We empirically determine effective temperatures and bolometric luminosities for a large sample of nearby M dwarfs, for which high accuracy optical and infrared photometry is available. We introduce a ...new technique which exploits the flux ratio in different bands as a proxy of both effective temperature and metallicity. Our temperature scale for late-type dwarfs extends well below 3000 K (almost to the brown dwarf limit) and is supported by interferometric angular diameter measurements above 3000 K. Our metallicities are in excellent agreement (usually within 0.2 dex) with recent determinations via independent techniques. A subsample of cool M dwarfs with metallicity estimates based on hotter Hipparcos common proper motion companions indicates our metallicities are also reliable below 3000 K, a temperature range unexplored until now. The high quality of our data allows us to identify a striking feature in the bolometric luminosity versus temperature plane, around the transition from K to M dwarfs. We have compared our sample of stars with theoretical models and conclude that this transition is due to an increase in the radii of the M dwarfs, a feature which is not reproduced by theoretical models.
Colour–magnitude diagrams (CMDs) of the Small Magellanic Cloud (SMC) star cluster NGC 419, derived from Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) data, reveal a well-delineated ...secondary clump located below the classical compact red clump typical of intermediate-age populations. We demonstrate that this feature belongs to the cluster itself, rather than to the underlying SMC field. Then, we use synthetic CMDs to show that it corresponds very well to the secondary clump predicted to appear as a result of He-ignition in stars just massive enough to avoid e--degeneracy settling in their H-exhausted cores. The main red clump instead is made of the slightly less massive stars which passed through e- degeneracy and ignited He at the tip of the red giant branch. In other words, NGC 419 is the rare snapshot of a cluster while undergoing the fast transition from classical to degenerate H-exhausted cores. At this particular moment of a cluster's life, the colour distance between the main-sequence turn-off and the red clump(s) depends sensitively on the amount of convective core overshooting, Λc. By coupling measurements of this colour separation with fits to the red clump morphology, we are able to estimate simultaneously the cluster mean age (1.35+0.11-0.04 Gyr) and overshooting efficiency (Λc= 0.47+0.14-0.04). Therefore, clusters like NGC 419 may constitute important marks in the age scale of intermediate-age populations. After eye inspection of other CMDs derived from HST/ACS data, we suggest that the same secondary clump may also be present in the Large Magellanic Cloud clusters NGC 1751, 1783, 1806, 1846, 1852 and 1917.
We present a detailed analysis of the behaviour of the red clump K-band absolute magnitude (MRCK) in simple and composite stellar populations, in light of its use as a standard candle for distance ...determinations. The advantage of using MRCK, following recent empirical calibrations of its value for the solar neighbourhood, arises from its very low sensitivity to extinction by interstellar dust. We show that, as in the case of the V- and I-band results, MRCK is a complicated function of the stellar metallicity Z and age t. In general, MRCK is more sensitive to t and Z than MRCI, for high t and low Z. Moreover, for ages above ∼1.5 Gyr, MRCK decreases with increasing Z, i.e. the opposite behaviour with respect to MRCV and MRCI. We provide data and equations that allow the determination of the K-band population correction ΔMRCK (the difference between the red clump brightness in the solar neighbourhood and in the population under scrutiny) for any generic stellar population. These data complement the results presented by Girardi & Salaris for the V- and I-band. We show how data from galactic open clusters consistently support our predicted ΔMRCV, ΔMRCI and ΔMRCK values. Multiband VIK population corrections for various galaxy systems are provided. They can be used in conjunction with the method devised by Alves et al., in order to derive simultaneously reddening and distance from the use of VIK observations of red clump stars. We have positively tested this technique on the Galactic globular cluster 47 Tuc, for which both an empirical parallax-based main-sequence-fitting distance and reddening estimates exist. We have also studied the case of using only V and I photometry, recovering consistent results for both reddening and distance. Application of this method to an OGLE-II field, and the results by Alves et al., confirm a Large Magellanic Cloud distance modulus of about 18.50, in agreement with the Hubble Space Telescope extragalactic distance scale zero-point.
We propose a zero-point photometric calibration of the data from the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) on board the Hubble Space Telescope, based on a spectrum of Vega and ...the most up-to-date in-flight transmission curves of the camera. This calibration is accurate at the level of a few hundredths of a magnitude. The main purpose of this effort is to transform the entire set of evolutionary models into a simple observational photometric system for ACS/WFC data, and to make them available to the astronomical community. We provide the zero-points for the most used ACS/WFC bands, and give basic recipes for calibrating both the observed data and the models. We also present the colour–magnitude diagram from ACS data of five Galactic globular clusters, spanning the metallicity range −2.2 <Fe/H < −0.04, and we provide fiducial points representing their sequences from several magnitudes below the turn-off to the red giant branch tip. The observed sequences are compared with the models in the newly defined photometric system.
We derive an empirical effective temperature and bolometric luminosity calibration for G and K dwarfs, by applying our own implementation of the Infrared Flux Method to multiband photometry. Our ...study is based on 104 stars for which we have excellent BV(RI)C
JHK
S photometry, excellent parallaxes and good metallicities.
Colours computed from the most recent synthetic libraries (ATLAS9 and MARCS) are found to be in good agreement with the empirical colours in the optical bands, but some discrepancies still remain in the infrared. Synthetic and empirical bolometric corrections also show fair agreement.
A careful comparison to temperatures, luminosities and angular diameters obtained with other methods in the literature shows that systematic effects still exist in the calibrations at the level of a few per cent. Our Infrared Flux Method temperature scale is 100-K hotter than recent analogous determinations in the literature, but is in agreement with spectroscopically calibrated temperature scales and fits well the colours of the Sun. Our angular diameters are typically 3 per cent smaller when compared to other (indirect) determinations of angular diameter for such stars, but are consistent with the limb-darkening corrected predictions of the latest 3D model atmospheres and also with the results of asteroseismology.
Very tight empirical relations are derived for bolometric luminosity, effective temperature and angular diameter from photometric indices.
We find that much of the discrepancy with other temperature scales and the uncertainties in the infrared synthetic colours arise from the uncertainties in the use of Vega as the flux calibrator. Angular diameter measurements for a well-chosen set of G and K dwarfs would go a long way to addressing this problem.
The red giant clump has been recently argued to be a reliable distance indicator for the galaxies in the Local Group. The accuracy of distance determinations based on this method, however, depends on ...the possible presence of systematic magnitude differences (ΔMIRC) between the local clump revealed by the Hipparcos colour—magnitude diagram (CMD), and the clump stars observed in distant galaxies. In this paper, we re-address the problem of these systematic ‘population’ effects. First, we present tables with the theoretically predicted I-band clump magnitude as a function of age and metallicity. Simple equations, taken from basic population synthesis theory, are provided for the easy computation of the mean clump magnitude for any given galaxy model. We use our models to explain in some detail what determines the distribution of masses, ages and metallicities of clump stars in a galaxy. Such an approach has so far been neglected in the analysis of clump data related with distance determinations. We point out that, in galaxies with recent/ongoing star formation (e.g. the discs of spirals), the age distribution of clump stars is strongly biased towards younger (∼1–3 Gyr) ages, and hence towards higher metallicities. Obviously, this does not happen in galaxies with predominantly old stellar populations (e.g. ellipticals and bulges). We construct detailed models for the clump population in the local (Hipparcos) sample, the bulge, Magellanic Clouds and Carina dSph galaxy. In all cases, star formation rates and chemical enrichment histories are taken from the literature. The Hipparcos model is shown to produce distributions of metallicities, colours, and magnitudes, that are similar to those derived from spectroscopic and Hipparcos data. The bulge, Magellanic Clouds, and Carina dSph models are used to analyse the values of ΔMIRC for these different stellar systems. We show how the clump—RR Lyrae data from Udalski are well reproduced by the models. However, despite the similarity between the models and data, the models indicate that the linear ΔMIRC versus Fe/H relations that have been derived from the same data (such as by Udalski and Popowski) are not general. In fact, the distribution of clump stars has several factors hidden in it — e.g. the age—metallicity relation, the rate of past star formation — that cannot be described by such relations. The model behaviour is also supported by empirical data for open clusters by Sarajedini and Twarog et al. We argue that Udalski's data for Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) star clusters do not allow a good assessment of the age dependence of the clump magnitude. Moreover, we remark that similar analyses of cluster data should better include clump stars with ages 1–2 Gyr, which turn out to be very important in determining the mean clump in galaxies with recent star formation. Finally, we provide revised clump distances to the bulge, Magellanic Clouds and Carina dSph, and further comment on their reliability. The largest ΔMIRC values are found for the Magellanic Clouds and Carina dSph, which turn out to be located at distance moduli ∼0.2–0.3 mag longer than indicated by works which ignore population effects. The Galactic bulge, instead, may be slightly closer (up to 0.1 mag in distance modulus) than indicated by previous works based on the red clump, the exact result depending on the use of either scaled-solar or α-enhanced stellar models.
The rich Small Magellanic Cloud (SMC) star cluster NGC 419 has recently been found to present both a broad main-sequence turn-off and a dual red clump of giants in the sharp colour–magnitude diagrams ...(CMDs) derived from the High-Resolution Channel of the Advanced Camera for Surveys onboard the Hubble Space Telescope. In this work, we apply to the NGC 419 data the classical method of star-formation-history (SFH) recovery through CMD reconstruction, deriving for the first time this function for a star cluster with multiple turn-offs. The values for the cluster metallicity, reddening, distance and binary fraction were varied within the limits allowed by present observations. The global best-fitting solution is an excellent fit to the data, reproducing all the CMD features with striking accuracy. The corresponding star-formation rate is provided together with estimates of its random and systematic errors. Star formation is found to last for at least 700 Myr, and to have a marked peak in the middle of this interval, for an age of 1.5 Gyr. Our findings argue in favour of multiple star-formation episodes (or continuous star formation) being at the origin of the multiple main-sequence turn-offs in Magellanic Cloud clusters with ages of about 1 Gyr. It remains to be tested whether alternative hypotheses, such as a main-sequence spread caused by rotation, could produce similarly good fits to the data.
Abstract
Our Universe is estimated to be 13.5 ± 2 billion light years old and the observable universe is over 90 billion light years wide. Over 100 billion stars are born and die each passing day. ...Since the very dawn of creation, the cycle of life and death is constant at every part of the universe. But we are too tiny to comprehend such measures. Our earth where we reside in is too small and the cosmos is too vast. For anyone to measure anything from outer space is nigh impossible, or so we thought until the early 1900s. In this article, star clusters namely NGC 188 and M41 was used to show one of many convenient methods on how to find the age of a cluster or rather an individual star in that cluster using the help of the color magnitude diagram (CMD). It was observed that the ages of NGC 188 were found to be 4 ± 0.5 billion years and M41 to be around 220±30 million years.
Using imaging that shows 4 mag of main-sequence stars, we have discovered that the Galactic globular cluster NGC 1851 is surrounded by a halo that is visible from the tidal radius of 700 arcsec (41 ...pc) to more than 4500 arcsec (>250 pc). This halo is symmetric and falls in density as a power law of r -1.24. It contains approximately 0.1% of the dynamical mass of NGC 1851. There is no evidence for tidal tails. Current models of globular cluster evolution do not explain this feature, although simulations of tidal influences on dwarf spheroidal galaxies qualitatively mimic these results. Given the state of published models, it is not possible to decide between creation of this halo from either isolated cluster evaporation or from tidal or disk shocking, or from destruction of a dwarf galaxy in which this object may have once been embedded.
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