Showing papers by "Massimo Marengo published in 2007"

Discovery of Two T Dwarf Companions with the Spitzer Space Telescope

Abstract: We report the discovery of T dwarf companions to the nearby stars HN Peg (G0 V, 18.4 pc, τ ~ 0.3 Gyr) and HD 3651 (K0 V, 11.1 pc, τ ~ 7 Gyr). During an ongoing survey of 5' × 5' fields surrounding stars in the solar neighborhood with the Infrared Array Camera aboard the Spitzer Space Telescope, we identified these companions as candidate T dwarfs based on their mid-infrared colors. Using near-infrared spectra obtained with SpeX at the NASA Infrared Telescope Facility, we confirm the presence of methane absorption that characterizes T dwarfs and measure spectral types of T2.5 ± 0.5 and T7.5 ± 0.5 for HN Peg B and HD 3651B, respectively. By comparing our Spitzer data to images from the Two Micron All Sky Survey obtained several years earlier, we find that the proper motions of HN Peg B and HD 3651B are consistent with those of the primaries, confirming their companionship. A comparison of their luminosities to the values predicted by theoretical evolutionary models implies masses of 0.021 ± 0.009 and 0.051 ± 0.014 M_⊙ for HN Peg B and HD 3651B, respectively. In addition, the models imply an effective temperature for HN Peg B that is significantly lower than the values derived for other T dwarfs at similar spectral types, which is the same behavior reported by Metchev & Hillenbrand for the young late L dwarf HD 203030B. Thus, the temperature of the L/T transition appears to depend on surface gravity. Meanwhile, HD 3651B is the first substellar companion directly imaged around a star that is known to harbor a close-in planet from radial velocity surveys. The discovery of this companion supports the notion that the high eccentricities of close-in planets like that near HD 3651 may be the result of perturbations by low-mass companions at wide separations.

The Dust and Gas Around β Pictoris

Abstract: We have obtained Spitzer IRS 5.5-35 ?m spectroscopy of the debris disk around ? Pictoris. In addition to the 10 ?m silicate emission feature originally observed from the ground, we also detect the crystalline silicate emission bands at 28 and 33.5 ?m. This is the first time that the silicate bands at wavelengths longer than 10 ?m have ever been seen in the ? Pictoris disk. The observed dust emission is well reproduced by a dust model consisting of fluffy cometary and crystalline olivine aggregates. We searched for line emission from molecular hydrogen and atomic [S I], Fe II, and Si II gas but detected none. We place a 3 ? upper limit of <17 M? on the H2 S(1) gas mass, assuming an excitation temperature of Tex = 100 K. This suggests that there is less gas in this system than is required to form the envelope of Jupiter. We hypothesize that some of the atomic Na I gas observed in Keplerian rotation around ? Pictoris may be produced by photon-stimulated desorption from circumstellar dust grains.

The Dust and Gas Around beta Pictoris

Abstract: We have obtained Spitzer IRS 5.5 - 35 micron spectroscopy of the debris disk around beta Pictoris. In addition to the 10 micron silicate emission feature originally observed from the ground, we also detect the crystalline silicate emission bands at 28 micron and 33.5 micron. This is the first time that the silicate bands at wavelengths longer than 10 micron have ever been seen in the beta Pictoris disk. The observed dust emission is well reproduced by a dust model consisting of fluffy cometary and crystalline olivine aggregates. We searched for line emission from molecular hydrogen and atomic [S I], Fe II, and Si II gas but detected none. We place a 3 sigma upper limit of <17 Earth masses on the H2 S(1) gas mass, assuming an excitation temperature of Tex = 100 K. This suggests that there is less gas in this system than is required to form the envelope of Jupiter. We hypothesize that some of the atomic Na I gas observed in Keplerian rotation around beta Pictoris may be produced by photon-stimulated desorption from circumstellar dust grains.

Spitzer-MIPS survey of the young stellar content in the Vela Molecular Cloud-D

Abstract: A new, unbiased Spitzer-MIPS imaging survey (~1.8 square degs) of the young stellar content of the Vela Molecular Cloud-D is presented. The survey is complete down to 5mJy and 250mJy at 24micron (mu) and 70mu, respectively. 849 sources are detected at 24mu and 52 of them also have a 70mu counterpart. The VMR-D region is one that we have already partially mapped in dust and gas millimeter emission, and we discuss the correlation between the Spitzer compact sources and the mm contours. About half of the 24mu sources are located inside the region delimited by the 12CO(1-0) contours (corresponding to only one third of the full area mapped with MIPS) with a consequent density increase of about 100% of the 24mu sources [four times for 70mu ones] moving from outside to inside the CO contours. About 400 sources have a 2MASS counterpart. So we have constructed a Ks vs. Ks-[24] diagram and identified the protostellar population. We find an excess of Class I sources in VMR-D in comparison with other star forming regions. This result is reasonably biased by the sensitivity limits, or, alternatively, may reflect a very short lifetime (<=10^6yr) of the protostellar content in this cloud. The MIPS images have identified embedded cool objects in most of the previously identified starless cores; in addition, there are 6 very young, possibly Class 0 objects identified. Finally we report finding of the driving sources for a set of five out of six very compact protostellar jets previously discovered in near-infrared images.

The Dust and Gas Around Beta Pictoris

Abstract: We have obtained Spitzer IRS 5.5 - 35 micron spectroscopy of the debris disk around beta Pictoris. In addition to the 10 micron silicate emission feature originally observed from the ground, we also detect the crystalline silicate emission bands at 28 micron and 33.5 micron. This is the first time that the silicate bands at wavelengths longer than 10 micron have ever been seen in the beta Pictoris disk. The observed dust emission is well reproduced by a dust model consisting of fluffy cometary and crystalline olivine aggregates. We searched for line emission from molecular hydrogen and atomic [S I], Fe II, and Si II gas but detected none. We place a 3 sigma upper limit of <17 Earth masses on the H2 S(1) gas mass, assuming an excitation temperature of Tex = 100 K. This suggests that there is less gas in this system than is required to form the envelope of Jupiter. We hypothesize that some of the atomic Na I gas observed in Keplerian rotation around beta Pictoris may be produced by photon-stimulated desorption from circumstellar dust grains.

Spitzer MIPS Survey of the Young Stellar Content in the Vela Molecular Ridge-D

Abstract: A new, unbiased Spitzer MIPS imaging survey (~1.8 deg2) of the young stellar content of the Vela Molecular Ridge-D (VMR-D) is presented. The survey is complete down to 5 and 250 mJy at 24 and 70 μm, respectively. A total of 849 sources are detected at 24 μm, and 52 of them also have a 70 μm counterpart. The VMR-D region is one that we have already partially mapped in dust and gas millimeter emission, and we discuss the correlation between the Spitzer compact sources and the millimeter contours. About half of the 24 μm sources are located inside the region delimited by the 12CO(1-0) contours, corresponding to only one-third of the full area mapped with MIPS. Therefore, the 24 μm source density increases by about 100% moving from outside to inside the CO contours. For the 70 μm sources, the corresponding density increase is a factor of 4. About 400 sources of these have a 2MASS counterpart, and we have used this to construct a Ks versus Ks - [24] diagram and to identify the protostellar population inside the cloud. We find an excess of Class I sources in VMR-D in comparison with other star-forming regions. This result could be reasonably biased by the sensitivity limits at 2.2 and 24 μm or, alternatively, may reflect a very short lifetime (106 yr) for the protostellar content in this molecular cloud. The MIPS images have identified embedded cool objects in most of the previously identified starless cores in the region; in addition, there are six very young, possibly Class 0 objects identified. Finally, we report finding the driving sources for a set of five out of six very compact protostellar jets that had been previously discovered in near-IR images of VMR-D.

Observations of Extrasolar Planets During the non-Cryogenic Spitzer Space Telescope Mission

Abstract: Precision infrared photometry from Spitzer has enabled the first direct studies of light from extrasolar planets, via observations at secondary eclipse in transiting systems. Current Spitzer results include the first longitudinal temperature map of an extrasolar planet, and the first spectra of their atmospheres. Spitzer has also measured a temperature and precise radius for the first transiting Neptune-sized exoplanet, and is beginning to make precise transit timing measurements to infer the existence of unseen low mass planets. The lack of stellar limb darkening in the infrared facilitates precise radius and transit timing measurements of transiting planets. Warm Spitzer will be capable of a precise radius measurement for Earth-sized planets transiting nearby M-dwarfs, thereby constraining their bulk composition. It will continue to measure thermal emission at secondary eclipse for transiting hot Jupiters, and be able to distinguish between planets having broad band emission versus absorption spectra. It will also be able to measure the orbital phase variation of thermal emission for close-in planets, even non-transiting planets, and these measurements will be of special interest for planets in eccentric orbits. Warm Spitzer will be a significant complement to Kepler, particularly as regards transit timing in the Kepler field. In addition to studying close-in planets, Warm Spitzer will have significant application in sensitive imaging searches for young planets at relatively large angular separations from their parent stars.

Observations of Extrasolar Planets During the non‐Cryogenic Spitzer Space Telescope Mission

Abstract: Precision infrared photometry from Spitzer has enabled the first direct studies of light from extrasolar planets, via observations at secondary eclipse in transiting systems. Current Spitzer results include the first longitudinal temperature map of an extrasolar planet, and the first spectra of their atmospheres. Spitzer has also measured a temperature and precise radius for the first transiting Neptune‐sized exoplanet, and is beginning to make precise transit timing measurements to infer the existence of unseen low mass planets. The lack of stellar limb darkening in the infrared facilitates precise radius and transit timing measurements of transiting planets. Warm Spitzer will be capable of a precise radius measurement for Earth‐sized planets transiting nearby M‐dwarfs, thereby constraining their bulk composition. It will continue to measure thermal emission at secondary eclipse for transiting hot Jupiters, and be able to distinguish between planets having broad band emission vs. absorption spectra. It will also be able to measure the orbital phase variation of thermal emission for close‐in planets, even non‐transiting planets, and these measurements will be of special interest for planets in eccentric orbits. Warm Spitzer will be a significant complement to Kepler, particularly as regards transit timing in the Kepler field. In addition to studying close‐in planets, Warm Spitzer will have significant application in sensitive imaging searches for young planets at relatively large angular separations from their parent stars.

Spitzer/IRAC Characterization of Galactic AGB Stars

Abstract: The Spitzer Space Telescope and in particular its InfraRed Array Camera (IRAC) is an ideal facility to study the distribution of AGB stars in our own and other galaxies because of its efficiency in surveying vast areas of the sky and its ability to detect sources with infrared excess. The IRAC colors of AGB stars, however, are not well known because cool stars have numerous molecular absorption features in the spectral region covered by the IRAC photometric system. The presence and strength of these features depends on the chemistry of the stellar atmosphere and the mass loss rate and can change with time due to the star's variability. To characterize the IRAC colors of AGB stars, we are carrying out a Spitzer Guaranteed Time Observation program to observe a sample of AGB stars with IRAC. The results will be made available to the community in the form of template magnitudes and colors for each target with the goal of aiding the identification of AGB stars in already available and future IRAC surveys. We present here the first results of this project.

Stellar Astronomy in the Warm Spitzer Era

Abstract: We consider the impact on the study of normal stars of large‐scale pointed and mapping observations at 36 μm and 45 μm with the Spitzer IRAC imager Deep observations at these wave‐lengths are particularly sensitive to very cool stellar and substellar objects, both as companions to other stars and in the field A wide‐angle survey can be expected to detect 50–100 cool T dwarfs and up to 5 “Y” dwarfs in the field, and AGB stars throughout the Galactic halo Pointed observations of white dwarfs at these wavelengths will be sensitive to unresolved cool companions and to circumstellar dust disk remnants of planetary system objects The cumulative photometry of normal stars in the imaging fields will be invaluable for understanding stellar colors and atmospheres