The M 4 Core Project with HST is designed to exploit the Hubble Space Telescope (HST) to investigate the central regions of M 4, the Globular Cluster closest to the Sun. In this paper we combine ...optical and near-infrared photometry to study multiple stellar populations in M 4. We detected two sequences of M-dwarfs containing ∼38 per cent (MS
I) and ∼62 per cent (MS
II) of MS stars below the main-sequence (MS) knee. We compare our observations with those of NGC 2808, which is the only other GCs where multiple MSs of very low-mass stars have been studied to date. We calculate synthetic spectra for M-dwarfs, assuming the chemical composition mixture inferred from spectroscopic studies of stellar populations along the red giant branch, and different helium abundances, and we compare predicted and observed colours. Observations are consistent with two populations, one with primordial abundance and another with enhanced nitrogen and depleted oxygen.
We use Hubble Space Telescope multicolour photometry of the globular cluster 47 Tucanae to uncover a population of 24 objects with no previous classification that are outliers from the single-star ...model tracks in the colour–magnitude diagram and yet are likely cluster members. By comparing those sources with evolutionary models and X-ray source catalogues, we were able to show that the majority of those sources are likely binary systems that do not have any X-ray source detected nearby, most possibly formed by a white dwarf and a main-sequence star and a small number of possible double-degenerate systems.
Photometric metallicities in Boötes I Hughes, J; Wallerstein, G; Dotter, A ...
Monthly Notices of the Royal Astronomical Society,
03/2014, Letnik:
439, Številka:
1
Journal Article
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We present new Strömgren and Washington data sets for the Boötes I dwarf galaxy, and combine them with the available Sloan Digital Sky Survey photometry. The goal of this project is to refine a ...ground-based, practical, accurate method to determine age and metallicity for individual stars in Boötes I that can be selected in an unbiased imaging survey, without having to take spectra. With few bright upper red giant branch stars and distances of about 35-250 kpc, the ultra-faint dwarf galaxies (UDFs) present observational challenges in characterizing their stellar population. Other recent studies have produced spectra and proper motions, making Boötes I an ideal test case for our photometric methods. We produce photometric metallicities from Strömgren and Washington photometry, for stellar systems with a range of −1.0 > Fe/H > −3.5. Needing to avoid the collapse of the metallicity sensitivity of the Strömgren m
1-index on the lower red giant branch, we replace the Strömgren v filter with the broader Washington C filter to minimize observing time. We construct two indices: m
* = (C − T
1)0 − (T
1 − T
2)0 and m
** = (C − b)0 − (b − y)0. We find that CT
1
by is the most successful filter combination, for individual stars with Fe/H < −2.0, to maintain ∼0.2 dex Fe/H-resolution over the whole red giant branch. The m
**-index would be the best choice for space-based observations because the (C − y) colour is not sufficient to fix metallicity alone in an understudied system. Our photometric metallicites of stars in the central regions of Boötes I confirm that there is a metallicity spread of at least −1.9 > Fe/H > −3.7. The best-fitting Dartmouth isochrones give a mean age, for all the Boötes I stars in our data set, of 11.5 ± 0.4 Gyr. From ground-based telescopes, we show that the optimal filter combination is CT
1
by, avoiding the v filter entirely. We demonstrate that we can break the isochrones' age-metallicity degeneracy with the CT
1
by filters, using stars with log g = 2.5 − 3.0, which have less than a 2 per cent change in their (C − T
1) colour due to age, over a range of 10-14 Gyr.
We present a new distance determination to the Galactic globular cluster 47 Tucanae by fitting the spectral energy distributions of its white dwarfs (WDs) to pure hydrogen atmosphere WD models. Our ...photometric data set is obtained from a 121-orbit Hubble Space Telescope program using the Wide Field Camera 3 UVIS/IR channels, capturing F390W, F606W, F110W, and F160W images. These images cover more than 60 arcmin super(2) and extend over a radial range of 5-13.7 arcmin (6.5-17.9 pc) within the globular cluster. Using a likelihood analysis, we obtain a best-fitting unreddened distance modulus of (m - M) sub(o) = 13.36 + or - 0.02 + or - 0.06 corresponding to a distance of 4.69 + or - 0.04 + or -0.13 kpc, where the first error is random and the second is systematic. We also search the WD photometry for infrared excess in the F160W filter, indicative of low-mass companions, and find no convincing cases within our sample.
We use photometric and spectroscopic observations of the eclipsing binary V69-47 Tuc to derive the masses, radii, and luminosities of the component stars. Based on measured systemic velocity, ...distance, and proper motion, the system is a member of the globular cluster 47 Tuc. The system has an orbital period of 29.5d and the orbit is slightly eccentric with e = 0.056. We obtain Mp = 0.8762 ? 0.0048 M , Rp = 1.3148 ? 0.0051 R , Lp = 1.94 ? 0.21 L for the primary and Ms = 0.8588 ? 0.0060 M , Rs = 1.1616 ? 0.0062 R , Ls = 1.53 ? 0.17 L for the secondary. These components of V69 are the first Population II stars with masses and radii derived directly and with an accuracy of better than 1%. We measure an apparent distance modulus of (m -- M) V = 13.35 ? 0.08 to V69. We compare the absolute parameters of V69 with five sets of stellar evolution models and estimate the age of V69 using mass-luminosity-age, mass-radius-age, and turnoff mass-age relations. The masses, radii, and luminosities of the component stars are determined well enough that the measurement of ages is dominated by systematic differences between the evolutionary models, in particular, the adopted helium abundance. By comparing the observations to Dartmouth model isochrones we estimate the age of V69 to be 11.25 ? 0.21(random) ? 0.85(systematic) Gyr assuming Fe/H = --0.70, Delta *a/Fe = 0.4, and Y = 0.255. The determination of the distance to V69, and hence to 47 Tuc, can be further improved when infrared eclipse photometry is obtained for the variable.
Multi-epoch observations with the Advanced Camera for Surveys on the Hubble Space Telescope provide a unique and comprehensive probe of stellar dynamics within NGC 6397. We are able to confront ...analytic models of the globular cluster with the observed stellar proper motions. The measured proper motions probe well along themain sequence from 0.8 to below 0.1 M sub(middot in circle) as well as white dwarfs younger than 1 Gyr. The observed proper motions show no evidence for anisotropy in the velocity distribution; furthermore, the observations agree in detail with a straightforward model of the stellar distribution function. We do not find any evidence that the young white dwarfs have received a natal kick in contradiction with earlier results. The proper motions of the youngest white dwarfs appear to resemble those of the most massive main-sequence stars, providing the first direct constraint on the relaxation time of the stars in a globular cluster of greater than or about 0.7 Gyr.
By means of high-resolution cosmological hydrodynamical simulations of Milky Way (MW) like disc galaxies, we conduct an analysis of the associated stellar metallicity distribution functions (MDFs). ...After undertaking a kinematic decomposition of each simulation into spheroid and disc subcomponents, we compare the predicted MDFs to those observed in the solar neighbourhood and the Galactic bulge. The effects of the star formation density threshold are visible in the star formation histories, which show a modulation in their behaviour driven by the threshold. The derived MDFs show median metallicities lower by 0.2-0.3 dex than the MDF observed locally in the disc and in the Galactic bulge. Possible reasons for this apparent discrepancy include the use of low stellar yields and/or centrally concentrated star formation. The dispersions are larger than the one of the observed MDF; this could be due to simulated discs being kinematically hotter relative to the MW. The fraction of low-metallicity stars is largely overestimated, visible from the more negatively skewed MDF with respect to the observational sample. For our fiducial MW analogue, we study the metallicity distribution of the stars born in situ relative to those formed via accretion (from disrupted satellites), and demonstrate that this low-metallicity tail to the MDF is populated primarily by accreted stars. Enhanced supernova and stellar radiation energy feedback to the surrounding interstellar media of these pre-disrupted satellites is suggested as an important regulator of the MDF skewness.
We present a detailed analysis of the upper main sequence of the ~1.3 Gyr old open cluster Trumpler 20. High-accuracy BVI photometry combined with the Very Large Telescope/FLAMES medium-resolution ...spectroscopy of 954 stars is essential to understanding the unusual appearance of the color-magnitude diagram (CMD), initially suggesting multiple populations in Trumpler 20. We show that differential reddening is a dominant contributor to the apparent splitting/widening of the main-sequence turnoff region. At its extreme, the excess differential reddening reaches A (B - V) ~ 0.1 while the adopted minimum reddening for the cluster is E(B - V) = 0.36. A unique sample of measured projected rotational velocities indicates that stellar rotation is high near the main-sequence turnoff, reaching v sin i ~ 180 km s super(-1). By dividing the upper main-sequence stars into two equal groups of slow and fast rotators, we find that fast rotators have a marginal blueshift of delta (V - I) ~ -0.01, corresponding to a difference in the median v sin i of ~60 km s super(-1) between these subsamples. We conclude that stellar rotation has an insignificant effect on the morphology of the upper main sequence of this intermediate-age open cluster. Trumpler 20 appears to contain a single coeval population of stars but there is evidence that the red clump is extended.
We present an empirical determination of the white dwarf cooling sequence in the globular cluster 47 Tucanae. Using spectral models, we determine temperatures for 887 objects from Wide Field Camera 3 ...data, as well as 292 objects from data taken with the Advanced Camera for Surveys. We make the assumption that the rate of white dwarf formation in the cluster is constant. Stellar evolution models are then used to determine the rate at which objects are leaving the main sequence, which must be the same as the rate at which objects are arriving on the white dwarf sequence in our field. The result is an empirically derived relation between temperature (T sub(eff)) and time (t) on the white dwarf cooling sequence. Comparing this result to theoretical cooling models, we find general agreement with the expected slopes between 20,000 K and 30,000 K and between 6000 K and 20,000 K, but the transition to the Mestel cooling rate of T sub(eff) is proportional to t super(-0.4) is found to occur at hotter temperatures, and more abruptly than is predicted by any of these models.
Pharmaceutical companies have invested heavily to equip their sales organizations with interactive visual aids (IVAs) running on tablet PCs designed to increase sales productivity and reduce risks ...associated with regulatory non-compliance. However, research indicates that the majority of sales representatives within these organizations do not use the new technology. In this study, sales representatives from a major US-based pharmaceutical company are surveyed to better understand factors impacting IVA adoption. Using diffusion of innovation as the theoretical lens, findings from this study make significant contributions to both theory and practice. Results of the study indicate that relative advantage and compatibility positively affect usage intention while voluntariness negatively affects usage intention. In addition, evidence is found to support that both image and behavioral evaluation, in the form of financial compensation, positively affects relative advantage while behavioral evaluation and monitoring negatively affects voluntariness.
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