arXiv subjects

E. E. Becklin

Publications and source records attributed to E. E. Becklin.

At least 73 records · Page 4Linked to original sources

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.

astro-ph↗

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.

astro-ph↗

arXiv2001

Infrared Views of the TW Hya Disk

The face-on disk around TW Hya is imaged in scattered light at wavelengths of 1.1 and 1.6 micron using the coronagraph in the Near Infrared Camera and Multi Object Spectrometer aboard the Hubble Space Telescope. Stellar light scattered from the optically thick dust disk is seen from 20-230 AU. The surface brightness declines as a power law of r^(-2.6+/-0.1) between 45 and 150 AU. The scattering profile indicates that the disk is flared, not geometrically flat. The disk, while spatially unresolved in thermal radiation at wavelengths of 12 and 18 micron in observations from the W. M. Keck Observatory, shows amorphous and crystalline silicate emission in its spectrum. A disk with silicate grains of a ~1 micron in size in its surface layers can explain the shape of the mid-infrared spectrum. Much larger grains in the disk interior are necessary to fit the millimeter-wave spectral energy distribution, and hence grain growth from an original interstellar size population may have occurred.

astro-ph↗

arXiv2001

Are Starburst Galaxies the Hosts of Gamma-Ray Bursts ?

We present deep 2.2 micron imaging of twelve gamma-ray burst host galaxies. Template spectral energy distributions are fit to the multiband photometry between visible and near-infrared wavelengths to derive a better constraint on the stellar mass of these galaxies. The internal extinction in the host galaxies is estimated using the rest-frame ultraviolet (UV) slope. We find that the extinction corrected star-formation rates (SFRs) of the galaxies are significantly larger than rates derived from emission lines in the visible or the UV continuum. The ratio between the extinction corrected SFRs and stellar mass for 7 of the host galaxies is high compared to local starbursts and 3 of the hosts have derived far-infrared luminosities comparable to infrared luminous galaxies. In addition, existing observational data reveal that at least 6 of the 11 putative hosts seem to be disturbed or have companion galaxies within a projected angular separation of about 2.5 arcsec. If we assume that the host and the companion are at similar redshifts, this corresponds to a physical separation of less than 20 kpc, providing some evidence for an ongoing/recent tidal encounter. We conclude that tidally-induced starbursts such as those found in infrared luminous galaxies might be popular birthplaces for gamma-ray bursts. The age of the stellar population in 4 out of 6 galaxies is rather young, of order 10 Myr. This favors models where gamma-ray bursts are due to the core-collapse of isolated, massive stars and explosion of the resultant black hole-accretion disk system.

astro-ph↗

arXiv2001

The Altitude of an Infrared Bright Cloud Feature on Neptune from Near-Infrared Spectroscopy

We present 2.03-2.30 micron near-infrared spectroscopy of Neptune taken 1999 June 2 (UT) with the W.M. Keck Observatory's near-infrared spectrometer (NIRSPEC) during the commissioning of the instrument. The spectrum is dominated by a bright cloud feature, possibly a storm or upwelling, in the southern hemisphere at approximately 50 degrees S latitude. The spectrum also includes light from a dimmer northern feature at approximately 30 degrees N latitude. We compare our spectra (R ~ 2000) of these two features with a simple model of Neptune's atmosphere. Given our model assumption that the clouds are flat reflecting layers, we find that the top of the bright southern cloud feature sat at a pressure level of 0.14 (+0.05, -0.03) bar, and thus this cloud did not extend into the stratosphere (P < 0.1 bar). A similar analysis of the dimmer northern feature gives a cloud-top pressure of 0.084 +/- 0.026 bar. This suggests that the features we observed efficiently transport methane to the base of the stratosphere, but do not directly transport methane to the upper stratosphere (P < 0.01-0.001 bar) where photolysis occurs. Our observations do not constrain how far these clouds penetrate down into the troposphere. We find that our model fits to the data restrict the fraction of molecular hydrogen in ortho/para thermodynamic equilibrium to greater than 0.8.

astro-ph↗

arXiv2001

H2 and CO emission from disks around TTauri and HerbigAe pre-main-sequence stars and from debris disks around young stars: warm and cold circumstellar gas

We present ISO-SWS observations of H2 pure-rotational line emission from the disks around low and intermediate mass pre-main-sequence stars as well as from young stars thought to be surrounded by debris disks. We detect `warm' (T ~ 100-200 K) H2 gas around many sources, including tentatively the debris-disk objects. The mass of this warm gas ranges from ~1E-4 Solar mass up to 8E-3 Solar mass, and can constitute a non-negligible fraction of the total disk mass. Complementary single-dish 12CO 3-2, 13CO 3-2 and 12CO 6-5 observations have been obtained as well. These transitions probe cooler gas at T ~ 20-80 K. Most objects show a double-peaked CO emission profile characteristic of a disk in Keplerian rotation, consistent with interferometer data on the lower-J lines. The ratios of the 12CO 3-2/ 13CO 3-2 integrated fluxes indicate that 12CO 3-2 is optically thick but that 13CO 3-2 is optically thin or at most moderately thick. The 13CO 3-2 lines have been used to estimate the cold gas mass. If a H2/CO conversion factor of 1E4 is adopted, the derived cold gas masses are factors of 10-200 lower than those deduced from 1.3 millimeter dust emission assuming a gas/dust ratio of 100,in accordance with previous studies. The warm gas is typically 1-10 % of the total mass deduced from millimeter continuum emission, but can increase up to 100% or more for the debris-disk objects. Thus, residual molecular gas may persist into the debris-disk phase. No significant evolution in the H2, CO or dust masses is found for stars with ages in the range of 1E6-1E7 years, although a decrease is found for the older debris-disk star beta Pictoris. Existing models fail to explain the amount of warm gas quantitatively.

astro-ph↗

arXiv2001

High Resolution Mid-Infrared Imaging of Infrared Luminous Starburst Galaxies

Observations for seven infrared luminous starburst galaxies are reported in the mid-infrared from 8 - 18 microns using the Keck Telescopes with spatial resolution approaching the diffraction limit. All of the galaxies observed show evidence of strong interactions based on optical morphologies. For these galaxies, a substantial fraction, usually more than 50%, of the infrared luminosity is generated in regions ranging in sizes from 100pc -- 1 Kpc. Nuclear starbursts often dominate the infrared luminosity, but this is not always true. In some galaxies, most notably NGC 6090, substantial infrared luminosity greatly in excess of the nuclear luminosity is generated in regions associated with the physical interaction between two galaxies. The radio emission is a good tracer of the location of high luminosity young stars. The visual/ultraviolet radiation output of the nearby star forming galaxies is dominated by emission from regions that are generally not producing the copious infrared luminosity of the systems. The regions of high infrared luminosity in local starburst galaxies are significantly smaller than the galaxies as a whole. The integrated spectral energy distributions (SEDs) of these galaxies are very different from the SEDs of the regions of star formation. If the SEDs of star-forming regions in these galaxies reflect the SEDs found in forming galaxies at high redshift, the distant galaxies should be dominated by the mid and far-infrared luminosity output far more than the integrated luminous output of nearby starburst galaxies would suggest.

astro-ph↗

arXiv2001

Finding Brown Dwarf Companions with HST/NICMOS

We present the results of a HST/NICMOS coronagraphic survey for the direct detection of substellar companions within the young TW Hydrae and Tucana Associations. At the distance of these associations, the lower mass limit of detection, based on models, is well into the high mass planet region for separations > 30 AU. Results presented here include spectra and proper motion verification of two brown dwarf companions, TWA 5B and HR 7329B, located 100 AU and 180 AU from their primaries, respectively. We also present a possible exo-solar giant planet candidate located 125 AU from TWA 6. These few examples demonstrate that the young associations remain fertile ground for discovery and environmental study of planetary systems.

astro-ph↗

arXiv2001

Dust to Dust: Evidence for Planet Formation?

We discuss the properties of several circumstellar debris disk systems imaged with the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer in a survey of young stars with known far-IR excesses. These dusty disks around young ($\sim$ 5--8 Myr) unembedded stars exhibit morphological anisotropies and other characteristics which are suggestive of recent or on-going planet formation. We consider evidence for the evolution of populations of collisionally produced disk grains in light of the significant presence of remnant primordial gas in the optically thick disk of the classical T-Tauri star TW Hya; the dust-dominated and Kuiper-belt like circumstellar ring about the young main-sequence star HR 4796A; and a possible ``intermediate'' case, the complex disk around the Herbig AeBe star HD 141569A. Only a small number of debris disks have been imaged thus far in scattered light. Yet all show structures that may be indicative of dust reprocessing, possibly as a result of planet formation, and speak to the contemporary competing scenarios of disk/planet evolution and formation time scales.

astro-ph↗

arXiv2001

Dusty Debris Around Solar-Type Stars: Temporal Disk Evolution

Using ISO-ISOPHOT we carried out a survey of almost 150 stars to search for evidence of emission from dust orbiting young main sequence stars, both in clusters and isolated systems. Over half of the detections are new examples of dusty stellar systems, and demonstrate that such dust can be detected around numerous stars older than a few x 10^6 years. Fluxes at 60 microns and either 90 or 100 microns for the new excess sources together with improved fluxes for a number of IRAS-identified sources are presented. Analysis of the excess luminosity relative to the stellar photosphere shows a systematic decline of this excess with stellar age consistent with a power law index of -2.

astro-ph↗

arXiv2000

NICMOS Coronagraphic Observations of 55 Cancri

We present new near-infrared (1.1-micron) observations of the circumstellar environment of the planet-bearing star 55 Cancri. With these Hubble Space Telescope images we are unable to confirm the observation of bright scattered radiation at longer NIR wavelengths previously reported by Trilling & Brown (1998), and Trilling, Brown & Rivkin (2000). NICMOS coronagraphic images with detection sensitivities to ~ 100 microJy arcsec$^{-2}$ at 1.1 microns in the region 28 - 60 AU from the star fail to reveal any significant excess flux in point- spread-function (PSF) subtracted images taken in two Hubble Space Telescope orbits. These new observations place flux densities in the 19-28 AU zone at a factor of ten or more below the reported ground-based observations. Applying a suite of a dozen well-matched coronagraphic reference PSFs, including one obtained in the same orbits as the observations of 55 Cnc, yielded consistently null results in detecting a disk. We also searched for, and failed to find, a suggested flux-excess anisotropy in the ratio of ~1.7:1 in the circumstellar background along and orthogonal to the plane of the putative disk. We suggest that, if such a disk does exist, then the total 1.1-micron spectral flux density in an annular zone 28 - 42 AU from the star must be no more than ~0.4mJy, at least ten times smaller than suggested by Trilling and Brown, upon which their very large estimate for the total dust mass (0.4~$M_{\earth}$) was based. Based on the far infrared and submillimeter flux of this system and observations of scattered light and thermal emission from other debris disks, we also expect the intensity of the scattered light to be at least an order of magnitude below our upper limits.

astro-ph↗

arXiv2000

The Accelerations of Stars Orbiting the Milky Way's Central Black Hole

Recent measurements, of the velocities of stars near the center of the Milky Way have provided the strongest evidence for the presence of a supermassive black hole in a galaxy, but the observational uncertainties poorly constrain many of the properties of the black hole. Determining the accelerations of stars in their orbits around the center provides much more precise information about the position and mass of the black hole. Here we report measurements of the accelerations for three stars located ~0.005 pc from the central radio source Sgr A*; these accelerations are comparable to those experienced by the Earth as it orbits the Sun. These data increase the inferred minimum mass density in the central region of the Galaxy by an order of magnitude relative to previous results and localized the dark mass to within 0.05 +- 0.04 arcsec of the nominal position of Sgr A*. In addition, the orbital period of one of the observed stars could be as short as 15 years, allowing us the opportunity in the near future to observe an entire period.

astro-ph↗

arXiv2000

Substellar Companions to Nearby Stars from NICMOS Surveys

A coronagraphic imaging survey of 65 nearby stars was conducted using the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope by the Instrument Definition Team. Using these guaranteed time observations we searched for very low mass stellar and substellar (extra-solar giant planet and brown dwarf) companions to young main sequence and late M dwarf stars. Nineteen additional stars, similarly observed in our circumstellar disk program, were also examined for evidence of companions. We discuss the large depth and dynamic range of these surveys which have given rise to several putative and confirmed detections. Using follow-up spectrographic observations obtained with the Space Telescope Imaging Spectrograph (STIS) we have begun to characterize the physical nature of the objects found in the companion search program.

astro-ph↗

arXiv2000

Stellar Companions and the Age of HD 141569 and its Circumstellar Disk

We investigate the stellar environment of the Beta Pictoris-like star HD 141569 with optical spectroscopy and near infrared imaging. The B9.5Ve primary, and an M2 and an M4 type star both located in projection less than nine arseconds away have the same radial velocity and proper motion and therefore almost certainly form a triple star system. From their x-ray flux, lithium absorption and location on pre-main-sequence evolutionary tracks, the companions appear to be 5 Myr old. HD 141569 A is now one of the few stars with a circumstellar disk that has a well-determined age. The circumstellar disk is composed of secondary debris material (Fisher et al. 2000), thus placing an upper limit on the time needed for disk processing. These three stars may be part of an association of young stars.

astro-ph↗

arXiv2000

Measurement of [OIII] Emission in Lyman Break Galaxies

Measurements of [OIII] emission in Lyman Break galaxies (LBGs) at z>3 are presented. Four galaxies were observed with narrow-band filters using the Near-IR Camera on the Keck I 10-m telescope. A fifth galaxy was observed spectroscopically during the commissioning of NIRSPEC, the new infrared spectrometer on Keck II. The emission-line spectrum is used to place limits on the metallicity. Comparing these new measurements with others available from the literature, we find that strong oxygen emission in LBGs may suggest sub-solar metallicity for these objects. The [OIII]5007 line is also used to estimate the star formation rate (SFR) of the LBGs. The inferred SFRs are higher than those estimated from the UV continuum, and may be evidence for dust extinction.

astro-ph↗

arXiv2000

A Candidate Substellar Companion to HR 7329

We present the discovery of a candidate substellar companion from a survey of nearby, young stars with the NICMOS coronagraph on the Hubble Space Telescope. The H ~ 12 mag object was discovered approximately 4" from the young A0V star HR 7329. Using follow-up spectroscopy from STIS, we derive a spectral type between M7V and M8V with an effective temperature of ~ 2600 K. We estimate that the probability of a chance alignment with a foreground dwarf star of this nature is ~ 10^(-8) and therefore suggest the object (HR 7329B) is physically associated with HR 7329 with a projected separation of 200 AU. Current brown dwarf cooling models indicate a mass of less than 50 Jupiter masses for HR 7329B based on age estimates of < 30 Myr for HR7329A.

astro-ph↗

arXiv2000

J-Band Infrared Spectroscopy of a Sample of Brown Dwarfs Using Nirspec on Keck II

Near-infrared spectroscopic observations of a sample of very cool, low-mass objects are presented with higher spectral resolution than in any previous studies. Six of the objects are L-dwarfs, ranging in spectral class from L2 to L8/9, and the seventh is a methane or T-dwarf. These new observations were obtained during commissioning of NIRSPEC, the first high-resolution near-infrared cryogenic spectrograph for the Keck II 10-meter telescope on Mauna Kea, Hawaii. Spectra with a resolving power of R=2500 from 1.135 to 1.360 microns (approximately J-band) are presented for each source. At this resolution, a rich spectral structure is revealed, much of which is due to blending of unresolved molecular transitions. Strong lines due to neutral potassium (K I), and bands due to iron hydride (FeH) and steam (H2O) change significantly throughout the L sequence. Iron hydride disappears between L5 and L8, the steam bands deepen and the K I lines gradually become weaker but wider due to pressure broadening. An unidentified feature occurs at 1.22 microns which has a temperature dependence like FeH but has no counterpart in the available FeH opacity data. Because these objects are 3-6 magnitudes brighter in the near-infrared compared to the I-band, spectral classification is efficient. One of the objects studied (2MASSW J1523+3014) is the coolest L-dwarf discovered so far by the 2-Micron All-Sky Survey (2MASS), but its spectrum is still significantly different from the methane-dominated objects such as Gl229B or SDSS 1624+0029.

astro-ph↗

arXiv2000

The Rest-Frame Optical Spectrum of MS 1512-cB58

Moderate resolution, near-IR spectroscopy of MS1512-cB58 is presented, obtained during commissioning of the the Near IR Spectrometer (NIRSPEC) on the Keck II telescope. The strong lensing of this z=2.72 galaxy by the foreground cluster MS1512+36 makes it the best candidate for detailed study of the rest-frame optical properties of Lyman Break Galaxies. A redshift of z=2.7290+/-0.0007 is inferred from the emission lines, in contrast to the z=2.7233 calculated from UV observations of interstellar absorption lines. Using the Balmer line ratios, we find an extinction of E(B-V)=0.27. Using the line strengths, we infer an SFR=620+/-18 Msun/yr (H_0=75, q_0=0.1, Lambda =0), a factor of 2 higher than that measured from narrow-band imaging observations of the galaxy, but a factor of almost 4 lower than the SFR inferred from the UV continuum luminosity. The width of the Balmer lines yields a mass of M_vir=1.2x10^10 Msun. We find that the oxygen abundance is 1/3 solar, in good agreement with other estimates of the metallicity. However, we infer a high nitrogen abundance, which may argue for the presence of an older stellar population.

astro-ph↗