Mass accretion rates from multiband photometry in the Carina Nebula: The case of Trumpler 14

open8siWe present a study of the mass accretion rates of pre-main sequence (PMS) stars in the cluster Trumpler 14 (Tr 14) in the Carina Nebula. Using optical multiband photometry we were able to identify 356 PMS stars showing Hα excess emission with equivalent width EW(Hα) > 20 Å. We interpret this observational feature as an indication that these objects are still actively accreting gas from their circumstellar medium. From a comparison of the HR diagram with PMS evolutionary models we derive ages and masses of the PMS stars. We find that most of the PMS objects are younger than 10 Myr with a median age of ~3 Myr. Surprisingly, we also find that ~20% of the mass accreting objects are older than 10 Myr. For each PMS star in Trumpler 14 we determine the mass accretion rate (Ṁacc) and discuss its dependence on mass and age. We finally combine the optical photometry with near-IR observations to build the spectral energy distribution (SED) for each PMS star in Tr 14. The analysis of the SEDs suggests the presence of transitional discs in which a large amount of gas is still present and sustains accretion onto the PMS object at ages older than 10 Myr. Our results, discussed in light of recent recent discoveries with Herschel of transitional discs containing a massive gas component around the relatively old PMS stars TW Hydrae, 49 Ceti, and HD 95086, support a new scenario n which old and evolved debris discs still host a significant amount of gas.openBeccari, G.; De Marchi, G.; Panagia, N.; Valenti, E.; Carraro, G.; Romaniello, M.; Zoccali, M.; Weidner, C.Beccari, G.; De Marchi, G.; Panagia, N.; Valenti, E.; Carraro, Giovanni; Romaniello, M.; Zoccali, M.; Weidner, C

Contribution of White Dwarfs to Cluster Masses

I present a literature search through 31 July 1997 of white dwarfs (WDs) in open and globular clusters. There are 36 single WDs and 5 WDs in binaries known among 13 open clusters, and 340 single WDs and 11 WDs in binaries known among 11 globular clusters. From these data I have calculated WD mass fractions for four open clusters (the Pleiades, NGC 2168, NGC 3532, and the Hyades) and one globular cluster (NGC 6121). I develop a simple model of cluster evolution that incorporates stellar evolution but not dynamical evolution to interpret the WD mass fractions. I augment the results of my simple model with N-body simulations incorporating stellar evolution (Terlevich 1987; de la Feunte Marcos 1996; Vesperini & Heggie 1997). I find that even though these clusters undergo moderate to strong kinematical evolution the WD mass fraction is relatively insensitive to kinematical evolution. By comparing the cluster mass functions to that of the Galactic disk, and incorporating plausibility arguments for the mass function of the Galactic halo, I estimate the WD mass fraction in these two populations. I assume the Galactic disk is ~10 Gyrs old (Winget et al. 1987; Liebert, Dahn, & Monet 1988; Oswalt et al. 1996) and that the Galactic halo is ~12 Gyrs old (Reid 1997b; Gratton et al. 1997; Chaboyer et al. 1998), although the WD mass fraction is insensitive to age in this range. I find that the Galactic halo should contain 8 to 9% (alpha = -2.35) or perhaps as much as 15 to 17% (alpha = -2.0) of its stellar mass in the form of WDs. The Galactic disk WD mass fraction should be 6 to 7% (alpha = -2.35), consistent with the empirical estimates of 3 to 7% (Liebert, Dahn, & Monet 1988; Oswalt et al. 1996). (abridged)Comment: 20 pages, uuencoded gunzip'ed latex + 3 postscrip figures, to be published in AJ, April, 199

Pre-main-sequence stars older than 8 Myr in the Eagle nebula

Attention is given to a population of 110 stars in the NGC 6611 cluster of the Eagle Nebula that have prominent near-infrared (NIR) excess and optical colours typical of pre-main sequence (PMS) stars older than 8 Myr. At least half of those for which spectroscopy exists have a Halpha emission line profile revealing active accretion. In principle, the V-I colours of all these stars would be consistent with those of young PMS objects (< 1 Myr) whose radiation is heavily obscured by a circumstellar disc seen at high inclination and in small part scattered towards the observer by the back side of the disc. However, using theoretical models it is shown here that objects of this type can only account for a few percent of this population. In fact, the spatial distribution of these objects, their X-ray luminosities, their optical brightness, their positions in the colour-magnitude diagram and the weak Li absorption lines of the stars studied spectroscopically suggest that most of them are at least 8 times older than the ~1 Myr-old PMS stars already known in this cluster and could be as old as ~30 Myr. This is the largest homogeneous sample to date of Galactic PMS stars considerably older than 8 Myr that are still actively accreting from a circumstellar disc and it allows us to set a lower limit of 7% to the disc frequency at ~16 Myr in NGC 6611. These values imply a characteristic exponential lifetime of ~6 Myr for disc dissipation.Comment: 12 pages, 5 figures, accepted for publication in Monthly Notices of the Astronomical Societ

Heavy element abundances in giant stars of the globular clusters M4 and M5

We present a comprehensive abundance analysis of 27 heavy elements in bright giant stars of the globular clusters M4 and M5 based on high resolution, high signal-to-noise ratio spectra obtained with the Magellan Clay Telescope. We confirm and expand upon previous results for these clusters by showing that (1) all elements heavier than, and including, Si have constant abundances within each cluster, (2) the elements from Ca to Ni have indistinguishable compositions in M4 and M5, (3) Si, Cu, Zn, and all s-process elements are approximately 0.3 dex overabundant in M4 relative to M5, and (4) the r-process elements Sm, Eu, Gd, and Th are slightly overabundant in M5 relative to M4. The cluster-to-cluster abundance differences for Cu and Zn are intriguing, especially in light of their uncertain nucleosynthetic origins. We confirm that stars other than Type Ia supernovae must produce significant amounts of Cu and Zn at or below the clusters' metallicities. If intermediate-mass AGB stars or massive stars are responsible for the Cu and Zn enhancements in M4, the similar [Rb/Zr] ratios and (preliminary) Mg isotope ratios in both clusters may be problematic for either scenario. For the elements from Ba to Hf, we assume that the s- and r-process contributions are scaled versions of the solar s- and r-process abundances. We quantify the relative fractions of s- and r-process material for each cluster and show that they provide an excellent fit to the observed abundances.Comment: Accepted for publication in Ap

Deep Photometry of the Globular Cluster M5: Distance Estimates from White Dwarf and Main Sequence Stars

We present deep VI photometry of stars in the globular cluster M5 (NGC 5904) based on images taken with the Hubble Space Telescope. The resulting color-magnitude diagram reaches below V ~ 27 mag, revealing the upper 2-3 magnitudes of the white dwarf cooling sequence, and main sequence stars eight magnitudes and more below the turn-off. We fit the main sequence to subdwarfs of known parallax to obtain a true distance modulus of (m-M)_0 = 14.45 +/- 0.11 mag. A second distance estimate based on fitting the cluster white dwarf sequence to field white dwarfs with known parallax yielded (m-M)_0 = 14.67 +/- 0.18 mag. We couple our distance estimates with extensive photometry of the cluster's RR Lyrae variables to provide a calibration of the RR Lyrae absolute magnitude yielding M_V(RR) = 0.42 +/- 0.10 mag at [Fe/H] = -1.11 dex. We provide another luminosity calibration in the form of reddening-free Wasenheit functions. Comparison of our calibrations with predictions based on recent models combining stellar evolution and pulsation theories shows encouraging agreement. (Abridged)Comment: AASTeX, 29 pages including 5 figures. Complete photometry data and FITS-format images are available at http://physics.bgsu.edu/~layden/ASTRO/PUBL/published.html . Accepted for publication in the Astrophysical Journal, 2005 October 20. Replaced errant wording in last sentence of paragraph 4 of conclusion

Apollo asteroids (1566) Icarus and 2007 MK6: Icarus family members?

Although it is more complicated to search for near-Earth object (NEO) families than main belt asteroid (MBA) families, since differential orbital evolution within a NEO family can cause current orbital elements to drastically differ from each other, we have found that Apollo asteroids (1566) Icarus and the newly discovered 2007 MK6 are almost certainly related. Specifically, their orbital evolutions show a similar profile, time shifted by only ~1000 yr, based on our time-lag theory. The dynamical relationship between Icarus and 2007 MK6 along with a possible dust band, the Taurid-Perseid meteor swarm, implies the first detection of an asteroidal NEO family, namely the "Icarus asteroid family".Comment: 11 pages, 1 figure, to appear on Astrophysical Journal Letters (journal info added

HST measures of Mass Accretion Rates in the Orion Nebula Cluster

The present observational understanding of the evolution of the mass accretion rates (Macc) in pre-main sequence stars is limited by the lack of accurate measurements of Macc over homogeneous and large statistical samples of young stars. Such observational effort is needed to properly constrain the theory of star formation and disk evolution. Based on HST/WFPC2 observations, we present a study of Macc for a sample of \sim 700 sources in the Orion Nebula Cluster, ranging from the Hydrogen-burning limit to M\ast \sim 2M\odot. We derive Macc from both the U-band excess and the H{\alpha} luminosity (LH{\alpha}), after determining empirically both the shape of the typical accretion spectrum across the Balmer jump and the relation between the accretion luminosity (Lacc) and LH{\alpha}, that is Lacc/L\odot = (1.31\pm0.03)\cdotLH{\alpha}/L\odot + (2.63\pm 0.13). Given our large statistical sample, we are able to accurately investigate relations between Macc and the parameters of the central star such as mass and age. We clearly find Macc to increase with stellar mass, and decrease over evolutionary time, but we also find strong evidence that the decay of Macc with stellar age occurs over longer timescales for more massive PMS stars. Our best fit relation between these parameters is given by: log(Macc/M\odot\cdotyr)=(-5.12 \pm 0.86) -(0.46 \pm 0.13) \cdot log(t/yr) -(5.75 \pm 1.47)\cdot log(M\ast/M\odot) + (1.17 \pm 0.23)\cdot log(t/yr) \cdot log(M\ast/M\odot). These results also suggest that the similarity solution model could be revised for sources with M\ast > 0.5M\odot. Finally, we do not find a clear trend indicating environmental effects on the accretion properties of the sources.Comment: 17 pages, 15 figures, accepted for publication in Ap

The tidal tails of NGC 2298

We present an implementation of the matched-filter technique to detect tidal tails of globular clusters. The method was tested using SDSS data for the globular cluster Palomar 5 revealing its well known tidal tails. We also ran a simulation of a globular cluster with a tidal tail where we successfully recover the tails for a cluster at the same position and with the same characteristics of NGC 2298. Based on the simulation we estimate that the matched-filter increases the contrast of the tail relative to the background of stars by a factor of 2.5 for the case of NGC 2298. We also present the photometry of the globular cluster NGC 2298 using the MOSAIC2 camera installed on the CTIO 4m telescope. The photometry covers ~ 3deg2 reaching V ~ 23. A fit of a King profile to the radial density profile of NGC 2298 shows that this cluster has a tidal radius of 15.91' \pm 1.07' which is twice as in the literature. The application of the matched-filter to NGC 2298 reveals several extra-tidal structures, including a leading and trailing tail. We also find that NGC 2298 has extra-tidal structures stretching towards and against the Galactic disk, suggesting strong tidal interaction. Finally, we assess how the matched-filter performs when applied to a globular cluster with and without mass segregation taken into account. We find that disregarding the effects of mass segregation may significantly reduce the detection limit of the matched-filter.Comment: 11 pages, 9 figures, 1 table. Accepted for publication on MNRAS main journa