Convective line shifts for the Gaia RVS from the CIFIST 3D model atmosphere grid

To derive space velocities of stars along the line of sight from wavelength shifts in stellar spectra requires accounting for a number of second-order effects. For most stars, gravitational redshifts, convective blueshifts, and transverse stellar motion are the dominant contributors. We provide theoretical corrections for the net velocity shifts due to convection expected for the measurements from the Gaia Radial Velocity Spectrometer (RVS). We used a set of three-dimensional time-dependent simulations of stellar surface convection computed with CO5BOLD to calculate spectra of late-type stars in the Gaia RVS range and to infer the net velocity offset that convective motions will induce in radial velocities derived by cross-correlation. The net velocity shifts derived by cross-correlation depend both on the wavelength range and spectral resolution of the observations. Convective shifts for Gaia RVS observations are less than 0.1 km/s for late-K-type stars, and they increase with stellar mass, reaching about 0.3 km/s or more for early F-type dwarfs. This tendency is the result of an increase with effective temperature in both temperature and velocity fluctuations in the line-forming region. Our simulations also indicate that the net RVS convective shifts can be positive (i.e. redshifts) in some cases. Overall, the blueshifts weaken slightly with increasing surface gravity, and are enhanced at low metallicity. Gravitational redshifts amount up to 0.7 km/s and dominate convective blueshifts for dwarfs, but become much weaker for giants.Comment: 13 pages, to appear in A&A; model fluxes available from ftp://leda.as.utexas.edu/pub/callende/Gaia3D and soon from CD

Velocities from Cross-Correlation: A Guide for Self-Improvement

The measurement of Doppler velocity shifts in spectra is a ubiquitous theme in astronomy, usually handled by computing the cross-correlation of the signals, and finding the location of its maximum. This paper addresses the problem of the determination of wavelength or velocity shifts among multiple spectra of the same, or very similar, objects. We implement the classical cross-correlation method and experiment with several simple models to determine the location of the maximum of the cross-correlation function. We propose a new technique, 'self-improvement', to refine the derived solutions by requiring that the relative velocity for any given pair of spectra is consistent with all others. By exploiting all available information, spectroscopic surveys involving large numbers of similar objects may improve their precision significantly. As an example, we simulate the analysis of a survey of G-type stars with the SDSS instrumentation. Applying 'self-improvement' refines relative radial velocities by more than 50% at low signal-to-noise ratio. The concept is equally applicable to the problem of combining a series of spectroscopic observations of the same object, each with a different Doppler velocity or instrument-related offset, into a single spectrum with an enhanced signal-to-noise ratio.Comment: 7 pages, 3 figures, uses emulateapj.cls; to appear in the Astronomical Journal; see http://hebe.as.utexas.edu/stools/ to obtain the companion softwar

Searching for Planets in the Hyades V: Limits on Planet Detection in the Presence of Stellar Activity

We present the results of a radial velocity survey of a sample of Hyades stars, and discuss the effects of stellar activity on radial velocity measurements. The level of radial velocity scatter due to rotational modulation of stellar surface features for the Hyades is in agreement with the predictions of Saar & Donahue (1997)- the maximum radial velocity rms of up to ~50 m/s, with an average rms of ~16 m/s. In this sample of 94 stars, we find 1 new binary, 2 stars with linear trends indicative of binary companions, and no close-in giant planets. We discuss the limits on extrasolar planet detection in the Hyades and the constraints imposed on radial velocity surveys of young stars.Comment: To appear in the June 2004 issue of A

Detailed analysis of Balmer lines in cool dwarf stars

An analysis of H alpha and H beta spectra in a sample of 30 cool dwarf and subgiant stars is presented using MARCS model atmospheres based on the most recent calculations of the line opacities. A detailed quantitative comparison of the solar flux spectra with model spectra shows that Balmer line profile shapes, and therefore the temperature structure in the line formation region, are best represented under the mixing length theory by any combination of a low mixing-length parameter alpha and a low convective structure parameter y. A slightly lower effective temperature is obtained for the sun than the accepted value, which we attribute to errors in models and line opacities. The programme stars span temperatures from 4800 to 7100 K and include a small number of population II stars. Effective temperatures have been derived using a quantitative fitting method with a detailed error analysis. Our temperatures find good agreement with those from the Infrared Flux Method (IRFM) near solar metallicity but show differences at low metallicity where the two available IRFM determinations themselves are in disagreement. Comparison with recent temperature determinations using Balmer lines by Fuhrmann (1998, 2000), who employed a different description of the wing absorption due to self-broadening, does not show the large differences predicted by Barklem et al. (2000). In fact, perhaps fortuitously, reasonable agreement is found near solar metallicity, while we find significantly cooler temperatures for low metallicity stars of around solar temperature.Comment: 17 pages, 9 figures, to appear in A&

The puzzling interpretation of NIR indices: The case of NaI2.21

We present a detailed study of the Na I line strength index centered in the $K$-band at $22100$, {\AA} (NaI2.21 hereafter) relying on different samples of early-type galaxies. Consistent with previous studies, we find that the observed line strength indices cannot be fit by state-of-art scaled-solar stellar population models, even using our newly developed models in the NIR. The models clearly underestimate the large NaI2.21 values measured for most early-type galaxies. However, we develop a Na-enhanced version of our newly developed models in the NIR, which - together with the effect of a bottom-heavy initial mass function - yield NaI2.21 indices in the range of the observations. Therefore, we suggest a scenario in which the combined effect of [Na/Fe] enhancement and a bottom-heavy initial mass function are mainly responsible for the large NaI2.21 indices observed for most early-type galaxies. To a smaller extent, also [C/Fe] enhancement might contribute to the large observed NaI2.21 values.Comment: 13 pages, 4 figures, accepted for publication in MNRA

HS 1857+5144 : a hot and young pre-cataclysmic variable

Aims. We report the discovery of a new white dwarf/M dwarf binary, HS 1857+5144, identified in the Hamburg Quasar Survey (HQS). Methods. Time-resolved optical spectroscopy and photometry were carried out to determine the properties of this new cataclysmic variable progenitor (pre-CV). Results. The light curves of HS 1857+5144 display a sinusoidal variation with a period of Porb = 383.52 min and peak-to-peak amplitudes of 0.7 mag and 1.1mag in the B-band and R-band, respectively. The large amplitude of the brightness variation results from a reflection effect on the heated inner hemisphere of the companion star, suggesting a very high temperature of the white dwarf. Our radial velocity study confirms the photometric period as the orbital period of the system. A model atmosphere fit to the spectrum of the white dwarf obtained at minimum light provides limits to its mass and temperature of Mwd 0.6−1.0 M and Twd 70 000−100 000 K, respectively. The detection of He II λ4686 absorption classifies the primary star of HS 1857+5144 as a DAO white dwarf. Combining the results from our spectroscopy and photometry, we estimate the mass of the companion star and the binary inclination to be Msec 0.15−0.30 M and i 45◦−55◦, respectively. Conclusions. We classify HS 1857+5144 as one of the youngest pre-CV known to date. The cooling age of the white dwarf suggests that the present system has just emerged from a common envelope phase ∼105 yr ago. HS 1857+5144 will start mass transfer within or below the 2–3 h period gap

IMF and [Na/Fe] abundance ratios from optical and NIR Spectral Features in Early-type Galaxies

We present a joint analysis of the four most prominent sodium-sensitive features (NaD, NaI8190, NaI1.14, and NaI2.21), in the optical and Near-Infrared spectral range, of two nearby, massive (sigma~300km/s), early-type galaxies (named XSG1 and XSG2). Our analysis relies on deep VLT/X-Shooter long-slit spectra, along with newly developed stellar population models, allowing for [Na/Fe] variations, up to 1.2dex, over a wide range of age, total metallicity, and IMF slope. The new models show that the response of the Na-dependent spectral indices to [Na/Fe] is stronger when the IMF is bottom heavier. For the first time, we are able to match all four Na features in the central regions of massive early-type galaxies, finding an overabundance of [Na/Fe], in the range 0.5-0.7dex, and a bottom-heavy IMF. Therefore, individual abundance variations cannot be fully responsible for the trends of gravity-sensitive indices, strengthening the case towards a non-universal IMF. Given current limitations of theoretical atmosphere models, our [Na/Fe] estimates should be taken as upper limits. For XSG1, where line strengths are measured out to 0.8Re, the radial trend of [Na/Fe] is similar to [Mg/Fe] and [C/Fe], being constant out to 0.5Re, and decreasing by 0.2-0.3dex at 0.8Re, without any clear correlation with local metallicity. Such a result seems to be in contrast with the predicted increase of Na nucleosynthetic yields from AGB stars and TypeII SNe. For XSG1, the Na-inferred IMF radial profile is consistent, within the errors, with that derived from TiO features and the Wing-Ford band, presented in a recent paper.Comment: 22 pages, 8 figure, accepted for publication in MNRAS. The new Na-enhanced models will be available soon at http://miles.iac.es

The Observed Trend of Boron and Oxygen in Field Stars of the Disk

Oxygen abundances are derived in a sample of 13 field F and G dwarfs and subgiants with metallicities in the range of -0.75 < [Fe/H] < +0.15. This is the same sample of stars for which boron abundances have been derived earlier from archived spectra obtained with the Hubble Space Telescope. In a log-log comparison of the B versus the O abundances, a slope of m(BO)=1.39 is found, indicating that in the disk, the abundance of B relative to O is intermediate between primary and secondary production (hybrid behavior). This relation of B versus O for disk stars is compared to the same relation for halo stars.Comment: 14 pages, 3 figures, 1 table. In press to The Astronomical Journal (July 2001

Surface abundances of light elements for a large sample of early B-type stars - IV. The magnesium abundance in 52 stars - a test of metallicity

From high-resolution spectra a non-LTE analysis of the MgII 4481.2 A feature is implemented for 52 early and medium local B stars on the main sequence (MS). The influence of the neighbouring line AlIII 4479.9 A is considered. The magnesium abundance is determined; it is found that log e(Mg) = 7.67 +- 0.21 on average. It is shown that uncertainties in the microturbulent parameter Vt are the main source of errors in log e(Mg). When using 36 stars with the most reliable Vt values derived from OII and NII lines, we obtain the mean abundance log e(Mg) = 7.59 +- 0.15. The latter value is precisely confirmed for several hot B stars from an analysis of the MgII 7877 A weak line. The derived abundance log e(Mg) = 7.59 +- 0.15 is in excellent agreement with the solar magnesium abundance log e_sun(Mg) = 7.55 +- 0.02, as well as with the proto-Sun abundance log e_ps(Mg) = 7.62 +- 0.02. Thus, it is confirmed that the Sun and the B-type MS stars in our neighbourhood have the same metallicity.Comment: 9 pages, 6 figures. Has been accepted for publication at MNRA