arXiv subjects

I. Song

Publications and source records attributed to I. Song.

At least 37 records · Page 2Linked to original sources

arXiv2011

YSOVAR: the first sensitive, wide-area, mid-IR photometric monitoring of the ONC

We present initial results from time series imaging at infrared wavelengths of 0.9 sq. degrees in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 micron data over 40 consecutive days. We extracted light curves with ~3% photometric accuracy for ~2000 ONC members ranging from several solar masses down to well below the hydrogen burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (Ic) and/or near-infrared (JKs) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence (PMS) eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 & 4.5 micron variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009, and we highlight our light curves for AA-Tau analogs - YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs.

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arXiv2010

The Absence of Cold Dust and the Mineralogy and Origin of the Warm Dust Encircling BD +20 307

Spitzer Space Telescope photometry and spectroscopy of BD +20 307 show that all of the dust around this remarkable Gyr-old spectroscopic binary arises within 1 AU. No additional cold dust is needed to fit the infrared excess. Peaks in the 10 and 20 micron spectrum are well fit with small silicates that should be removed on a timescale of years from the system. This is the dustiest star known for its age, which is >1 Gyr. The dust cannot arise from a steady-state collisional cascade. A catastrophic collision of two rocky, planetary-scale bodies in the terrestrial zone is the most likely source for this warm dust because it does not require a reservoir of planetesimals in the outer system.

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arXiv2009

The LAOG-Planet Imaging Surveys

With the development of high contrast imaging techniques and infrared detectors, vast efforts have been devoted during the past decade to detect and characterize lighter, cooler and closer companions to nearby stars, and ultimately image new planetary systems. Complementary to other observing techniques (radial velocity, transit, micro-lensing, pulsar-timing), this approach has opened a new astrophysical window to study the physical properties and the formation mechanisms of brown dwarfs and planets. I here will briefly present the observing challenge, the different observing techniques, strategies and samples of current exoplanet imaging searches that have been selected in the context of the LAOG-Planet Imaging Surveys. I will finally describe the most recent results that led to the discovery of giant planets probably formed like the ones of our solar system, offering exciting and attractive perspectives for the future generation of deep imaging instruments.

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arXiv2009

Mid-Infrared Variability of protostars in IC 1396A

We have used Spitzer/IRAC to conduct a photometric monitoring program of the IC1396A dark globule in order to study the mid-IR (3.6 - 8 micron) variability of the heavily embedded Young Stellar Objects (YSOs) present in that area. We obtained light curves covering a 14 day timespan with a twice daily cadence for 69 YSOs, and continuous light curves with approximately 12 second cadence over 7 hours for 38 YSOs. Typical accuracies for our relative photometry were 1-2% for the long timespan data and a few mmag, corresponding to less than 0.5%, for the 7 hour continuous "staring-mode" data. More than half of the YSOs showed detectable variability, with amplitudes from ~0.05 mag to ~0.2 mag. About thirty percent of the YSOs showed quasi-sinusoidal light curve shapes with apparent periods from 5-12 days and light curve amplitudes approximately independent of wavelength over the IRAC bandpasses. We have constructed models which simulate the time dependent spectral energy distributions of Class I and I I YSOs in order to attempt to explain these light curves. Based on these models, the apparently periodic light curves are best explained by YSO models where one or two high latitude photospheric spots heat the inner wall of the circumstellar disk, and where we view the disk at fairly large inclination angle. Disk inhomogeneities, such as increasing the height where the accretion funnel flows to the stellar hotspot, enhances the light curve modulations. The other YSOs in our sample show a range of light curve shapes, some of which are probably due to varying accretion rate or disk shadowing events. One star, IC1396A-47, shows a 3.5 hour periodic light curve; this object may be a PMS Delta Scuti star.

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arXiv2009

Kinematic analysis and membership status of TWA22ABR

TWA22 was initially regarded as a member of the TW Hydrae association (TWA). In addition to being one of the youngest (~8Myr) and nearest (~20pc) stars to Earth, TWA22 has proven to be very interesting after being resolved as a tight, very low-mass binary. This binary can serve as a very useful dynamical calibrator for pre-main sequence evolutionary models. However, its membership in the TWA has been recently questioned despite due to the lack of accurate kinematic measurements. Based on proper motion, radial velocity, and trigonometric parallax measurements, we aim here to re-analyze the membership of TWA22 to young, nearby associations. Using the ESO NTT/SUSI2 telescope, we observed TWA22AB during 5 different observing runs over 1.2 years to measure its trigonometric parallax and proper motion. This is a part of a larger project measuring trigonometric parallaxes and proper motions of most known TWA members at a sub-milliarcsec level. HARPS at the ESO 3.6m telescope was also used to measure the system's radial velocity over 2 years. We report an absolute trigonometric parallax of TWA22AB, 57.0mas, corresponding to a distance 17.5pc from Earth. Measured proper motions of TWA22AB are -175.8mas/yr in right ascension and -21.3mas/yr in declination. Finally, from HARPS measurements, we obtain a radial velocity 14.8km/s.

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arXiv2009

Abundant Circumstellar Silica Dust and SiO Gas Created by a Giant Hypervelocity Collision in the ~12 Myr HD172555 System

The fine dust detected by IR emission around the nearby Beta Pic analogue star HD172555 is very peculiar. The dust mineralogy is composed primarily of highly refractory, non-equilibrium materials, with approximately three-quarters of the Si atoms in silica (SiO2) species. Tektite and obsidian lab thermal emission spectra (non-equilibrium glassy silicas found in impact and magmatic systems) are required to fit the data. The best-fit model size distribution for the observed fine dust is dn/da = a-3.95 +/- 0.10. This steep a size distribution, with abundant micron-sized particles, argues for a fresh source of material within the last 0.1 Myr. The location of the dust with respect to the star is at 5.8 +/- 0.6 AU (equivalent to 1.9 +/- 0.2 AU from the Sun), within the terrestrial planet formation region but at the outer edge of any possible terrestrial habitability zone. The mass of fine dust is 4 x 10^19 - 2 x 10^20 kg, equivalent to a 150 - 200 km radius asteroid. Significant emission features centered at 4 and 8 um due to fluorescing SiO gas are also found. Roughly 10^22 kg of SiO gas, formed by vaporizing silicate rock, is also present in the system, and a separate population of very large, cool grains, massing 10^21 - 10^22 kg and equivalent to the largest sized asteroid currently found in the Solar System's main asteroid belt, dominates the solid circumstellar material by mass. The makeup of the observed dust and gas, and the noted lack of a dense circumstellar gas disk, strong primary x-ray activity, or an extended disk of Beta-meteroids argues that the source of the observed circumstellar materials is a giant hypervelocity (> 10 km sec^-1) impact between large rocky planetesimals, similar to the ones which formed the Moon and which stripped the surface crustal material off of Mercury's surface.

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arXiv2009

VLT/NACO Deep imaging survey of young, nearby austral stars

Since November 2002, we have conducted the largest deep imaging survey of the young, nearby associations of the southern hemisphere. Our goal is detection and characterization of substellar companions at intermediate (10--500 AU) physical separations. We have observed a sample of 88 stars, mostly G to M dwarfs, that we essentially identify as younger than 100 Myr and closer to Earth than 100 pc. The VLT/NACO adaptive optics instrument of the ESO Paranal Observatory was used to explore the faint circumstellar environment between typically 0.1 and 10''. We report the discovery of 17 new close (0.1-5.0'') multiple systems. HIP108195AB and C (F1III-M6), HIP84642AB (a~14 AU, K0-M5) and TWA22AB (a~1.8 AU; M6-M6) confirmed comoving systems. TWA22AB is likely to be a astrometric calibrator that can be used to test evolutionary predictions. Among our complete sample, a total of 65 targets observed with deep coronagraphic imaging. About 240 faint candidates were detected around 36 stars. Follow-up observations VLT or HST for 83% of these stars enabled us to identify a fraction of contaminants. The latest results about the substellar companions to GSC08047-00232, AB Pic and 2M1207, confirmed during this survey and published earlier, are reviewed. Finally, the statistical analysis of our complete set of coronagraphic limits enables us to place constraints on the physical and properties of giant planets between typically 20 and 150 AU.

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arXiv2009

The young, tight and low mass binary TWA22AB: a new calibrator for evolutionary models ? Orbit, spectral types and temperatures determination

Tight binaries discovered in young, nearby associations, with known distances, are ideal targets to provide dynamical mass measurements to test the physics of evolutionary models at young ages and very low masses. We report for the first time the binarity of TWA22, possible new dynamical calibrator for evolutionary models at young ages. Based on an accurate trigonometric distance (17.53 +- 0.21 pc) determination, we infer a total dynamical mass of 220 +- 21 MJup for the system. From the resolved near-infrared integral-field spectroscopy, we find an effective temperature Teff=2900+200-200 K for TWA22 A and Teff=2900+200-100 K for TWA22 B and surface gravities between 4.0 and 5.5 dex. From our photometry and a M6 +- 1 spectral type for both components, we find luminosities of log(L/Lsun)=-2.11 +- 0.13 dex and log(L/Lsun)=-2.30 +- 0.16 dex for TWA22 A and B respectively. By comparing these parameters with evolutionary models, we question the age and the multiplicity of this system. We also discuss a possible underestimation of the mass predicted by evolutionary models for young stars close to the substellar boundary.

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arXiv2009

New Debris Disks Around Young, Low Mass Stars Discovered With The Spitzer Space Telescope

(abridged) We present 24 and 70 micron MIPS observations of 70 A through M-type dwarfs with estimated ages from 8 Myr to 1.1 Gyr, as part of a Spitzer guaranteed time program. Our sample is selected from stars with common youth indicators such as lithium abundance, X-ray activity, chromospheric activity, and rapid rotation. We compare our MIPS observations to empirically derived K-[24] colors as a function of the stellar effective temperature to identify 24 and 70 micron excesses. We confirm the previously published 70 micron excesses for HD 92945, HD 112429, and AU Mic. We present the discovery of 70 micron excesses for five stars: HD 7590, HD 10008, HD 59967, HD 73350, and HD 135599. We also present the detection of 24 micron excesses for ten stars: HD 10008, GJ 3400A, HD 73350, HD 112429, HD 123998, HD 175742, AT Mic, BO Mic, HD 358623 and Gl 907.1. We find that large 70 micron excesses are less common around stars with effective temperatures of less than 5000 K (3.7%) than around stars with effective temperatures between 5000 K and 6000 K (21.4%), despite the cooler stars having a younger median age in our sample (12 vs.340 Myr). We find that the previously reported excess for TWA 13A at 70 microns is due to a nearby background galaxy. In the Appendix, we present an updated analysis of dust grain removal time-scales due to grain-grain collisions and radiation pressure, Poynting-Robertson drag, stellar wind drag and planet-dust dynamical interaction.

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arXiv2009

A Substantial Dust Disk Surrounding an Actively Accreting First-Ascent Giant Star

We report identification of the first unambiguous example of what appears to be a new class of first-ascent giant stars that are actively accreting gas and dust and that are surrounded by substantial dusty disks. These old stars, who are nearing the end of their lives, are experiencing a rebirth into characteristics typically associated with newborn stars. The F2-type first-ascent giant star TYC 4144 329 2 is in a wide separation binary system with an otherwise normal G8 IV star, TYC 4144 329 1. From Keck near-infrared imaging and high-resolution spectroscopy we are able to determine that these two stars are $\sim$1 Gyr old and reside at a distance of $\sim$550 pc. One possible explanation for the origin of the accreting material is common-envelope interaction with a low-mass stellar or sub-stellar companion. The gaseous and dusty material around TYC 4144 329 2, as it is similar to the primordial disks observed around young classical T Tauri stars, could potentially give rise to a new generation of planets and/or planetesimals.

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arXiv2008

Direct Imaging of Multiple Planets Orbiting the Star HR 8799

Direct imaging of exoplanetary systems is a powerful technique that can reveal Jupiter-like planets in wide orbits, can enable detailed characterization of planetary atmospheres, and is a key step towards imaging Earth-like planets. Imaging detections are challenging due to the combined effect of small angular separation and large luminosity contrast between a planet and its host star. High-contrast observations with the Keck and Gemini telescopes have revealed three planets orbiting the star HR 8799, with projected separations of 24, 38, and 68 astronomical units. Multi-epoch data show counter-clockwise orbital motion for all three imaged planets. The low luminosity of the companions and the estimated age of the system imply planetary masses between 5 and 13 times that of Jupiter. This system resembles a scaled-up version of the outer portion of our Solar System.

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arXiv2008

Color Gradients Detected in the HD 15115 Circumstellar Disk

We report HST/NICMOS coronagraphic images of the HD 15115 circumstellar disk at 1.1\micron. We find a similar morphology to that seen in the visible and at H band--an edge-on disk that is asymmetric in surface brightness. Several aspects of the 1.1\micron data are different, highlighting the need for multi-wavelength images of each circumstellar disk. We find a flattening to the western surface brightness profile at 1.1\micron interior to 2\arcsec (90 AU) and a warp in the western half of the disk. We measure the surface brightness profiles of the two disk lobes and create a measure of the dust scattering efficiency between 0.55-1.65\micron at 1\arcsec, 2\arcsec, and 3\arcsec. At 2\arcsec the western lobe has a neutral spectrum up to 1.1\micron and a strong absorption or blue spectrum $>$1.1\micron, while a blue trend is seen in the eastern lobe. At 1\arcsec the disk has a red F110W-H color in both lobes.

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arXiv2008

SSPMJ1102-3431 brown dwarf characterization from accurate proper motion and trigonometric parallax

In 2005, Scholz and collaborators (Scholz et al. 2005) discovered, in a proper motion survey, a young brown dwarf SSSPMJ1102-3431(SSSPMJ1102) of spectral type M8.5, probable member of the TW Hydrae Association (TWA) and possible companion of the T Tauri star TW Hya. The physical characterization of SSSPMJ1102 was based on the hypothesis that it forms a binary system with TW Hya. The recent discovery of a probable giant planet inside the TW Hya protoplanetary disk with a very short-period (Setiawan et al. 2008) and a disk around SSSPMJ1102 (Riaz and Gizis 2008) make it especially interesting and important to measure well the physical parameters of SSSPMJ1102. Trigonometric parallax and proper motion measurements of SSSPMJ1102 are necessary to test for TWA membership and, thus, to determine the mass and age of this young brown dwarf and the possibility that it forms a wide binary system with TW Hya. Two years of regular observations at the ESO NTT/SUSI2 telescope, have enabled us to determine the trigonometric parallax and proper motion of SSSPMJ1102. Our parallax and proper motion determination allow us to precisely describe the physical properties of this low mass object and to confirm its TWA membership. Our results are not incompatible with the hypothesis that SSSPMJ1102 is a binary companion of the star TW Hya.

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arXiv2008

Optical Spectroscopy of Type Ia Supernovae

We present 432 low-dispersion optical spectra of 32 Type Ia supernovae (SNe Ia) that also have well-calibrated light curves. The coverage ranges from 6 epochs to 36 epochs of spectroscopy. Most of the data were obtained with the 1.5m Tillinghast telescope at the F. L. Whipple Observatory with typical wavelength coverage of 3700-7400A and a resolution of ~7A. The earliest spectra are thirteen days before B-band maximum; two-thirds of the SNe were observed before maximum brightness. Coverage for some SNe continues almost to the nebular phase. The consistency of the method of observation and the technique of reduction makes this an ideal data set for studying the spectroscopic diversity of SNe Ia.

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arXiv2008

Spitzer/MIPS Observations of Stars in the Beta Pictoris Moving Group

We present Multiband Imaging Photometer for Spitzer (MIPS) observations at 24 and 70 microns for 30 stars, and at 160 microns for a subset of 12 stars, in the nearby (~30 pc), young (~12 Myr) Beta Pictoris Moving Group (BPMG). In several cases, the new MIPS measurements resolve source confusion and background contamination issues in the IRAS data for this sample. We find that 7 members have 24 micron excesses, implying a debris disk fraction of 23%, and that at least 11 have 70 micron excesses (disk fraction of >=37%). Five disks are detected at 160 microns (out of a biased sample of 12 stars observed), with a range of 160/70 flux ratios. The disk fraction at 24 and 70 microns, and the size of the excesses measured at each wavelength, are both consistent with an "inside-out" infrared excess decrease with time, wherein the shorter-wavelength excesses disappear before longer-wavelength excesses, and consistent with the overall decrease of infrared excess frequency with stellar age, as seen in Spitzer studies of other young stellar groups. Assuming that the infrared excesses are entirely due to circumstellar disks, we characterize the disk properties using simple models and fractional infrared luminosities. Optically thick disks, seen in the younger TW Hya and eta Cha associations, are entirely absent in the BPMG. Additional flux density measurements at 24 and 70 microns are reported for nine Tucanae-Horologium Association member stars. Since this is <20% of the association membership, limited analysis on the complete disk fraction of this association is possible.

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arXiv2005

A companion to AB Pic at the planet/brown dwarf boundary

We report deep imaging observations of the young, nearby star AB Pic, a member of the large Tucana-Horologium as sociation. We have detected a faint, red source 5.5" South of the star with JHK colors compatible with that of a young substellar L dwarf. Follow-up observations at two additional epochs confirm, with a confidence level of 4.7sigma, that the faint red object is a companion to AB Pic rather than it being a stationary background object. A low resolution K-band spectrum indicates an early-L spectral type for the companion. Finally, evolutionary model predictions based on the JHK photometry of AB Pic b indicate a mass of 13 to 14 MJup if its age is ~30Myr. Is AB Pic b a massive planet or a minimum mass brown dwarf?

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arXiv2005

Giant Planet Companion to 2MASSW J1207334-393254

We report new VLT/NACO imaging observations of the young, nearby brown dwarf 2MASSW J1207334-393254 and its suggested planetary mass companion (2M1207 b). Three epochs of VLT/NACO measurements obtained over nearly one year show that the planetary mass companion candidate shares the same proper motion and, with a high confidence level, is not a stationary background object. This result confirms the status of 2M1207 b as of planetary mass (5 times the mass of Jupiter) and the first image of a planetary mass companion in a different system than our own. This discovery offers new perspectives for our understanding of chemical and physical properties of planetary mass objects as well as their mechanisms of formation.

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arXiv2005

Can AB Dor C be a Tight Binary Brown Dwarf?

AB Dor C, a low-mass companion to the young star AB Dor A has recently been imaged. Direct detection and astrometric observations have been used to solve the AB Dor C orbit and obtain a dynamical mass estimate. Near-infrared magnitudes and effective temperatures from models have then been compared with observed values. Models and observations disagree at more than the 2 sigma level. This result prompts Close et al. 2005 to claim that models are overestimating young low-mass object luminosities by roughly a factor of two. This claim is based on the hypothesis that the detected source, AB Dor C, is a single object. Another possible interpretation of the data that was not considered is that AB Dor C is an unresolved binary brown dwarf. Considering that 21 brown dwarfs companions have been found to date around stars and that three of them are confirmed or potential brown dwarf binaries (Gl569Ba & Bb, GL564B & C and Indi Ba & Bb), this suggests that brown dwarf binaries may constitute a significant fraction (14% +/- 8%) of the brown dwarf population around stars. For these reasons, the possibility that AB Dor C is an unresolved binary brown dwarf deserves attention.

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