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

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

Quasipolyhedral sets in linear semiinfinite inequality systems

AbstractThis paper provides an extension to linear semiinfinite systems of a well-known property of finite linear inequality systems, the so-called Weyl property, which characterizes the extreme points of the solution set as those solution points such that the gradient vectors of the active constraints form a complete set. A class of linear semiinfinite systems which satisfy this property is identified, the p-systems. It is also shown that any p-system contains an equivalent minimal subsystem

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

Chitosan induces delayed grapevine defense mechanisms and protects grapevine against Botrytis cinerea

In the present study, a commercial chitosan soluble in acid solution and obtained from shrimp shell waste, with a molecular weight of 173 kDa and a degree of acetylation of 17%, named as chitosan (173/17), was investigated. Chitosan is a well-known biopolymer whose antimicrobial properties are highly influenced by the molecular weight, degree of acetylation as well as the preparation and derivatization methods used. Chitosan (173/17) was applied on grapevine leaves before Botrytis cinerea inoculation to verify its effectiveness as a preventive treatment against the fungal infection. The expression of a set of defense marker genes, as well as accumulation of stilbene phytoalexins, was investigated. Thanks to its fungistatic and filmogenic properties, chitosan (173/17) protected grapevine leaves against B. cinerea. Moreover, it induced grapevine defense response: three days after the treatment an induction of the jasmonic acid and ethylene-mediated response, a repression of the salicylic acid-mediated signaling, and a transient accumulation of trans-resveratrol were registered. Our data indicate that chitosan (173/17), when used in preventive application, is able to protect grapevine against B. cinerea infection. The effectiveness of chitosan (173/17) as a natural ecofriendly product for the control of B. cinerea on grapevine was demonstrated

Star Formation in the Starburst Cluster in NGC 3603

We have used new, deep, visible and near infrared observations of the compact starburst cluster in the giant HII region NGC 3603 and its surroundings with the WFC3 on HST and HAWK-I on the VLT to study in detail the physical properties of its intermediate mass (~ 1 - 3 M_sun) stellar population. We show that after correction for differential extinction and actively accreting stars, and the study of field star contamination, strong evidence remains for a continuous spread in the ages of pre-main sequence stars in the range ~ 2 to ~ 30 Myr within the temporal resolution available. Existing differences among presently available theoretical models account for the largest possible variation in shape of the measured age histograms within these limits. We also find that this isochronal age spread in the near infrared and visible Colour-Magnitude Diagrams cannot be reproduced by any other presently known source of astrophysical or instrumental scatter that could mimic the luminosity spread seen in our observations except, possibly, episodic accretion. The measured age spread and the stellar spatial distribution in the cluster are consistent with the hypothesis that star formation started at least 20-30 Myrs ago progressing slowly but continuously up to at least a few million years ago. All the stars in the considered mass range are distributed in a flattened oblate spheroidal pattern with the major axis oriented in an approximate South-East - North-West direction, and with the length of the equatorial axis decreasing with increasing age. This asymmetry is most likely due to the fact that star formation occurred along a filament of gas and dust in the natal molecular cloud oriented locally in this direction.Comment: 21 pages, 19 figures, accepted for publication in Astrophysics & Space Scienc

Cataclysmic Variables and a New Class of Faint UV Stars in the Globular Cluster NGC 6397

We present evidence that the globular cluster NGC 6397 contains two distinct classes of centrally-concentrated UV-bright stars. Color-magnitude diagrams constructed from U, B, V, and I data obtained with the HST/WFPC2 reveal seven UV-bright stars fainter than the main-sequence turnoff, three of which had previously been identified as cataclysmic variables (CVs). Lightcurves of these stars show the characteristic ``flicker'' of CVs, as well as longer-term variability. A fourth star is identified as a CV candidate on the basis of its variability and UV excess. Three additional UV-bright stars show no photometric variability and have broad-band colors characteristic of B stars. These non-flickering UV stars are too faint to be extended horizontal branch stars. We suggest that they could be low-mass helium white dwarfs, formed when the evolution of a red giant is interrupted, due either to Roche-lobe overflow onto a binary companion, or to envelope ejection following a common-envelope phase in a tidal-capture binary. Alternatively, they could be very-low-mass core-He-burning stars. Both the CVs and the new class of faint UV stars are strongly concentrated toward the cluster center, to the extent that mass segregation from 2-body relaxation alone may be unable to explain their distribution.Comment: 11 pages plus 3 eps figures; LaTeX using aaspp4.sty; to appear in The Astrophysical Journal Letter

Factors affecting the incidence of first-quality wheels of Trentingrana cheese

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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