arXiv subjects

A. M. Ghez

Publications and source records attributed to A. M. Ghez.

69 records · Page 4Linked to original sources

arXiv2003

The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy's Central Black Hole: A Paradox of Youth

We have obtained the first detection of spectral absorption lines in one of the high-velocity stars in the vicinity of the Galaxy's central supermassive black hole. Both Brgamma (2.1661 micron) and He I (2.1126 micron) are seen in absorption in S0-2 with equivalent widths (2.8+-0.3 Ang & 1.7+-0.4 Ang) and an inferred stellar rotational velocity (220+-40 km/s) that are consistent with that of an O8-B0 dwarf, which suggests that it is a massive (~15 Msun), young (<10 Myr) main sequence star. This presents a major challenge to star formation theories, given the strong tidal forces that prevail over all distances reached by S0-2 in its current orbit (130 - 1900 AU) and the difficulty in migrating this star inward during its lifetime from further out where tidal forces should no longer preclude star formation. The radial velocity measurements (-510+-40 km/s) and our reported proper motions for S0-2 strongly constrain its orbit, providing a direct measure of the black hole mass of 4.1(+-0.6)x10^6(Ro/8kpc)^3 Msun. The Keplerian orbit parameters have uncertainities that are reduced by a factor of 2-3 compared to previously reported values and include, for the first time, an independent solution for the dynamical center; this location, while consistent with the nominal infrared position of Sgr A*, is localized to a factor of 5 more precisely (+-2 milli-arcsec). Furthermore, the ambiguity in the inclination of the orbit is resolved with the addition of the radial velocity measurement, indicating that the star is behind the black hole at the time of closest approach and counter-revolving against the Galaxy. With further radial velocity measurements in the next few years, the orbit of S0-2 will provide the most robust estimate of the distance to the Galactic Center.

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arXiv2002

Limits on the Short Term Variability of Sagittarius A* in the Near-Infrared

The recent detection of a 3-hr X-ray flare by the Chandra Observatory has raised the possibility of enhanced emission over a broad range of wavelengths from Sgr A*, the suspected 2.6 x 10^6 solar mass black hole at the Galactic Center, during a flaring event. We have, therefore, reconstructed 3-hr data sets from 2 micron speckle and adaptive optics images (theta_core = 50 - 100 mas) obtained with the W. M. Keck 10-m telescopes between 1995 and 2001. In 25 separate observations, no evidence of any significant excess emission associated with Sgr A* was detected. The lowest of our detection limits gives an observed limit for the quiescent state of Sgr A* of 0.09+/-0.005 mJy, or, equivalently, a dereddened value of 2.0+/-0.1 mJy, which is a factor of 2 lower than the best previously published quiescent value. Under the assumption that there are random 3-hr flares producing both enhanced X-ray and near-infrared emission, our highest limit constrains the variable state of Sgr A* to <0.8 mJy (observed) or 19 mJy (dereddened). These results suggest that the model favored by Markoff et al. (2001), in which the flare is produced through local heating of relativistic particles surrounding Sgr A* (e.g., a sudden magnetic reconnection event), is unlikely, because it predicts peak 2 micron emission of 300 mJy, well above our detection limit.

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arXiv2002

Stellar Companions to Stars with Planets

A combination of high-resolution and wide-field imaging reveals two binary stars and one triple star system among the sample of the first 11 stars with planets detected by radial velocity variations. High resolution speckle or adaptive optics (AO) data probe subarcsecond scales down to the diffraction limit of the Keck 10-m or Lick 3-m, and direct images or AO images are sensitive to a wider field, extending to 10" or 38", depending upon the camera. One of the binary systems -- HD 114762 -- was not previously known to be a spatially resolved multiple system; additional data taken with the combination of Keck adaptive optics and NIRSPEC are used to characterize the new companion. The second binary system -- Tau Boo -- was a known multiple with two conflicting orbital solutions; the current data will help constrain the discrepant estimates of periastron time and separation. Another target -- 16 Cyg B -- was a known common proper motion binary, but the current data resolve a new third component, close to the wide companion 16 Cyg A. Both the HD 114762 and 16 Cyg B systems harbor planets in eccentric orbits, while the Tau Boo binary contains an extremely close planet in a tidally-circularized orbit. Although the sample is currently small, the proportion of binary systems is comparable to that measured in the field over a similar separation range. Incorporating the null result from another companion search project lowers the overall fraction of planets in binary systems, but the detections in our survey reveal that planets can form in binaries separated by less than 50 AU.

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arXiv2002

Adaptive Optics Near-Infrared Spectroscopy of the Sgr A* Cluster

We present K-band $λ/Δλ$ ~ 2600 spectroscopy of five stars (K ~ 14 - 16 mag) within 0.''5 of Sgr A*, the radio source associated with the compact massive object suspected to be a 2.6 x 10$^{6}$ \msun black hole at the center of our Galaxy. High spatial resolution of ~ 0.''09, and good strehl ratios of ~ 0.2 achieved with adaptive optics on the 10-meter Keck telescope make it possible to measure moderate-resolution spectra of these stars individually for the first time. Two stars (S0-17 and S0-18) are identified as late-type stars by the detection of CO bandhead absorption in their spectra. Their absolute K magnitudes and CO bandhead absorption strengths are consistent with early K giants. Three stars (S0-1, S0-2, and S0-16), with r$_{proj}$ $<$ 0.0075 pc (~ 0.''2) from Sgr A*, lack CO bandhead absorption, confirming the results of earlier lower spectral and lower spatial resolution observations that the majority of the stars in the Sgr A* Cluster are early-type stars. The absolute K magnitudes of the early-type stars suggest that they are late O - early B main sequence stars of ages $<$ 20 Myr. The presence of young stars in the Sgr A* Cluster, so close to the central supermassive black hole, poses the intriguing problem of how these stars could have formed, or could have been brought, within its strong tidal field.

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arXiv2002

NICMOS Images of the GG Tau Circumbinary Disk

We present deep, near-infrared images of the circumbinary disk surrounding the pre-main-sequence binary star, GG Tau A, obtained with NICMOS aboard the Hubble Space Telescope. The spatially resolved proto-planetary disk scatters roughly 1.5% of the stellar flux, with a near-to-far side flux ratio of ~1.4, independent of wavelength, and colors that are comparable to the central source; all of these properties are significantly different from the earlier ground-based observations. New Monte Carlo scattering simulations of the disk emphasize that the general properties of the disk, such as disk flux, near side to far side flux ratio and integrated colors, can be approximately reproduced using ISM-like dust grains, without the presence of either circumstellar disks or large dust grains, as had previously been suggested. A single parameter phase function is fitted to the observed azimuthal variation in disk flux, providing a lower limit on the median grain size of 0.23 micron. Our analysis, in comparison to previous simulations, shows that the major limitation to the study of grain growth in T Tauri disk systems through scattered light lies in the uncertain ISM dust grain properties. Finally, we use the 9 year baseline of astrometric measurements of the binary to solve the complete orbit, assuming that the binary is coplanar with the circumbinary ring. We find that the estimated 1 sigma range on disk inner edge to semi-major axis ratio, 3.2 < Rin/a < 6.7, is larger than that estimated by previous SPH simulations of binary-disk interactions.

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arXiv2002

Spatially Resolved Observations of the Galactic Center Source, IRS 21

We present diffraction-limited 2-25 micron images obtained with the W. M. Keck 10-m telescopes that spatially resolve the cool source, IRS 21, one of a small group of enigmatic objects in the central parsec of our Galaxy that have eluded classification. Modeled as a Gaussian, the azimuthally-averaged intensity profile of IRS 21 has a half-width half-maximum (HWHM) size of 650+/-80 AU at 2.2 microns and an average HWHM size of 1600+/-200 AU at mid-infrared wavelengths. These large apparent sizes imply an extended distribution of dust. The mid-infrared color map indicates that IRS 21 is a self-luminous source rather than an externally heated dust clump as originally suggested. The spectral energy distribution has distinct near- and mid-infrared components. A simple radiative transfer code, which simultaneously fits the near- and mid- infrared photometry and intensity profiles, supports a model in which the near-infrared radiation is scattered and extincted light from an embedded central source, while the mid-infrared emission is from thermally re-radiating silicate dust. We argue that IRS 21 (and by analogy the other luminous sources along the Northern Arm) is a massive star experiencing rapid mass loss and plowing through the Northern Arm, thereby generating a bow shock, which is spatially resolved in our observations.

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arXiv2001

Resolved Near-Infrared Spectroscopy of the Mysterious Pre-Main Sequence Binary Systems T Tau S

We obtained new near-infrared images of the prototypical pre-main sequence triple system T Tau, as well as the first resolved medium-resolution spectra of the close pair T Tau S. At the time of our observations, the tight binary had a 13 AU projected separation and showed significant motion since its discovery, three years before. The orbit cannot be strongly constrained yet, but the observed motion of T Tau Sb with respect to T Tau Sa suggests that the system is at least as massive as T Tau N itself. This may indicate that T Tau N is not the most massive star in the system. The spectrum of T Tau Sa, which is totally featureless except for a strong Br gamma emission line, identifies this component with the ``infrared companion'', whose exact nature remains obscure but may be the consequence of it being the most massive component of the system. Contrasting sharply with T Tau Sa, the spectrum of T Tau Sb shows numerous photospheric features consistent with an early-M spectral type. The presence of a strong Br gamma emission line and of a significant veiling continuum classifies this object as a deeply embedded T Tauri star. From these observations, we conclude that both components of T Tau S are embedded in their own dense circumstellar cocoon of material, which are probably fed by a much more extended structure.

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arXiv2001

A High Angular Resolution Multiplicity Survey of the Open Clusters alpha Persei and Praesepe

Two hundred and forty-two members of the Praesepe and alpha Persei clusters ranging from B (~5Msun) to early-M (~0.5 Msun) have been surveyed with high angular resolution imaging. The 39 binary and 1 quadruple systems detected encompass separations from 0".053 to 7".28; 28 of the systems are new detections and there are 9 candidate substellar companions. The main results from the survey are: * Over the projected separation range of 26-581 AU and magnitude differences of DeltaK < 4.0 mag, the companion star fraction (CSF) for alpha Persei is 0.09 +/- 0.03 and for Praesepe is 0.10 +/- 0.03. This fraction is consistent with the field G-dwarf value, implying that there is not a systematic decline in multiplicity with age on timescales of at 4 +1/-1.5 AU, significantly smaller value than both the field G-dwarf and the nearby T Tauri distributions. A simple population synthesis model suggests that the G-dwarf binary population is a combination of ~30% dark cloud and ~70% giant molecular cloud systems. * An exploration of the binary star properties reveals a cluster CSF that increases with decreasing target mass and a cluster mass ratio distribution that rises more sharply for higher mass stars, but is independent of binary separation. These observational trends are consistent with several models of capture in small clusters and simulations of accretion following fragmentation in a cluster environment. * Among the cluster A stars, there is a higher fraction of binaries in the subset with X-ray detections, consistent with the hypothesis that lower mass companions are the true source of X-ray emission. * In alpha Persei, the rotational velocities for solar-type binaries with separations less that 60 AU the rotational evolution of young stars.

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arXiv2001

Observational Constraints on the Formation and Evolution of Binary Stars

We present a high spatial resolution UV to NIR survey of 44 young binary stars in Taurus with separations of 10-1000 AU. The primary results include: (1) The relative ages of binary star components are more similar than the relative ages of randomly paired single stars, supporting coeval formation. (2) Only one of the companion masses is substellar, and hence the apparent overabundance of T Tauri star companions relative to main-sequence star companions can not be explained by a wealth of substellar secondaries that would have been missed in main-sequence surveys. (3) Roughly 10% of T Tauri binary star components have very red NIR colors (K-L > 1.4) and unusually high mass accretion rates. This phenomenon does not appear to be restricted to binary systems, however, since a comparable fraction of single T Tauri stars exhibit the same properties. (4) Although the disk lifetimes of single stars are roughly equal to their stellar ages, the disk lifetimes of binary stars are an order of magnitude less than their ages. (5) The accretion rates for both single and binary T Tauri stars appear to be moderately mass dependent. (6) Although most classical T Tauri star binaries retain both a circumprimary and a circumsecondary disk, there are several systems with only a circumprimary disk. Together with the relative accretion rates, this suggests that circumprimary disks survive longer, on average, than circumsecondary disks. (7) The disk lifetimes, mass ratios, and relative accretion signatures of the closest binaries (10-100 AU) suggest that they are being replenished from a circumbinary reservoir with low angular momentum. Overall, these results support fragmentation as the dominant binary star formation mechanism.

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arXiv2001

An Adaptive Optics Survey for Companions to Stars with Extra-Solar Planets

We have undertaken an adaptive optics (AO) imaging survey of extrasolar planetary systems and stars showing interesting radial velocity trends from high precision radial velocity searches. AO increases the resolution and dynamic range of an image, substantially improving the detectability of faint close companions. This survey is sensitive to objects less luminous than the bottom of the main sequence at separations as close as 1 arcsec. We have detected stellar companions to the planet bearing stars HD 114762 and Tau Boo. We have also detected a companion to the non-planet bearing star 16 Cyg A.

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arXiv2000

Keck Diffraction-Limited Imaging of the Young Quadruple Star System HD 98800

This paper presents diffraction-limited 1-18 micron images of the young quadruple star system HD 98800 obtained with the W. M. Keck 10-m telescopes using speckle and adaptive optics imaging at near-IR wavelengths and direct imaging at mid-IR wavelengths. The two components of the visual binary, A and B, both themselves spectroscopic binaries, were separable at all wavelengths, allowing us to determine their stellar and circumstellar properties. Combining these observations with spectroscopic data from the literature, we derive an age of 10 Myr, masses of 0.93 and 0.64 M_sun and an inclination angle of 58 deg for the spectroscopic components of HD 98800 B, and an age of 10 Myr and a mass of 1.1 M_sun for HD 98800 Aa. Our data confirm that the large mid-IR excess is entirely associated with HD 98800 B. This excess exhibits a black body temperature of 150 K and a strong 10 micron silicate emission feature. The theoretical equilibrium radius of large, perfectly absorbing, 150 K grains around HD 98800 B is 2.4 AU, suggesting a circum-spectroscopic binary distribution. Our observations set important upper limits on the size of the inner dust radius of ~2 AU (mid-IR data) and on the quantity of scattered light of <10% (H-band data). For an inner radius of 2 AU, the dust distribution must have a height of at least 1 AU to account for the fractional dust luminosity of \~20% L_B. Based on the scattered light limit, the dust grains responsible for the excess emission must have an albedo of <0.33. The presence of the prominent silicate emission feature at 10 microns implies dust grain radii of >2 microns. The total mass of the dust, located in a circumbinary disk around the HD 98800 B, is >0.002 M_earth. The orbital dynamics of the A-B pair are likely responsible for the disk geometry.

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arXiv1999

Deep 10 and 18 micron Imaging of the HR 4796A Circumstellar Disk: Transient Dust Particles & Tentative Evidence for a Brightness Asymmetry

We present new 10.8 and 18.2 micron images of HR 4796A, a young A0V star that was recently discovered to have a spectacular, nearly edge-on, circumstellar disk prominent at ~20 microns (Jayawardhana et al. 1998; Koerner et al. 1998). These new images, obtained with OSCIR at Keck II, show that the disk's size at 10 microns is comparable to its size at 18 microns. Therefore, the 18 micron-emitting dust may also emit some, or all, of the 10 micron radiation. Using these multi-wavelength images, we determine a "characteristic" diameter of 2-3 microns for the mid-infrared-emitting dust particles if they are spherical and composed of astronomical silicates. Particles this small are expected to be blown out of the system by radiation pressure in a few hundred years, and therefore these particles are unlikely to be primordial. Dynamical modeling of the disk (Wyatt et al. 2000) indicates that the disk surface density is relatively sharply peaked near 70 AU, which agrees with the mean annular radius deduced by Schneider et al. (1999) from their NICMOS images. We present evidence (~1.8 sigma significance) for a brightness asymmetry that may result from the presence of the hole and the gravitational perturbation of the disk particle orbits by the low-mass stellar companion or a planet. This "pericenter glow," which must still be confirmed, results from a very small (a few AU) shift of the disk's center of symmetry relative to the central star HR 4796A; one side of the inner boundary of the annulus is shifted towards HR 4796A, thereby becoming warmer and more infrared-emitting. The possible detection of pericenter glow implies that the detection of even complex dynamical effects of planets on disks is within reach.

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arXiv1998

High Proper Motion Stars in the Vicinity of Sgr A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy

Over a two year period (1995-1997), we have conducted a diffraction-limited imaging study at 2.2 microns of the inner 6"x6" of the Galaxy's central stellar cluster using the Keck 10-m telescope. The K band images obtained reveal a large population of faint stars. We use an unbiased approach for identifying and selecting stars to be included in this proper motion study, which results in a sample of 90 stars with brightness ranging from K=9-17 and velocities as large as 1,400+-100 km/sec. Compared to earlier work (Eckart et al. 1997; Genzel et al. 1997), the source confusion is reduced by a factor of 9, the number of stars with proper motion measurement in the central 25 arcsec^2 of our galaxy is doubled, and the accuracy of the velocity measurements in the central 1 arcsec^2 is improved by a factor of 4. The peaks of both the stellar surface density and the velocity dispersion are consistent with the position of the unusual radio source and blackhole candidate, Sgr A*, suggesting that Sgr A* is coincident (+-0."1) with the dynamical center of the Galaxy. As a function of distance from Sgr A*, the velocity dispersion displays a falloff well fit by Keplerian motion about a central dark mass of 2.6(+-0.2)x10^6 Mo confined to a volume of at most 10^-6 pc^3, consistent with earlier results. Although uncertainties in the measurements mathematically allow for the matter to be distributed over this volume as a cluster, no realistic cluster is physically tenable. Thus, independent of the presence of Sgr A*, the large inferred central density of at least 10^12 Mo/pc^3, which exceeds the volume-averaged mass densities found at the center of any other galaxy, leads us to the conclusion that our Galaxy harbors a massive central black hole.

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arXiv1998

The Multiplicity of the Hyades and its Implications for Binary Star Formation and Evolution

A 2.2 micron speckle imaging survey of 167 bright (K $< 8.5$ mag) Hyades members reveals a total of 33 binaries with separations spanning 0."044 to 1."34 and magnitude differences as large as 5.5 mag. Of these binaries, 13 are new detections. Combining the results of this survey with previous radial velocity, optical speckle, and direct imaging Hyades surveys, the detected multiplicity of the sample is: 98 singles, 59 binaries, and 10 triples. A statistical analysis of this sample investigates a variety of multiple star formation and evolution theories. Accounting for the inability to detect high flux ratio binaries results in an implied companion star fraction (csf) of 0.30 +/- 0.06 in this separation range 0."1 to 1."07 (5 to 50 AU). The Hyades csf is intermediate between the values derived from observations of T Tauri stars (T Tauri csf=0.40 +/- 0.08) and solar neighborhood G-dwarfs (G-dwarf csf=0.14 +/- 0.03). This result allows for an evolution of the csf from an initially high value for the pre-main sequence to that found for main sequence stars. Within the Hyades, the csf is independent of the radial distance from the cluster center and the primary star mass. The distribution of mass ratios is best fit by a power law q^(-1.3 +/- 0.3) and shows no dependence on the primary mass, binary separation, or the radial distance from the cluster center. Overall, the Hyades data are consistent with scale-free fragmentation, but inconsistent with capture in small clusters and disk-assisted capture in small clusters. Without testable predictions, scale-dependent fragmentation and disk fragmentation cannot be assessed with the Hyades data.

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arXiv1995

Mid-Infrared Imaging of Young Stellar Objects

We present arcsecond resolution mid-infrared (8--13 $μ$m) images and photometry of four young stellar objects (YSOs)~--- L1551-IRS5, HL~Tau, AS~205, and AS~209 (V1121~Oph)~--- taken with the Berkeley Mid-Infrared Camera. For AS~205, a known T Tauri binary, we also present near-infrared JHK images and HKL$^{\prime}$ speckle imaging data. All three single stars are unresolved in our mid-IR images. Our data is the first to resolve in the mid-IR both components of the close binary AS~205 (projected separation $\sim$1.3$^{\prime \prime}$ (210~A.U.)). AS~205~North is the IR brighter star in our data while published observations find it to be the optically fainter star. Pre-main sequence evolutionary models suggest the AS~205 system is non-coeval; we discuss possible explanations for this result and comment on the circumstellar material and evolutionary status of this young binary. Nearly all of our objects exhibit changes in their mid-IR flux in measurements separated by intervals of days up to many years; the variations range from 30--300\%. We speculate that the cause of the variability lies in the accretion disks of these objects; the data suggest disk accretion rate fluctuations of nearly an order of magnitude. The existence of large mid-IR variability argues that simultaneous multiwavelength observations are needed for a proper analysis of YSO spectral energy distributions.

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