-oxygen (C/O) ratio for lower metallicity carbon stars. In contrast, the HCN equivalent width is low: fewer than half of the extra-galactic carbon stars show the 3.5micron HCN band, and only a few LMC stars show high HCN equivalent width. HCN abundances are limited by both nitrogen and carbon elemental abundances. The amount of synthesized nitrogen depends on the initial mass, and stars with high luminosity (i.e. high initial mass) could have a high HCN abundance. CH bands are found in both the extra-galactic and Galactic carbon stars. None of the oxygen-rich LMC stars show SiO bands, except one possible detection in a low quality spectrum. The limits on the equivalent widths of the SiO bands are below the expectation of up to 30angstrom for LMC metallicity. Several possible explanations are discussed. The observations imply that LMC and SMC carbon stars could reach mass-loss rates as high as their Galactic counterparts, because there are more carbon atoms available and more carbonaceous dust can be formed. On the other hand, the lack of SiO suggests less dust and lower mass-loss rates in low-metallicity oxygen-rich stars. The effect on the ISM dust enrichment is discussed.
Small amounts of star formation in elliptical galaxies are suggested by several results: surprisingly young ages from optical line indices, cooling X-ray gas, and mid-IR dust emission. Such star formation has previously been difficult to directly detect, but using UV Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) imaging, we have identified individual young stars and star clusters in four nearby ellipticals. This technique is orders of magnitude more sensitive than other methods, allowing detections of star formation to 10^(-5) Msun/yr. Ongoing star formation is detected in all galaxies, including three ellipticals that have previously exhibited potential signposts of star forming conditions (NGC 4636, NGC 4697, and NGC 4374), as well as the typical red and dead NGC 3379. The current star formation in our closest targets, where we are most complete, is between 1-8x10^(-5) Msun/yr. The star formation history was roughly constant from 0.5-1.5 Gyr (at 3-5x10^(-4) Msun/yr), but decreased by a factor of several in the past 0.3 Gyr. Most star clusters have a mass between 10^2 - 10^4 Msun. The specific star formation rates of ~10^(-16) yr^(-1) (at the present day) or ~10^(-14) yr^(-1) (when averaging over the past Gyr) imply that a fraction 10^(-8) of the stellar mass is younger than 100 Myr and 10^(-5) is younger than 1 Gyr, quantifying the level of frosting of recent star formation over the otherwise passive stellar population. There is no obvious correlation between either the presence or spatial distribution of postulated star formation indicators and the star formation we detect.
Runaway stars are characterised by their remarkably high space velocities, and the study of their formation mechanisms has attracted considerable interest. Young, nearby runaway stars are the most favorable for identifying their place of origin, and for searching for possible associated objects such as neutron stars. Usually the research field of runaway stars focuses on O- and B-type stars, because these objects are better detectable at larger distances than late-type stars. Early-type runaway stars have the advantage, that they evolve faster and can therefore better be confirmed to be young. In contrast to this, the catalogue of young runaway stars within 3 kpc by Tetzlaff, Neuhauser, & Hohle (2011) contains also stars of spectral type A and later. The objects in this catalogue were originally classified as young ($le 50$ Myr) runaway stars by using Hipparcos data to estimate the ages from their location in the Hertzsprung-Russell diagram and evolutionary models. In this article, we redetermine and/or constrain their ages not only by using the more precise second data release of the Gaia mission, but also by measuring the equivalent width of the lithium (6708 $unicode{xC5}$) line, which is a youth indicator. Therefore, we searched for lithium absorption in the spectra of 51 target stars, taken at the University Observatory Jena between March and September 2020 with the Echelle spectrograph FLECHAS, and within additional TRES-spectra from the Fred L. Whipple Observatory. The main part of this campaign with its 308 reduced spectra, accessible at VizieR, was already published. In this work, which is the continuation and completion of the in 2015 initiated observing campaign, we found three additional young runaway star candidates.
Young nearby stars are good candidates in the search for planets with both radial velocity (RV) and direct imaging techniques. This, in turn, allows for the computation of the giant planet occurrence rates at all separations. The RV search around young stars is a challenge as they are generally faster rotators than older stars of similar spectral types and they exhibit signatures of magnetic activity (spots) or pulsation in their RV time series. Specific analyses are necessary to characterize, and possibly correct for, this activity. Our aim is to search for planets around young nearby stars and to estimate the giant planet (GP) occurrence rates for periods up to 1000 days. We used the HARPS spectrograph on the 3.6m telescope at La Silla Observatory to observe 89 A-M young (< 600 Myr) stars. We used our SAFIR (Spectroscopic data via Analysis of the Fourier Interspectrum Radial velocities ) software to compute the RV and other spectroscopic observables. Then, we computed the companion occurrence rates on this sample. We confirm the binary nature of HD177171, HD181321 and HD186704. We report the detection of a close low mass stellar companion for HIP36985. No planetary companion was detected. We obtain upper limits on the GP (< 13 MJup) and BD (13-80 MJup) occurrence rates based on 83 young stars for periods less than 1000 days, which are set, 2_-2^+3 % and 1_-1^+3 %.
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