Simple relations for deriving the oxygen abundance in H ii
regions with intensities of the three strong emission lines R
2, R
3, and N
2 (R calibration) or S
2, R
3, and N
2 (S calibration) in their ...spectra are suggested. A sample of 313 reference H ii regions of the counterpart method (C method) is used as calibrating data points. Relations for the determination of nitrogen abundances, the R calibration, are also constructed. We find that the oxygen and nitrogen abundances in high-metallicity H ii regions can be estimated using the intensities of the two strong lines R
2 and N
2 (or S
2 and N
2 for oxygen) only. The corresponding two-dimensional relations are provided. There are considerable advantages of the suggested calibration relations as compared to the existing ones. First, the oxygen and nitrogen abundances estimated through the suggested calibrations agree with the T
e
-based abundances within ∼0.1 dex over the whole metallicity range, i.e. the relative accuracy of the calibration-based abundances is 0.1 dex. Although we constructed distinct relations for high- and low-metallicity objects, the separation between these two can be simply obtained from the intensity of the N
2 line. Moreover, the applicability ranges of the high- and low-metallicity relations overlap for adjacent metallicities, i.e. the transition zone disappears. Secondly, the oxygen abundances produced by the two suggested calibrations are in remarkable agreement with each other. In fact, the R-based and S-based oxygen abundances agree within ∼0.05 dex in the majority of cases for more than three thousand H ii region spectra.
Context. Ongoing and future massive spectroscopic surveys will collect large numbers (106–107) of stellar spectra that need to be analyzed. Highly automated software is needed to derive stellar ...parameters and chemical abundances from these spectra. Aims. We developed a new method of estimating the stellar parameters Teff, log g, M/H, and elemental abundances. This method was implemented in a new code, SP_Ace (Stellar Parameters And Chemical abundances Estimator). This is a highly automated code suitable for analyzing the spectra of large spectroscopic surveys with low or medium spectral resolution (R = 2000–20 000). Methods. After the astrophysical calibration of the oscillator strengths of 4643 absorption lines covering the wavelength ranges 5212–6860 Å and 8400–8924 Å, we constructed a library that contains the equivalent widths (EW) of these lines for a grid of stellar parameters. The EWs of each line are fit by a polynomial function that describes the EW of the line as a function of the stellar parameters. The coefficients of these polynomial functions are stored in a library called the “GCOG library”. SP_Ace, a code written in FORTRAN95, uses the GCOG library to compute the EWs of the lines, constructs models of spectra as a function of the stellar parameters and abundances, and searches for the model that minimizes the χ2 deviation when compared to the observed spectrum. The code has been tested on synthetic and real spectra for a wide range of signal-to-noise and spectral resolutions. Results. SP_Ace derives stellar parameters such as Teff, log g, M/H, and chemical abundances of up to ten elements for low to medium resolution spectra of FGK-type stars with precision comparable to the one usually obtained with spectra of higher resolution. Systematic errors in stellar parameters and chemical abundances are presented and identified with tests on synthetic and real spectra. Stochastic errors are automatically estimated by the code for all the parameters. A simple Web front end of SP_Ace can be found at http://dc.g-vo.org/SP_ACE, while the source code will be published soon.
Aims. We present empirical color transformations between the Sloan Digital Sky Survey (SDSS) ugriz photometry and the Johnson-Cousins UBVRI system and Beckers RGU system, respectively. Owing to the ...magnitude of data that is becoming available in the SDSS photometric system it is particularly important to be able to convert between this new system and traditional photometric systems. Unlike earlier published transformations we based our calculations on stars actually measured by the SDSS with the SDSS 2.5-m telescope. The photometric database of the SDSS provides in a sense a single- epoch set of "tertiary standards" covering more than one quarter of the sky. Our transformations should facilitate their use to easily and reliably derive the corresponding approximate Johnson-Cousins or RGU magnitudes. Methods. The SDSS survey covers a number of areas that were previously established as standard fields in the Johnson-Cousins system, in particular, fields established by Landolt and by Stetson. We used these overlapping fields to create well-photometered star samples on which our calculated transformations are based. For the RGU photometry we used fields observed in the framework of the new Basel high-latitude field star survey. Results. We calculated empirical color transformations between SDSS photometry and Johnson-Cousins UBVRI and Beckers RGU system. For all transformations we found linear relations to be sufficient. Furthermore we showed that the transformations between the Johnson-Cousins and the SDSS system have a slight dependence on metallicity.
We present a study of the hierarchical clustering of the young stellar clusters in six local (3-15 Mpc) star-forming galaxies using Hubble Space Telescope broadband WFC3/UVIS UV and optical images ...from the Treasury Program LEGUS (Legacy ExtraGalactic UV Survey). We identified 3685 likely clusters and associations, each visually classified by their morphology, and we use the angular two-point correlation function to study the clustering of these stellar systems. We find that the spatial distribution of the young clusters and associations are clustered with respect to each other, forming large, unbound hierarchical star-forming complexes that are in general very young. The strength of the clustering decreases with increasing age of the star clusters and stellar associations, becoming more homogeneously distributed after ∼40-60 Myr and on scales larger than a few hundred parsecs. In all galaxies, the associations exhibit a global behavior that is distinct and more strongly correlated from compact clusters. Thus, populations of clusters are more evolved than associations in terms of their spatial distribution, traveling significantly from their birth site within a few tens of Myr, whereas associations show evidence of disruption occurring very quickly after their formation. The clustering of the stellar systems resembles that of a turbulent interstellar medium that drives the star formation process, correlating the components in unbound star-forming complexes in a hierarchical manner, dispersing shortly after formation, suggestive of a single, continuous mode of star formation across all galaxies.
We present evidence for the contribution of high-mass globular clusters to the stellar halo of the Galaxy. Using SDSS-II/SEGUE spectra of over 1900 G- and K-type halo giants, we identify for the ...first time a subset of stars with CN bandstrengths significantly larger, and CH bandstrengths lower, than the majority of halo field stars, at fixed temperature and metallicity. Since CN bandstrength inhomogeneity and the usual attendant abundance variations are presently understood as a result of star formation in globular clusters, we interpret this subset of halo giants as a result of globular cluster dissolution into the Galactic halo. We find that 2.5% of our sample is CN-strong, and can infer based on recent models of globular cluster evolution that the fraction of halo field stars initially formed within globular clusters may be as large as $50\%$.
Abstract
Using the Radial Velocity Experiment (RAVE) survey, we recently brought to light a gradient in the mean galactocentric radial velocity of stars in the extended solar neighbourhood. This ...gradient likely originates from non-axisymmetric perturbations of the potential, among which a perturbation by spiral arms is a possible explanation. Here, we apply the traditional density wave theory and analytically model the radial component of the two-dimensional velocity field. Provided that the radial velocity gradient is caused by relatively long-lived spiral arms that can affect stars substantially above the plane, this analytic model provides new independent estimates for the parameters of the Milky Way spiral structure. Our analysis favours a two-armed perturbation with the Sun close to the inner ultra-harmonic 4:1 resonance, with a pattern speed and a small amplitude per cent of the background potential (14 per cent of the background density). This model can serve as a basis for numerical simulations in three dimensions, additionally including a possible influence of the Galactic bar and/or other non-axisymmetric modes.
We use data from the Radial Velocity Experiment (RAVE) and the Tycho-Gaia astrometric solution (TGAS) catalogue to compute the velocity fields yielded by the radial (V-R), azimuthal (V-phi), and ...vertical (V-z) components of associated Galactocentric velocity. We search in particular for variation in all three velocity components with distance above and below the disc midplane, as well as how each component of V-z (line-of-sight and tangential velocity projections) modifies the obtained vertical structure. To study the dependence of velocity on proper motion and distance, we use two main samples: a RAVE sample including proper motions from the Tycho-2, PPMXL, and UCAC4 catalogues, and a RAVE-TGAS sample with inferred distances and proper motions from the TGAS and UCAC5 catalogues. In both samples, we identify asymmetries in V-R and V-z. Below the plane, we find the largest radial gradient to be partial derivative V-R/partial derivative R = -7.01 +/- 0.61 km s(-1) kpc(-1), in agreement with recent studies. Above the plane, we find a similar gradient with partial derivative V-R/partial derivative R = -9.42 +/- 1.77 km s(-1) kpc(-1). By comparing our results with previous studies, we find that the structure in V-z is strongly dependent on the adopted proper motions. Using the Galaxia Milky Way model, we demonstrate that distance uncertainties can create artificial wave-like patterns. In contrast to previous suggestions of a breathing mode seen in RAVE data, our results support a combination of bending and breathing modes, likely generated by a combination of external or internal and external mechanisms.
ABSTRACT Data Release 5 (DR5) of the Radial Velocity Experiment (RAVE) is the fifth data release from a magnitude-limited ( ) survey of stars randomly selected in the Southern Hemisphere. The RAVE ...medium-resolution spectra ( ) covering the Ca-triplet region (8410-8795 ) span the complete time frame from the start of RAVE observations in 2003 to their completion in 2013. Radial velocities from 520,781 spectra of 457,588 unique stars are presented, of which 255,922 stellar observations have parallaxes and proper motions from the Tycho-Gaia astrometric solution in Gaia DR1. For our main DR5 catalog, stellar parameters (effective temperature, surface gravity, and overall metallicity) are computed using the RAVE DR4 stellar pipeline, but calibrated using recent K2 Campaign 1 seismic gravities and Gaia benchmark stars, as well as results obtained from high-resolution studies. Also included are temperatures from the Infrared Flux Method, and we provide a catalog of red giant stars in the dereddened color interval (0.50, 0.85) for which the gravities were calibrated based only on seismology. Further data products for subsamples of the RAVE stars include individual abundances for Mg, Al, Si, Ca, Ti, Fe, and Ni, and distances found using isochrones. Each RAVE spectrum is complemented by an error spectrum, which has been used to determine uncertainties on the parameters. The data can be accessed via the RAVE Web site or the VizieR database.
Context. The dynamical evolution of a single globular cluster and also of the entire Galactic globular cluster system has been studied theoretically in detail. In particular, simulations show how the ...“lost” stars are distributed in tidal tails emerging from the clusters. Aims. We investigate the distribution of Galactic globular cluster stars on the sky to identify such features as tidal tails. The Sloan Digital Sky Survey provides consistent photometry of a large part of the sky to study the projected two-dimensional structure of the 17 globular clusters in its survey area. Methods. We used a color–magnitude weighted counting algorithm to map (potential) cluster member stars on the sky. Results. We recover the already known tidal tails of Pal 5 and NGC 5466. For NGC 4147 we find a two-arm morphology. Possible indications of tidal tails are also seen around NGC 5053 and NGC 7078, confirming earlier suggestions. Moreover, we find potential tails around NGC 5904 and Pal 14. Especially for Palomar clusters other than Pal 5, deeper data are needed to confirm or to rule out the existence of tails. For many of the remaining clusters in our sample we observe a pronounced extratidal halo, which is particularly large for NGC 7006 and Pal 1. In some cases, the extratidal halos may be associated with the stream of the Sagittarius dwarf spheroidal galaxy (e.g., NGC 4147, NGC 5024, NGC 5053).
Abstract We present a study of the effective (half-light) radii and other structural properties of a systematically selected sample of young, massive star clusters (≥5 × 10 3 and ≤200 Myr) in two ...nearby spiral galaxies, NGC 628 and NGC 1313. We use Hubble Space Telescope ( HST ) WFC3/UVIS and archival ACS/WFC data obtained by the Legacy Extragalactic UV Survey (LEGUS), an HST Treasury Program. We measure effective radii with GALFIT, a two-dimensional image-fitting package, and with a new technique to estimate effective radii from the concentration index of observed clusters. The distribution of effective radii from both techniques spans ∼0.5–10 pc and peaks at 2–3 pc for both galaxies. We find slight positive correlations between effective radius and cluster age in both galaxies, but no significant relationship between effective radius and galactocentric distance. Clusters in NGC 1313 display a mild increase in effective radius with cluster mass, but the trend disappears when the sample is divided into age bins. We show that the vast majority of the clusters in both galaxies are much older than their dynamical times, suggesting they are gravitationally bound objects. We find that about half of the clusters in NGC 628 are underfilling their Roche lobes, based on their Jacobi radii. Our results suggest that the young, massive clusters in NGC 628 and NGC 1313 are expanding, due to stellar mass loss or two-body relaxation, and are not significantly influenced by the tidal fields of their host galaxies.