Transformations between the theoretical and observational planes in the HST-NICMOS and WFPC2 photometric systems

Color-temperature relations and bolometric corrections in the HST-NICMOS F1110W, F160W and F222M and in the WFPC2 F439W, F555W and F814W photometric systems, using two different sets of model atmospheres, have been derived. This database of homogeneous, self-consistent transformations between the theoretical and observational planes also allows combinations of visual and infrared quantities, without any further transformation between the two different photometric systems. The behavior of the inferred quantities with varying the stellar parameters, the adopted model atmospheres and the instrumental configurations are investigated. Suitable relations to transform colors and bolometric corrections from HST to ground-based photometric systems are also provided.Comment: 22 pages, 14 figure

Revised metallicity classes for low-mass stars: dwarfs (dM), subdwarfs (sdM), extreme subdwarfs (esdM), and ultra subdwarfs (usdM)

The current classification system of M stars on the main sequence distinguishes three metallicity classes (dwarfs - dM, subdwarfs - sdM, and extreme subdwarfs - esdM). The spectroscopic definition of these classes is based on the relative strength of prominent CaH and TiO molecular absorption bands near 7000A, as quantified by three spectroscopic indices (CaH2, CaH3, and TiO5). We re-examine this classification system in light of our ongoing spectroscopic survey of stars with proper motion \mu > 0.45 "/yr, which has increased the census of spectroscopically identified metal-poor M stars to over 400 objects. Kinematic separation of disk dwarfs and halo subdwarfs suggest deficiencies in the current classification system. Observations of common proper motion doubles indicates that the current dM/sdM and sdM/esdM boundaries in the [TiO5,CaH2+CaH3] index plane do not follow iso-metallicity contours, leaving some binaries inappropriately classified as dM+sdM or sdM+esdM. We propose a revision of the classification system based on an empirical calibration of the TiO/CaH ratio for stars of near solar metallicity. We introduce the parameter \zeta_{TiO/CaH} which quantifies the weakening of the TiO bandstrength due to metallicity effect, with values ranging from \zeta_{TiO/CaH}=1 for stars of near-solar metallicity to \zeta_{TiO/CaH}~0 for the most metal-poor (and TiO depleted) subdwarfs. We redefine the metallicity classes based on the value of the parameter \zeta_{TiO/CaH}; and refine the scheme by introducing an additional class of ultra subdwarfs (usdM). We introduce sequences of sdM, esdM, and usdM stars to be used as formal classification standards.Comment: 15 pages, accepted for publication in the Astrophysical Journa

HST observations of the LMC field around SN87A: distance determinations with Red Clump and Tip of the Red Giant Branch stars

We have used HST-WFPC2 multiband observations of a field around SN 1987A in the Large Magellanic Cloud to measure its distance from the Sun. The observations allowed us to carefully determine the interstellar extinction along the line of sight to a large number of stars and to measure the LMC distance by using two stellar distance indicators: the Red Clump and the Tip of the Red Giant Branch. From an application of the Red Clump method we obtain a distance modulus (m-M)o,rc(LMC)=18.59+-0.04+-0.08 mag (statistical plus systematic error), in good agreement with the distance derived by using the Tip of the Red Giant Branch stars, namely (m-M)o,trgb(LMC)=18.69+-0.25+-0.06 mag (statistical plus systematic error). Both values agree well with the distance to the SN 1987A as determined from a study of its inner ring fluorescent echo ((m-M)(SN87A)=18.55+-0.05 mag, Panagia 1998), thus excluding distance moduli lower than 18.43 to a 99.7% significance level. Differences with respect to previous results obtained using the same distance indicators are discussed.Comment: 7 pages including 3 figures; Astrophysical Journal, accepted for publicatio

SearchCal: a Virtual Observatory tool for searching calibrators in optical long baseline interferometry. I: The bright object case

In long baseline interferometry, the raw fringe contrast must be calibrated to obtain the true visibility and then those observables that can be interpreted in terms of astrophysical parameters. The selection of suitable calibration stars is crucial for obtaining the ultimate precision of interferometric instruments like the VLTI. We have developed software SearchCal that builds an evolutive catalog of stars suitable as calibrators within any given user-defined angular distance and magnitude around the scientific target. We present the first version of SearchCal dedicated to the bright-object case V<=10; K<=5). Star catalogs available at the CDS are consulted via web requests. They provide all the useful information for selecting of calibrators. Missing photometries are computed with an accuracy of 0.1 mag and the missing angular diameters are calculated with a precision better than 10%. For each star the squared visibility is computed by taking the wavelength and the maximum baseline of the foreseen observation into account.} SearchCal is integrated into ASPRO, the interferometric observing preparation software developed by the JMMC, available at the address: http://mariotti.fr

NICMOS Observations of the Pre-Main-Sequence Planetary Debris System HD 98800

Spectral energy distributions (SEDs) from 0.4 to 4.7 microns are presented for the two principal stellar components of HD~98800, A and B. The third major component, an extensive planetary debris system (PDS), emits > 20% of the luminosity of star B in a blackbody SED at 164 +/- 5K extending from mid-IR to millimeter-wavelengths. At 0.95 microns a preliminary upper limit of < 0.06 is obtained for the ratio of reflected light to the total from star B. This result limits the albedo of the PDS to < 0.3. Values are presented for the temperature, luminosity, and radius of each major systemic component. Remarkable similarities are found between the PDS and the interplanetary debris system around the Sun as it could have appeared a few million years after its formation.Comment: LaTeX, 9 pages with 1 encapsulated postscript figure and one specially formatted Table which is rendered as a postscript file and included as a figure. Accepted for publication in Astrophysical Journal Letter

A New Multiple Stellar System in the Solar Neighborhood

Adaptive optics corrected images obtained with the CIAO instrument at the Subaru 8.2-meter telescope show the presence of two subarsecond companions to the nearby (d=19.3 pc) young star GJ 900, which was previously classified as a single member of the IC 2391 supercluster. The two companions share the same proper motion as the primary and are redder. Their projected separations from the primary are 10 AU and 14.5 AU for B and C, respectively. The estimated masses for the two new companions depend strongly on the age of the system. For the range of ages found in the literature for IC 2391 supercluster members (from 35 Myr to 200 Myr), the expected masses range from 0.2 M$_\odot$ to 0.4 M$_\odot$ for the B component, and from 0.09 M$_\odot$ to 0.22 M$_\odot$ for the C component. The determination of the dynamical mass of the faintest component of GJ 900 will yield the age of the system using theoretical evolutionary tracks. The apparent separations of the GJ 900 system components meet the observational criterion for an unstable Trapezium-type system, but this could be a projection effect. Further observations are needed to establish the nature of this interesting low-mass multiple system.Comment: Scheduled for publication in the Astronomical Journal (August 2003

Photometric Identification of Type Ia Supernovae at Moderate Redshift

Large photometric surveys with the aim of identifying many Type Ia supernovae (SNe) at moderate redshift are challenged in separating these SNe from other SN types. We are motivated to identify Type Ia SNe based only on broadband photometric information, since spectroscopic determination of the SN type, the traditional method, requires significant amounts of time on large telescopes. We consider the possible observables provided by a large synoptic photometry survey. We examine the optical colors and magnitudes of many SN types from z=0.1 to z=1.0, using space-based ultraviolet spectra and ground-based optical spectra to simulate the photometry. We also discuss the evolution of colors over the SN outburst and the use of host galaxy characteristics to aid in the identification of Type Ia SNe. We consider magnitudes in both the SDSS photometric system and in a proposed filter system with logarithmically spaced bandpasses. We find that photometric information in four bands covering the entire optical spectrum appears capable of providing identification of Type Ia SNe based on their colors at a single observed epoch soon after maximum light, even without independent estimates of the SN redshift. Very blue filters are extremely helpful, as at moderate redshift they sample the restframe ultraviolet spectrum where the SN types are very different. We emphasize the need for further observations of SNe in the restframe ultraviolet to fully characterize, refine, and improve this method of SN type identification.Comment: AASTeX, 37 pages with 12 figures, being resubmitted to A.J. Figures 3, 4 and 9 updated, minor typos correcte

CG J1720-67.8: A Detailed Analysis of Optical and Infrared Properties of a New Ultracompact Group of Galaxies

We present here optical spectroscopy and BVRJHK(s) photometry of the recently discovered ultra-compact group of galaxies CG J1720-67.8. This work represents a considerable extension of the preliminary results we presented in a previous paper. Despite the complicated morphology of the group, a quantitative morphological classification of the three brightest members of the group is attempted based on photometric analysis. We find that one galaxy is consistent with a morphological type S0, while the other two are most probably late-type spirals that are already losing their identity due tothe interaction process. Information on the star formation activity and dust content derived from both spectroscopic data and optical and near-infrared colors are complemented with a reconstruction of far-infrared (FIR) maps from IRAS raw data. Enhanced star formation activity is revealed in all the group's members, including the early-type galaxy and the extended tidal tail, along which several tidal dwarf galaxy candidates are identified. The metallicity of the gaseous component is investigated and photoionization models are applied to the three main galaxies of the group, while a detailed study of the tidal dwarf candidates will appear in a companion paper. Subsolar metal abundances are found for all the three galaxies, the highest values being shown by the early-type galaxy (Z ~ 0.5 Zsolar).Comment: Accepted for publication in The Astrophysical Journa