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We have finally measured the evolutionary rate of cooling of the pulsating hydrogen atmosphere (DA) white dwarf ZZ Ceti (Ross 548), as reflected by the drift rate of the 213.13260694 s period. Using 41 yr of time-series photometry from 1970 November to 2012 January, we determine the rate of change of this period with time to be dP/dt = (5.2 ± 1.4) ×10−15 s s-ˡ employing the O−C method and (5.45 ± 0.79) ×10−15 s s-ˡ using a direct nonlinear least squares fit to the entire lightcurve. We adopt the dP/dt obtained from the nonlinear least squares program as our final determination, but augment the corresponding uncertainty to a more realistic value, ultimately arriving at the measurement of dP/dt = (5.5 ± 1.0) ×10−15 s s-ˡ. After correcting for proper motion, the evolutionary rate of cooling of ZZ Ceti is computed to be (3.3 ± 1.1) ×10−15 s s-ˡ. This value is consistent within uncertainties with the measurement of (4.19±0.73)×10−15 s s-ˡ for another similar pulsating DA white dwarf, G 117-B15A.Measuring the cooling rate of ZZ Ceti helps us refine our stellar structure and evolutionary models, as cooling depends mainly on the core composition and stellar mass. Calibrating white dwarf cooling curves with this measurement will reduce the theoretical uncertainties involved in white dwarf cosmochronometry. Should the 213.13 s period be trapped in the hydrogen envelope, then our determination of its drift rate compared to the expected evolutionary rate suggests an additional source of stellar cooling. Attributing the excess cooling to the emission of axions imposes a constraint on the mass of the hypothetical axion particle.

A recent JHKs study of several grand-design spiral galaxies shows a bimodal distribution of their system of star clusters and star forming complexes in colour-magnitude and colour-colour diagrams. In a comparison with stellar population models including gas, the (J-H) vs (H-Ks) diagram reveals that embedded clusters, still immersed in their parental clouds of gas and dust, generally have a redder (H-Ks) colour than older clusters, whose gas and dust have already been ejected. This bimodal behaviour is also evident in the colour-magnitude diagram MK vs (J-Ks), where the brightest clusters split into two sequences separating younger from older clusters. In addition, the reddening-free index Qd = (H-Ks) - 0.884 (J-H) has been shown to correlate with age for the young clusters and thus provided an effective way to differentiate the embedded clusters from the older ones. We aim to study the behaviour of these photometric indices for star cluster systems in the Local Group. We investigate the effectiveness of the Qd index in sorting out clusters of different ages at their early evolutionary stages. Surface photometry was carried out for 2MASS images of populous clusters younger than ~100Myr whose ages were available. Some clusters, particularly the embedded ones, were studied for the first time using this method. The integrated magnitudes and colours extracted from the surface photometry of the most populous clusters/complexes in the Local Group shows the expected bimodal distribution in the colour-colour and colour-magnitude diagrams. In particular, we confirm the index Qd as a powerful tool for distinguishing clusters younger than about 7Myr from older clusters. (abridged), Comment: 10 pages, 11 figures; accepted for publication in A&A

We report on the analysis of 436.1 hrs of nearly continuous high-speed photometry on the pulsating DB white dwarf GD358 acquired with the Whole Earth Telescope (WET) during the 2006 international observing run, designated XCOV25. The Fourier transform (FT) of the light curve contains power between 1000 to 4000 microHz, with the dominant peak at 1234 microHz. We find 27 independent frequencies distributed in 10 modes, as well as numerous combination frequencies. Our discussion focuses on a new asteroseismological analysis of GD358, incorporating the 2006 data set and drawing on 24 years of archival observations. Our results reveal that, while the general frequency locations of the identified modes are consistent throughout the years, the multiplet structure is complex and cannot be interpreted simply as l=1 modes in the limit of slow rotation. The high k multiplets exhibit significant variability in structure, amplitude and frequency. Any identification of the m components for the high k multiplets is highly suspect. The k=9 and 8 modes typically do show triplet structure more consistent with theoretical expectations. The frequencies and amplitudes exhibit some variability, but much less than the high k modes. Analysis of the k=9 and 8 multiplet splittings from 1990 to 2008 reveal a long-term change in multiplet splittings coinciding with the 1996 "sforzando" event, where GD358 dramatically altered its pulsation characteristics on a timescale of hours. We explore potential implications, including the possible connections between convection and/or magnetic fields and pulsations. We suggest future investigations, including theoretical investigations of the relationship between magnetic fields, pulsation, growth rates, and convection., 18 pages, 20 figures, 5 tables, accepted for publication in the Astrophysical Journal

We used time-resolved ultraviolet spectroscopy obtained with the FOS and STIS spectrographs of the Hubble Space Telescope (HST), together with archival IUE observations to measure the effective temperature (Teff), surface gravity (log g) and distance (d) of the pulsating DB white dwarf GD 358 with unprecedented accuracy, and to show that the temperature did not change during the 1996 sforzando, when the star changed basically to a single mode pulsator. We also measured for the first time for a DBV the spherical harmonic degree (l) for two modes, with k = 8 and k = 9, which was only possible because the stellar light curve was dominated by a single mode in 1996. The independent spectra provide the following values: Teff = 24 100 ± 400K, log g = 7.91 ± 0.26 and d = 42.7 ± 2.5 pc. The ultraviolet spectroscopic distance is in better agreement with the seismological value, than the one derived by parallax.

We report the discovery of three optical open clusters in the Milky Way. Two clusters are in Scutum (Cluster1 at l=18.44 degrees and b=-0.42 degrees, and Cluster2 at l=19.60 degrees and b=-1.02 degrees), thus projected not far from the Galactic center direction, and the other is in Canis Major (Cluster3 at l=235.61 degrees and b=-4.10 degrees), near the anti-center direction. Cluster3 is less populous than Clusters 1 and 2, but presents evidence of being a physical system. The objects were found optically by inspecting maps obtained from the Guide Star Catalogue and images from the Digitized Sky Survey. No previous identification of cluster has been reported in each area so far. The analysis was carried out with 2MASS photometry in J and H. For Cluster1 we derive an age of 25 Myr, a reddening E(B-V)=2.18 and a distance from the Sun 1.64kpc; for Cluster2, age of 500Myr, E(B-V)=0.91 and distance 2.19kpc; finally for Cluster3, age 32-100Myr, E(B-V)=0.94 and distance of 3.93kpc. Luminosity and mass functions are derived for Clusters1 and 2 which, in turn, allowed us to estimate their observed masses as 147 and 89 solar masses, respectively. Estimated total masses, by extrapolating the mass functions to 0.08 solar mass, amount to 382 and 614 solar masses, for the two clusters. Cluster3 has an observed mass of 55 solar masses. The present results indicate that further searches in the optical might still reveal new open clusters, and more so in infrared bands., accepted to Astronomy and Astrophysics, 9 figures

The spatial dependence of luminosity and mass functions of evolved open clusters is discussed in this work using J and H 2MASS photometry, which allows a wide spatial coverage and proper background determination. The target objects are the intermediate-age open cluster NGC3680 (l = 286.76° and b = 16.92°), which has been reported as being in an advanced state of dissolution, and NGC2180 (l = 203.85°, b = −7.01°), a possible dynamically evolved open cluster. At the 1 − σ level, NGC2180 presents a radial distribution of stars that can be represented by a King law. We conclude that, although in an advanced dynamical state (mass segregated), NGC3680 does not present strong signs of dissolution, having luminosity and mass functions very similar to those of the τage ≈ 3.2 Gyr open cluster M67. On the other hand, NGC2180 appears to have flat, eroded luminosity functions throughout its structure, suggesting that in addition to mass segregation, Galactic tidal stripping has been effective in depleting this object. Accordingly, NGC2180 may be the missing link between evolved open clusters and remnants. For NGC 2180 we infer a distance to the Sun dʘ = 0.91 ± 0.15 kpc, an age τage ≈ 710 Myr and an observed stellar mass of mobs ∼ 47 Mʘ. For NGC3680 we derive an age τage ≈ 1.6 Gyr, E(B − V) = 0.0 and dʘ = 1.00 ± 0.09 kpc, confirming previous estimates. The observed stellar mass mobs ≈ 130 Mʘ agrees with previous values. We study both clusters in the context of dynamical states estimated from diagnostic diagrams involving photometric and structural parameters. Both objects appear to be dynamically evolved systems. In particular, NGC2180 is closer to open cluster remnants than NGC3680.

The distribution of core radii of rich clusters in the Large Magellanic Cloud (LMC) systematically increases in both upper limit and spread with increasing cluster age. Cluster-to-cluster variations in the stellar initial mass function (IMF) have been suggested as an explanation.We discuss the implications of the observed degree of mass segregation in our sample clusters for the shape of the initial mass function. Our results are based on Hubble Space Telescope/WFPC2 observations of six rich star clusters in the LMC, selected to include three pairs of clusters of similar age, metallicity and distance from the LMC centre, and exhibiting a large spread in core radii between the clusters in each pair. All clusters show clear evidence of mass segregation: (i) their luminosity function slopes steepen with increasing cluster radius, and (ii) the brighter stars are characterized by smaller core radii. For all sample clusters, both the slope of the luminosity function in the cluster centres and the degree of mass segregation are similar to each other, within observational errors of a few tenths of power-law slope fits to the data. This implies that their initial mass functions must have been very similar, down to ~ 0.8–1.0 Mʘ. We therefore rule out variations in the IMF of the individual sample clusters as the main driver of the increasing spread of cluster core radii with cluster age.

We show the results of deep V and I HST photometry of 6 rich star clusters in the Large Magellanic Cloud with different ages and metallicities. The number of stars with measured magnitudes in each cluster varies from about 3000 to 10000. We build stellar density and surface brightness profiles for the clusters and extract half-light radii and other structural parameters for each. We also obtain luminosity functions, Phi (Mv), down to Mv ~ 6 (m/msun > 0.6), and investigate their dependence with distance from the cluster centre well beyond their half-light radius. In all clusters we find a systematic increase in the luminosity function slope with radial distance from the centre. Among the clusters displaying significant mass segregation are the two youngest in the sample: NGC 1805 and NGC 1818. For these two clusters we obtain present-day mass functions. The NGC 1818 mass function is in excellent agreement with that derived by other authors, also using HST data. They young cluster mass function slopes differ, that of NCG 1805 being systematically steeper than NGC 1818. Since these are very young stellar systems (age < 40 Myrs), these variations may reflect the initial conditions rather than evolution due to internal dynamics., 10 pages, 24 figures

Intrinsic infrared colors of stars in the Johnson 11 color system are derived. The database is a list of 3946 stars with observations in UBV RIJHKLMN, of all spectral types and luminosity classes, including carbon, T Tauri, and Wolf-Rayet stars. Intrinsic colors were derived from the zero-reddening curve that can be deÐned in the bluer side of temperature versus observed color diagrams. In a sample of stars of the same spectral type, the bluest stars are considered to have an observed color very near their intrinsic color. The comparison with former derivations from Johnson and Koornneef presents signiÐcant di†erences : new M and N colors, for all spectral types, are bluer than published values, the di†erences being more important for giants and supergiants ; the amplitude of color values in these new results is wider.

Observation of g-mode pulsations in the variable pre-white dwarf (GW Virginis) stars provides a unique means to probe their interiors and to study the late stages of stellar evolution. Multisite campaigns have in several cases proved highly successful in decoding preÈwhite dwarf light curves. Three previous attempts to untangle the pulsation spectrum of the coolest GW Virginis star, PG 0122+200, confirmed the existence of multiple g-modes but left the fundamental period spacing and therefore the star's mass and luminosity in doubt. We present an analysis based on new observations of PG 0122+200 obtained during a Whole Earth Telescope (WET) campaign conducted in the fall of 1996. Although our coverage was, because of bad weather, far poorer than in previous WET campaigns, we confirm the previous result that PG 0122+200 rotates once in 1.6±0.1 days. The most likely period spacing supported by the data implies a mass of 0.69±0.03 Mʘ. Based on the best seismology we can currently do, the cooling of PG 0122+200 is dominated by neutrino losses. This is not true for all pre-white dwarf stars and makes PG 0122+200 the prime candidate for learning useful physics. Constraints placed on the cooling rate of PG 0122+200 by future measurement of dII/dt could provide a unique test of the standard theory of lepton interactions in the (experimentally unexplored) region of phase-space appropriate to pre-white dwarf interiors.

As transition objects between the asymptotic giant branch (AGB) and cooling white dwarfs, the PG 1159 stars are key objects for understanding the late stages of stellar evolution. The pulsations exhibited by many members of the PG 1159 spectral class provide a mechanism for ferreting out this information. We present an analysis of the pulsation spectrum of the naked PG 1159 star PG 2131 +066, which we completely resolve using data from the Whole Earth Telescope (WET). The pulsation spectrum shows patterns that are expected from pulsation theory; comparison of these patterns with theoretical models indicate that PG 2131 has a mass of 0.61 ± 0.02 Mʘ, a luminosity of log (L/Lʘ) = 1.0 ± 0.2, and it lies ata distance of 470 +470 - 130pc. lt rotates with a period of 5.1 hr and shows evidence of a discontinuity in composition approximately 6 x 10- 3M* below the surface. Certain modes show complex structure in the power spectrum, while others show measur­ able frequency changes in 8 years. Fine structure in the prograde (m = -1) modes suggests the presence of nonlinear effects.

The Type Ia supernova SN 1992A in the SO galaxy NGC 1380 was observed as a target of opportunity by the International Ultraviolet Explorer (IUE) and with great alacrity by the Hubble Space Telescope (HST). Here we present the HST and IUE spectra and photometry that we obtained, as well as optical spectra obtained at the Cerro Tololo Inter-American Observatory (CTIO). The HST Faint Object Spectrograph (FOS) spectra, from 5 and 45 days past maximum light, are the best UV spectra of a Type Ia supernova and reveal for the first time with good signal-to-noise ratio the Type Ia spectral region blueward of ~2650 Å. The UV photometry taken in the F175W, F275W, and F342W bands defined by the HST filters shows light curves that resemble the Type Ia template U light curve. Using data from SN 1992A and SN 1990N, we have constructed a Type Ia template light curve for the flux region near 2750 Å that is quite detailed from 14 days before maximum light to 22 days after maximum light and that extends to 77 days after maximum light. This light curve also resembles the template U light curve. A high-resolution HST Goddard High Resolution Spectrograph (GHRS) spectrum of SN 1992A shows no evidence for any interstellar !ines attributable to NGC 1380 or to Galactic high-velocity clouds; low-velocity Galactic gas gives rise to relatively weak Mg 11 UV resonance absorption !ines. These results are consistent with low extinction along the line of sight to SN 1992A. We have done a parameterized LTE analysis of the SN 1992A spectra using a slightly modified version of Woosley's delayed-detonation model DD4. We find that the features in the region blueward of ~2650 Å in the HST spectra are P Cygni absorptions due to blends of iron peak element multiplets and the Mg II resonance multiplet. Newly synthesized magnesium, sulfur, and silicon probably extend to velocities at least as high as ~19,000 km s-ˡ. Newly synthesized Ni-Co may dominate the iron peak element abundances out to ~13,000 km s-ˡ in the ejecta of SN 1992A as in model DD4. However, further investigation of the extent of Ni-Co material in SN 1992A and other Type Ia supernovae is needed; this investigation could greatly help constrain Type Ia models. An analysis of the O I λ27773 line in SN 1992A and other Type Ia supernovae implies that the oxygen-rich layer in typical Type Ia's extends over a velocity range of at least ~11,000-19,000 km s-ˡ. None of the explosion models we considered, including model DD4, has an oxygen-rich layer that completely extends over this range. Model DD4, however, is promising, and further investigation of delayed-detonation models of its kind is merited. Spectropolarimetric observations taken by Spyromilio & Bailey of SN 1992A are consistent with the SN 1992A being spherically symmetric. We discuss these observations and the importance of taking spectropolarimetry of other Type Ia supernovae

The oldest white dwarf stars in the solar neighborhood are still observable, and their luminosity distribution can be used to determine directly the age of the Galactic disk and, somewhat less directly, the age of the universe. The observed number of white dwarfs in a given volume of space increases monotonically with decreasing luminosity, as expected from cooling rate considerations (hotter objects cool more quickly). However, their number drops abruptly at a luminosity of log (L/ L0 ) ""' -4.5, due to the finite age of our Galaxy. Comparing this sudden drop in the observed luminosity distribution with the best theoretical evolutionary white dwarf models available to us, and allowing for a mean pre-white-dwarf lifetime of 0.3 Gyr, we derive an age for the Galactic disk of 9.3 ± 2.0 Gyr. To obtain the age of the universe, we must add the time between the big bang and the first appearance of stars in the Galactic disk. Rather than assume a particular cosmological model, we prefer to choose a value (and stated error) that can include ali of the currently reasonable models describing this early era. On this basis we estimate the age of the universe to be 10.3 ± 2.2 Gyr. This new technique provides a way of determining tlie age of tlie Galactic disk that is largely independent of ali previous methods. Further, its current uncertainties can be materially reduced by direct measurements of the rate at which variable white dwarf stars cool tlirough the three known instability strips in the H-R diagram.

We have conducted time-series photometric observations of the binary star V471 Tauri using the Whole Earth Telescope observing network. Our purpose was to determine the mechanism responsible for causing the 555 and 277 s periodic luminosity variations exhibited by the white dwarf in this binary. Previous observers have proposed that either g-mode pulsations or rotation of an accreting magnetic white dwarf could cause the variations, but were unable to decide which was the correct model. Our observations have answered this question. Learning the cause of the white dwarf variations has been possible because of our discovery of a periodic signal at 562 s in the Johnson U-band flux of the binary. By identifying this signal as reprocessed radiation and using its phase to infer the phase of the shorter wavelength radiation which produces it, we have been able to compare the phase of the 555 s U-band variations to the phase of the X-ray variations. We have found that U-band maximum coincides with X-ray minimum. From this result we have concluded that the magnetic rotator model accurately describes the variations we observe, but that models involving g-mode pulsations do not.