- ID:
- ivo://CDS.VizieR/J/ApJ/736/146
- Title:
- Radial velocities of stars in Bootes I
- Short Name:
- J/ApJ/736/146
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We develop, implement, and characterize an enhanced data reduction approach which delivers precise, accurate, radial velocities from moderate resolution spectroscopy with the fiber-fed VLT/FLAMES+GIRAFFE facility. This facility, with appropriate care, delivers radial velocities adequate to resolve the intrinsic velocity dispersions of the very faint dwarf spheroidal (dSph) galaxies. Importantly, repeated measurements let us reliably calibrate our individual velocity errors (0.2km/s<={delta}_V_<=5km/s) and directly detect stars with variable radial velocities. We show, by application to the Bootes I dSph, that the intrinsic velocity dispersion of this system is significantly below 6.5km/s reported by previous studies. Our data favor a two-population model of Bootes I, consisting of a majority "cold" stellar component, with velocity dispersion 2.4^+0.9^_-0.5_km/s, and a minority "hot" stellar component, with velocity dispersion ~9km/s, although we cannot completely rule out a single component distribution with velocity dispersion 4.6^0.8^_-0.6_km/s. We speculate that this complex velocity distribution actually reflects the distribution of velocity anisotropy in Bootes I, which is a measure of its formation processes.
Number of results to display per page
Search Results
- ID:
- ivo://CDS.VizieR/J/ApJS/244/27
- Title:
- Radial velocity measurements in LAMOST-II
- Short Name:
- J/ApJS/244/27
- Date:
- 09 Dec 2021
- Publisher:
- CDS
- Description:
- The radial velocity (RV) is a basic physical quantity that can be determined through the Doppler shift of the spectrum of a star. The precision of the RV measurement depends on the resolution of the spectrum we used and the accuracy of wavelength calibration. In this work, radial velocities of the Large Sky Area Multi-Object Fibre Spectroscopic Telescope-II (LAMOST-II) medium-resolution (R~7500) spectra are measured for 1,594,956 spectra (each spectrum has two wavebands) through matching with templates. A set of RV standard stars are used to recalibrate the zero point of the measurement, and some reference sets with RVs derived from medium-/high-resolution observations are used to evaluate the accuracy of the measurement. By comparing with reference sets, the accuracy of our measurement can get 0.0277km/s with respect to radial velocities of standard stars. The intrinsic precision is estimated with the multiple observations of single stars, which can be achieved to 1.36km/s, 1.08km/s, and 0.91km/s for the spectra at signal-to-noise levels of 10, 20, and 50, respectively.
- ID:
- ivo://CDS.VizieR/J/ApJ/860/1
- Title:
- Radial velocity measurements of 20 EBs in LMC
- Short Name:
- J/ApJ/860/1
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a determination of the precise fundamental physical parameters of 20 detached, double-lined, eclipsing binary stars in the Large Magellanic Cloud (LMC) containing G- or early K-type giant stars. Eleven are new systems; the remaining nine are systems already analyzed by our team for which we present updated parameters. The catalog results from our long-term survey of eclipsing binaries in the Magellanic Clouds suitable for high-precision determination of distances (the Araucaria Project). The V-band brightnesses of the systems range from 15.4 to 17.7mag, and their orbital periods range from 49 to 773days. Six systems have favorable geometry showing total eclipses. The absolute dimensions of all eclipsing binary components are calculated with a precision of better than 3%, and all systems are suitable for a precise distance determination. The measured stellar masses are in the range 1.4 to 4.6M_{sun}_, and comparison with the MESA isochrones gives ages between 0.1 and 2.1Gyr. The systems show an age-metallicity relation with no evolution of metallicity for systems older than 0.6Gyr, followed by a rise to a metallicity maximum at age 0.5Gyr and then a slow metallicity decrease until 0.1Gyr. Two systems have components with very different masses: OGLE LMC-ECL-05430 and OGLE LMC-ECL-18365. Neither system can be fitted by a single stellar evolution isochrone, explained by a past mass transfer scenario in the case of ECL-18365 and a gravitational capture or hierarchical binary merger scenario in the case of ECL-05430. The longest-period system, OGLE LMC SC9_230659, shows a surprising apsidal motion that shifts the apparent position of the eclipses. This is a clear sign of a physical companion to the system; however, neither investigation of the spectra nor light-curve analysis indicates a third-light contribution larger than 2%-3%. In one spectrum of OGLE LMC-ECL-12669, we noted a peculiar dimming of one of the components by 65% well outside of the eclipses. We interpret this observation as arising from an extremely rare occultation event, as a foreground Galactic object covers only one component of an extragalactic eclipsing binary.
- ID:
- ivo://CDS.VizieR/J/AJ/157/63
- Title:
- Radius relations for low-metallicity M-dwarf stars
- Short Name:
- J/AJ/157/63
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- M subdwarfs are low-metallicity M dwarfs that typically inhabit the halo population of the Galaxy. Metallicity controls the opacity of stellar atmospheres; in metal-poor stars, hydrostatic equilibrium is reached at a smaller radius, leading to smaller radii for a given effective temperature. We compile a sample of 88 stars that span spectral classes K7 to M6 and include stars with metallicity classes from solar-metallicity dwarf stars to the lowest metallicity ultra subdwarfs to test how metallicity changes the stellar radius. We fit models to Palomar Double Spectrograph (DBSP) optical spectra to derive effective temperatures (T_eff_) and we measure bolometric luminosities (L_bol_) by combining broad wavelength-coverage photometry with Gaia parallaxes. Radii are then computed by combining the T_eff_ and L_bol_ using the Stefan-Boltzman law. We find that for a given temperature, ultra subdwarfs can be as much as five times smaller than their solar-metallicity counterparts. We present color-radius and color-surface brightness relations that extend down to [Fe/H] of -2.0 dex, in order to aid the radius determination of M subdwarfs, which will be especially important for the WFIRST exoplanetary microlensing survey.
- ID:
- ivo://CDS.VizieR/J/A+A/440/901
- Title:
- Reddening and metallicity of NGC 6752
- Short Name:
- J/A+A/440/901
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Accurate reddenings for Globular Clusters could be obtained by comparing the colour-temperature obtained using temperatures from reddening-free indicator (H{alpha}), with that given by standard colour-temperature calibrations. The large multiplexing opportunity offered by FLAMES at VLT2 allowed us to obtain spectra centered on H{alpha} at a resolution of R=6000 and 5<S/N<50 for 120 stars near the turn-off of NGC 6752 with GIRAFFE from a single 1300 seconds exposure on June 24th, 2004.
- ID:
- ivo://CDS.VizieR/J/ApJ/870/115
- Title:
- Reddening, distance modulus & Fe/H of RRLs in w Cen
- Short Name:
- J/ApJ/870/115
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We developed a new approach to provide accurate estimates of the metal content, reddening, and true distance modulus of RR Lyrae stars (RRLs). The method is based on homogeneous optical (BVI) and near-infrared (JHK) mean magnitudes and on predicted period-luminosity-metallicity relations (IJHK) and absolute mean magnitude-metallicity relations (BV). We obtained solutions for three different RRL samples in {omega}Cen: first overtone (RRc, 90), fundamental (RRab, 80), and global (RRc+RRab) in which the period of first overtones were fundamentalized. The metallicity distribution shows a well defined peak at [Fe/H]~-1.98 and a standard deviation of {sigma}=0.54dex. The spread is, as expected, metal-poor ([Fe/H]<=-2.3) objects. The current metallicity distribution is ~0.3dex more metal-poor than similar estimates for RRLs available in the literature. The difference vanishes if the true distance modulus we estimated is offset by -0.06/-0.07mag in true distance modulus. We also found a cluster true distance modulus of {mu}=13.720{+/-}0.002{+/-}0.030mag, where the former error is the error on the mean and the latter is the standard deviation. Moreover, we found a cluster reddening of E(B-V)=0.132{+/-}0.002{+/-}0.028mag and spatial variations of the order of a few arcmin across the body of the cluster. Both the true distance modulus and the reddening are slightly larger than similar estimates available in the literature, but the difference is within 1{sigma}. The metallicity dependence of distance diagnostics agrees with theory and observations, but firm constraints require accurate and homogeneous spectroscopic measurements.
- ID:
- ivo://CDS.VizieR/J/AJ/153/261
- Title:
- Red giant stellar parameters in the LMC bar
- Short Name:
- J/AJ/153/261
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report new spectroscopic observations obtained with the Michigan/Magellan Fiber System of 308 red giants (RGs) located in two fields near the photometric center of the bar of the Large Magellanic Cloud. This sample consists of 131 stars observed in previous studies (in one field) and 177 newly observed stars (in the second field) selected specifically to more reliably establish the metallicity and age distributions of the bar. For each star, we measure its heliocentric line-of-sight velocity, surface gravity, and metallicity from its high-resolution spectrum (effective temperatures come from photometric colors). The spectroscopic Hertzsprung-Russell diagrams- modulo small offsets in surface gravities-reveal good agreement with model isochrones. The mean metallicity of the 177-RG sample is [Fe/H]=-0.76+/-0.02 with a metallicity dispersion {sigma}=0.28+/-0.03. The corresponding metallicity distribution-corrected for selection effects-is well fitted by two Gaussian components: one metal-rich with a mean -0.66+/-0.02 and a standard deviation 0.17+/-0.01, and the other metal-poor with -1.20+/-0.24 and 0.41+/-0.06. The metal-rich and metal-poor populations contain approximately 85% and 15% of stars, respectively. We also confirm that the velocity dispersion in the bar center decreases significantly from 31.2+/-4.3 to 18.7+/-1.9km/s with increasing metallicity over the range -2.09 to -0.38. Individual stellar masses are estimated using the spectroscopic surface gravities and the known luminosities. We find that lower mass, hence older, RGs have larger metallicity dispersion and lower mean metallicity than the higher-mass, younger RGs. The estimated masses, however, extend to implausibly low values (~0.1M_{sun}_), making it impossible to obtain an absolute age-metallicity or age distribution of the bar.
- ID:
- ivo://CDS.VizieR/J/A+A/328/349
- Title:
- Role of Convection in A, F, and G stars
- Short Name:
- J/A+A/328/349
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We discuss the effects of convection on the theoretical uvby colours of A, F, and G stars. The standard mixing-length theory atlas9 models of Kurucz (1993, ATLAS9, SAO, Cambridge, USA), with and without approximate overshooting, are compared to models using the turbulent convection theory proposed by Canuto & Mazzitelli (1991ApJ...370..295C, 1992ApJ...389..724C) and implemented by Kupka (1996, ASPConf. Proc. 108, 73). Comparison with fundamental Teff and log g stars reveals that the Canuto & Mazzitelli models give results that are generally superior to standard mixing-length theory (MLT) without convective overshooting. MLT models with overshooting are found to be clearly discrepant. This is supported by comparisons of non-fundamental stars, with Teff obtained from the Infrared Flux Method and log g from stellar evolutionary models for open cluster stars. The Canuto & Mazzitelli theory gives values of (b-y)0 and c0 that are in best overall agreement with observations. Investigations of the m0 index reveal that all of the treatments of convection presented here give values that are significantly discrepant for models with Teff<6000K. It is unclear as to whether this is due to problems with the treatment of convection, missing opacity, or some other reason. None of the models give totally satisfactory m0 indices for hotter stars, but the Canuto & Mazzitelli models are in closest overall agreement above 7000K. Grids of uvby colours, based on the CM treatment of convection, are presented. These grids represent an improvement over the colours obtained from models using the mixing-length theory. The agreement with fundamental stars enables the colours to be used directly without the need for semi-empirical adjustments that were necessary with the earlier colour grids. For a description of the uvby photometric system, see e.g. <GCPD/04>
- ID:
- ivo://CDS.VizieR/J/A+A/634/L9
- Title:
- Rotation periods of 97 solar-like stars
- Short Name:
- J/A+A/634/L9
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The accurate determination of stellar rotation periods is important for estimating stellar ages and for understanding stellar activity and evolution. While rotation periods can be determined for about thirty thousand stars in the Kepler field, there are over one hundred thousand stars, especially with low photometric variability and irregular pattern of variations, for which rotational periods are unknown. Here we investigate the effect of metallicity on the detectability of rotation periods. This is done by synthesising light curves of hypothetical stars that are identical to our Sun with the exception of the metallicity. These light curves are then used as an input to the period determination algorithms. We find that the success rate for recovering the rotation signal has a minimum close to the solar metallicity value. This can be explained by the compensation effect of facular and spot contributions. In addition, selecting solar-like stars with near-solar effective temperature and photometric variability, and with metallicity between M/H=-0.35 and M/H=0.35 from the Kepler sample, we analyse the fraction of stars for which rotational periods have been detected as a function of metallicity. In agreement with our theoretical estimate we find a local minimum for the detection fraction close to the solar metallicity. We further report rotation periods of 87 solar-like Kepler stars for the first time.
- ID:
- ivo://CDS.VizieR/J/AJ/152/170
- Title:
- RRLs in globulars. IV. UBVRI photometry in Omega Cen
- Short Name:
- J/AJ/152/170
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- New accurate and homogeneous optical UBVRI photometry has been obtained for variable stars in the Galactic globular cluster {omega} Cen (NGC 5139). We secured 8202 CCD images covering a time interval of 24 years and a sky area of 84x48arcmin. The current data were complemented with data available in the literature and provided new, homogeneous pulsation parameters (mean magnitudes, luminosity amplitudes, periods) for 187 candidate {omega} Cen RR Lyrae (RRLs). Among them we have 101 RRc (first overtone) and 85 RRab (fundamental) variables, and a single candidate RRd (double-mode) variable. Candidate Blazhko RRLs show periods and colors that are intermediate between the RRc and RRab variables, suggesting that they are transitional objects. A comparison of the period distribution and the Bailey diagram indicates that RRLs in {omega} Cen show a long-period tail not present in typical Oosterhoff II (OoII) globulars. The RRLs in dwarf spheroidals and in ultra-faint dwarfs have properties between Oosterhoff intermediate and OoII clusters. Metallicity plays a key role in shaping the above evidence. These findings do not support the hypothesis that {omega} Cen is the core remnant of a spoiled dwarf galaxy. Using optical period-Wesenheit relations that are reddening-free and minimally dependent on metallicity we find a mean distance to {omega} Cen of 13.71+/-0.08+/-0.01mag (semi-empirical and theoretical calibrations). Finally, we invert the I-band period-luminosity-metallicity relation to estimate individual RRLs' metal abundances. The metallicity distribution agrees quite well with spectroscopic and photometric metallicity estimates available in the literature.