arXiv subjects

Inseok Song

Publications and source records attributed to Inseok Song.

At least 91 records · Page 5Linked to original sources

arXiv2012

New Members of the Scorpius Centaurus Complex and Ages of its sub-regions

We have spectroscopically identified about 100 G-, K- and M-type members of the Scorpius Centaurus complex. To deduce the age of these young stars we compare their Li $λ$6708 absorption line strengths against those of stars in the TW Hydrae association and $β$ Pictoris moving group. These line strengths indicate that Sco-Cen stars are younger than $β$ Pic stars whose ages of ~12 Myr have previously been derived from a kinematic traceback analysis. Our derived age,~10Myr, for stars in the LCC and UCL subgroups of ScoCen is younger than previously published ages based on the moving cluster method and upper main sequence fitting. The discrepant ages are likely due to an incorrect (or lack of) cross-calibration between model-dependent and model-independent age-dating methods.

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arXiv2012

Stellar Membership and Dusty Debris Disks in the alpha Persei Cluster

Because of proximity to the Galactic plane, reliable identification of members of the alpha Persei cluster is often problematic. Based primarily on membership evaluations contained in six published papers, we constructed a mostly complete list of high-fidelity members of spectral type G and earlier that lie within 3 arc degrees of the cluster center. Alpha Persei was the one nearby, rich, young open cluster not surveyed with the Spitzer Space Telescope. We examined the first and final data releases of the Wide Field Infrared Survey Explorer (WISE) and found 11, or perhaps 12, alpha Per cluster members that have excess mid-infrared emission above the stellar photosphere attributable to an orbiting dusty debris disk. The most unusual of these is V488 Per, a K-type star with an excess IR luminosity 16% (or more) of the stellar luminosity; this is a larger excess fraction than that of any other known dusty main sequence star. Much of the dust that orbits V488 Per is at a temperature of ~800 K; if these grains radiate like blackbodies, then they lie only ~0.06 AU from the star. The dust is probably the aftermath of a collision of two planetary embryos or planets with small semimajor axes; such orbital radii are similar to those of many of the transiting planets discovered by the Kepler satellite.

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arXiv2012

The VAST Survey -- II. Orbital motion monitoring of A-type star multiples

As a part of our ongoing Volume-limited A-Star (VAST) adaptive optics survey, we have obtained observations of 26 binary systems with projected separations <100 AU, 13 of which have sufficient historical measurements to allow for refinement of their orbital elements. For each system with an estimated orbit, the dynamical system mass obtained was compared with the system mass estimated from mass-magnitude relations. Discrepancies between the dynamical and theoretical system mass can be explained by the presence of a previously unresolved spectroscopic component, or by a non-solar metallicity of the system. Using this approach to infer the presence of additional companions, a lower limit to the fraction of binaries, triples, and quadruples can be estimated as 39, 46, and 15 per cent, for systems with at least one companion within 100 AU. The fraction of multiple systems with three or more components shows a relative increase compared to the fraction for Solar-type primaries resolved in previous volume-limited surveys. The observations have also revealed a pair of potentially young ($<$100 Myr) M-dwarf companions, which would make an ideal benchmark for the theoretical models during the pre-Main Sequence contraction phase for M-dwarfs. In addition to those systems with orbit fits, we report 13 systems for which further orbital monitoring observations are required, 11 of which are newly resolved as a part of the VAST survey.

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arXiv2011

Hunting the Coolest Dwarfs: Methods and Early Results

We present the methods and first results of a survey of nearby high proper motion main sequence stars to probe for cool companions with the Gemini camera at Lick Observatory. This survey uses a sample of old (age > 2 Gyr) stars as targets to probe for companions down to temperatures of 500 K. Multi-epoch observations allow us to discriminate comoving companions from background objects. So far, our survey successfully re-discovers the wide T8.5 companion to GJ 1263 and discovers a companion to the nearby M0V star GJ 660.1. The companion to GJ 660.1 (GJ 660.1B) is ~4 magnitudes fainter than its host star in the J-band and is located at a projected separation of ~120AU. Known trigonometric parallax and 2MASS magnitudes for the GJ 660.1 system indicate a spectral type for the companion of M9 +/- 2.

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arXiv2011

The Tucana/Horologium, Columba, AB Doradus, and Argus Associations: New Members and Dusty Debris Disks

We propose 35 star systems within ~70 pc of Earth as newly identified members of nearby young stellar kinematic groups; these identifications include the first A- and late-B type members of the AB Doradus moving group and field Argus Association. All but one of the 35 systems contain a bright solar- or earlier-type star that should make an excellent target for the next generation of adaptive optics (AO) imaging systems on large telescopes. AO imaging has revealed four massive planets in orbit around the λ Boo star HR 8799. Initially the planets were of uncertain mass due in large part to the uncertain age of the star. We find that HR 8799 is a likely member of the ~30 Myr old Columba Association implying planet masses ~6 times that of Jupiter. We consider Spitzer Space Telescope MIPS photometry of stars in the ~30 Myr old Tucana/Horologium and Columba Associations, the ~40 Myr old field Argus Association, and the ~70 Myr old AB Doradus moving group. The percentage of stars in these young stellar groups that display excess emission above the stellar photosphere at 24 and 70 μm wavelengths - indicative of the presence of a dusty debris disk - is compared with corresponding percentages for members of 11 open clusters and stellar associations with ages between 8 and 750 Myr, thus elucidating the decay of debris disks with time.

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arXiv2011

The Ultra Cool Brown Dwarf Companion of WD 0806-661: Age, Mass, and Formation Mechanism

We have combined multi-epoch images from the Infrared Side Port Imager on the CTIO 4-meter telescope to derive a 3-sigma limit of J=21.7 for the ultra cool brown dwarf companion to WD 0806-661 (GJ 3483). We find that J-[4.5]>4.95, redder than any other brown dwarf known to date. With theoretical evolutionary models and ages 1.5-2.7 Gyr, we estimate the brown dwarf companion to have mass <10-13 Jupiter masses and temperature <400 K, providing evidence that this is among the coolest brown dwarfs currently known. The range of masses for this object is consistent with that anticipated from Jeans-mass fragmentation and we present this as the likely formation mechanism. However, we find that substellar companions of similar mass (~7-17 Jupiter masses) are distributed over a wide range of semi-major axes, which suggests that giant planet and low-mass brown dwarf formation overlap in this mass range.

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arXiv2011

The VAST Survey - I. Companions and the unexpected X-ray detection of B6-A7 stars

With an adaptive optics imaging survey of 148 B6-A7 stars, we have tested the hypothesis that unresolved lower-mass companions are the source of the unexpected X-ray detections of stars in this spectral type range. The sample is composed of 63 stars detected in X-rays within the ROSAT All-Sky Survey and 85 stars that form a control sample; both subsets have the same restricted distribution of spectral type, age, X-ray sensitivity and separation coverage. A total of 68 companion candidates are resolved with separations ranging from 0.3" to 26.2", with 23 new detections. The multiple star frequency of the X-ray sample based on companions resolved within the ROSAT error ellipse is found to be 43 (+6,-6)%. The corresponding control sample multiple star frequency is three times lower at 12 (+4,-3)% -- a difference of 31\pm7%. These results are presented in the first of a series of papers based on our Volume-limited A-Star (VAST) survey -- a comprehensive study of the multiplicity of A-type stars.

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arXiv2010

Nearby Motionless Stars

We present methods and preliminary results of a relatively novel search for nearby stars. The method relies on photometric distance estimates as its primary search criterion, thus distinguishing itself from proper motion-based searches that have produced the bulk of nearby star discoveries.

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arXiv2010

The Spitzer Atlas of Stellar Spectra

We present the Spitzer Atlas of Stellar Spectra (SASS), which includes 159 stellar spectra (5 to 32 mic; R~100) taken with the Infrared Spectrograph on the Spitzer Space Telescope. This Atlas gathers representative spectra of a broad section of the Hertzsprung-Russell diagram, intended to serve as a general stellar spectral reference in the mid-infrared. It includes stars from all luminosity classes, as well as Wolf-Rayet (WR) objects. Furthermore, it includes some objects of intrinsic interest, like blue stragglers and certain pulsating variables. All the spectra have been uniformly reduced, and all are available online. For dwarfs and giants, the spectra of early-type objects are relatively featureless, dominated by Hydrogen lines around A spectral types. Besides these, the most noticeable photospheric features correspond to water vapor and silicon monoxide in late-type objects and methane and ammonia features at the latest spectral types. Most supergiant spectra in the Atlas present evidence of circumstellar gas. The sample includes five M supergiant spectra, which show strong dust excesses and in some cases PAH features. Sequences of WR stars present the well-known pattern of lines of HeI and HeII, as well as forbidden lines of ionized metals. The characteristic flat-top shape of the [Ne III] line is evident even at these low spectral resolutions. Several Luminous Blue Variables and other transition stars are present in the Atlas and show very diverse spectra, dominated by circumstellar gas and dust features. We show that the [8]-[24] Spitzer colors (IRAC and MIPS) are poor predictors of spectral type for most luminosity classes.

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arXiv2010

Shocks and a Giant Planet in the Disk Orbiting BP Piscium?

Spitzer IRS spectroscopy supports the interpretation that BP Piscium, a gas and dust enshrouded star residing at high Galactic latitude, is a first-ascent giant rather than a classical T Tauri star. Our analysis suggests that BP Piscium's spectral energy distribution can be modeled as a disk with a gap that is opened by a giant planet. Modeling the rich mid-infrared emission line spectrum indicates that the solid-state emitting grains orbiting BP Piscium are primarily composed of ~75 K crystalline, magnesium-rich olivine; ~75 K crystalline, magnesium-rich pyroxene; ~200 K amorphous, magnesium-rich pyroxene; and ~200 K annealed silica ('cristobalite'). These dust grains are all sub-micron sized. The giant planet and gap model also naturally explains the location and mineralogy of the small dust grains in the disk. Disk shocks that result from disk-planet interaction generate the highly crystalline dust which is subsequently blown out of the disk mid-plane and into the disk atmosphere.

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arXiv2010

The Age of the HD 15407 System and the Epoch of Final Catastrophic Mass Accretion onto Terrestrial Planets around Sun-like Stars

From optical spectroscopic measurements we determine that the HD 15407 binary system is ~80 Myr old. The primary, HD 15407A (spectral type F5V), exhibits strong mid-infrared excess emission indicative of a recent catastrophic collision between rocky planetary embryos or planets in its inner planetary system. Synthesis of all known stars with large quantities of dust in their terrestrial planet zone indicates that for stars of roughly Solar mass this warm dust phenomenon occurs at ages between 30 and 100 Myr. In contrast, for stars of a few Solar masses, the dominant era of the final assembling of rocky planets occurs earlier, between 10 and 30 Myr age. The incidence of the warm dust phenomenon, when compared against models for the formation of rocky terrestrial-like bodies, implies that rocky planet formation in the terrestrial planet zone around Sun-like stars is common.

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arXiv2008

The Minimum Jeans Mass, Brown Dwarf Companion IMF, and Predictions for Detection of Y-type Dwarfs

Cool L- and T-type objects were discovered first as companions to stars in 1988 and 1995, respectively. A certain example of the yet cooler Y-type spectral class (Teff <~ 500K?) has not been seen. Recent infrared imaging observations of stars and brown dwarfs indicate that substellar companions with large semi-major axes and with masses less than the brown dwarf/giant planet dividing line (~13.5 Mj) are rare. Theoretical considerations of Jeans mass fragmentation of molecular clouds are consistent with this minimum mass cutoff and also with the semi-major axis (hundreds of AU) characteristic of the lowest mass imaged companions. As a consequence, Y-class companions with large semi-major axes should be scarce around stars <2Gyr old, and also around substellar primaries of all ages. By focusing on brown dwarf companions to young stellar primaries, it is possible to derive a first estimate of the brown dwarf IMF over the entire range of brown dwarf masses (13 Mj to 79 Mj) -- the number of companion brown dwarfs is proportional to mass to the -1.2+-0.2 power.

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arXiv2008

Mid-Infrared Properties of Disk Averaged Observations of Earth with AIRS

We have investigated mid-infrared spectra of Earth obtained by the Atmospheric Infrared Sounder (AIRS) instrument on-board the AQUA spacecraft to explore the characteristics that may someday be observed in extrasolar terrestrial planets. We have used the AIRS infrared (R ~ 1200; 3.75-15.4 microns) spectra to construct directly-observed high-resolution spectra of the only known life bearing planet, Earth. The AIRS spectra are the first such spectra that span the seasons. We investigate the rotational and seasonal spectral variations that would arise due to varying cloud amount and viewing geometry and we explore what signatures may be observable in the mid-infrared by the next generation of telescopes capable of observing extrasolar terrestrial planets.

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arXiv2008

Warm dust in the terrestrial planet zone of a sun-like Pleiad: collisions between planetary embryos?

Only a few solar-type main sequence stars are known to be orbited by warm dust particles; the most extreme is the G0 field star BD+20 307 that emits ~4% of its energy at mid-infrared wavelengths. We report the identification of a similarly dusty star HD 23514, an F6-type member of the Pleiades cluster. A strong mid-IR silicate emission feature indicates the presence of small warm dust particles, but with the primary flux density peak at the non-standard wavelength of ~9 micron. The existence of so much dust within an AU or so of these stars is not easily accounted for given the very brief lifetime in orbit of small particles. The apparent absence of very hot (>~1000 K) dust at both stars suggests the possible presence of a planet closer to the stars than the dust. The observed frequency of the BD+20 307/HD 23514 phenomenon indicates that the mass equivalent of Earth's Moon must be converted, via collisions of massive bodies, to tiny dust particles that find their way to the terrestrial planet zone during the first few hundred million years of the life of many (most?) sun-like stars. Identification of these two dusty systems among youthful nearby solar-type stars suggests that terrestrial planet formation is common.

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arXiv2008

Gas and Dust Associated with the Strange, Isolated, Star BP Piscium

We have carried out a multiwavelength observational campaign demonstrating some of the remarkable properties of the infrared-bright variable star BP Psc. Surrounded by a compact dusty, gaseous disk, this little-studied late-G (or early-K) type star emits about 75% of its detected energy flux at infrared wavelengths. Evidence for accretion of gas in conjunction with narrow bi-polar jets and Herbig-Haro objects is apparently consistent with classification of BP Psc as a pre-main sequence star, as postulated in most previous studies. If young, then BP Psc would be one of the nearest and oldest known classical T Tauri stars. However, such an evolutionary classification encounters various problems that are absent or much less severe if BP Psc is instead a luminosity class III post-main sequence star. In this case, it would be the first known example of a first ascent giant surrounded by a massive molecular disk with accompanying rapid gas accretion and prominent jets and HH objects. In this model, the genesis of the massive dusty gaseous disk could be a consequence of the envelopment of a low mass companion star. Properties in the disk may be conducive to the current formation of planets, a gigayear or more after the formation of BP Psc itself.

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arXiv2007

An accurate distance to 2M1207Ab

In April 2004 the first image was obtained of a planetary mass companion (now known as 2M1207 b) in orbit around a self-luminous object different from our own Sun (the young brown dwarf 2MASSW J1207334-393254, hereafter 2M1207 A). 2M1207 b probably formed via fragmentation and gravitational collapse, offering proof that such a mechanism can form bodies in the planetary mass regime. However, the predicted mass, luminosity, and radius of 2M1207 b depend on its age, distance, and other observables such as effective temperature. To refine our knowledge of the physical properties of 2M1207 b and its nature, we obtained an accurate determination of the distance to the 2M1207 A and b system by measurements of its trigonometric parallax at the milliarcsec level. With the ESO NTT/SUSI2 telescope, in 2006 we began a campaign of photometric and astrometric observations to measure the trigonometric parallax of 2M1207 A. An accurate distance ($52.4\pm 1.1$ pc) to 2M1207A was measured. From distance and proper motions we derived spatial velocities fully compatible with TWA membership. With this new distance estimate, we discuss three scenarios regarding the nature of 2M1207 b: (1) a cool ($1150\pm150$ K) companion of mass $4\pm1$ M$_{\rm{Jup}}$, (2) a warmer ($1600\pm100$ K) and heavier ($8\pm2$ M$_{\rm{Jup}}$) companion occulted by an edge-on circum-secondary disk or (3) a hot protoplanet collision afterglow.

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arXiv2007

EF Cha: Warm Dust Orbiting a Nearby 10 Myr Old Star

Most Vega-like stars have far-infrared excess (60micron or longward in IRAS, ISO, or Spitzer MIPS bands) and contain cold dust (<~150K) analogous to the Sun's Kuiper-Belt region. However, dust in a region more akin to our asteroid belt and thus relevant to the terrestrial planet building process is warm and produces excess emission in mid-infrared wavelengths. By cross-correlating Hipparcos dwarfs with the MSX catalog, we found that EF Cha, a member of the recently identified, ~10 Myr old, ``Cha-Near'' Moving Group, possesses prominent mid-infrared excess. N-band spectroscopy reveals a strong emission feature characterized by a mixture of small, warm, amorphous and possibly crystalline silicate grains. Survival time of warm dust grains around this A9 star is <~ 1E5 yrs, much less than the age of the star. Thus, grains in this extra-solar terrestrial planetary zone must be of "second generation" and not a remnant of primodial dust and are suggestive of substantial planet formation activity. Such second generation warm excess occurs around ~ 13% of the early-type stars in nearby young stellar associations.

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arXiv2007

The Wide Brown Dwarf Binary Oph 1622-2405 and Discovery of A Wide, Low Mass Binary in Ophiuchus (Oph 1623-2402): A New Class of Young Evaporating Wide Binaries?

We imaged five objects near the star forming clouds of Ophiuchus with the Keck Laser Guide Star AO system. We resolved Allers et al. (2006)'s #11 (Oph 16222-2405) and #16 (Oph 16233-2402) into binary systems. The #11 object is resolved into a 243 AU binary, the widest known for a very low mass (VLM) binary. The binary nature of #11 was discovered first by Allers (2005) and independently here during which we obtained the first spatially resolved R~2000 near-infrared (J & K) spectra, mid-IR photometry, and orbital motion estimates. We estimate for 11A and 11B gravities (log(g)>3.75), ages (5+/-2 Myr), luminosities (log(L/Lsun)=-2.77+/-0.10 and -2.96+/-0.10), and temperatures (Teff=2375+/-175 and 2175+/-175 K). We find self-consistent DUSTY evolutionary model (Chabrier et al. 2000) masses of 17+4-5 MJup and 14+6-5 MJup, for 11A and 11B respectively. Our masses are higher than those previously reported (13-15 MJup and 7-8 MJup) by Jayawardhana & Ivanov (2006b). Hence, we find the system is unlikely a ``planetary mass binary'', (in agreement with Luhman et al. 2007) but it has the second lowest mass and lowest binding energy of any known binary. Oph #11 and Oph #16 belong to a newly recognized population of wide (>100 AU), young (<10 Myr), roughly equal mass, VLM stellar and brown dwarf binaries. We deduce that ~6+/-3% of young (<10 Myr) VLM objects are in such wide systems. However, only 0.3+/-0.1% of old field VLM objects are found in such wide systems. Thus, young, wide, VLM binary populations may be evaporating, due to stellar encounters in their natal clusters, leading to a field population depleted in wide VLM systems.

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