arXiv subjects

James E. Larkin

Publications and source records attributed to James E. Larkin.

83 records · Page 5Linked to original sources

arXiv2006

First High-Contrast Science with an Integral Field Spectrograph: the Sub-Stellar Companion to GQ Lup

We present commissioning data from the OSIRIS integral field spectrograph (IFS) on the Keck II 10 m telescope that demonstrate the utility of adaptive optics IFS spectroscopy in studying faint close-in sub-stellar companions in the haloes of bright stars. Our R~2000 J- and H-band spectra of the sub-stellar companion to the 1-10 Myr-old GQ Lup complement existing K-band spectra and photometry, and improve on the original estimate of its spectral type. We find that GQ Lup B is somewhat hotter (M6-L0) than reported in the discovery paper by Neuhauser and collaborators (M9-L4), mainly due to the surface-gravity sensitivity of the K-band spectral classification indices used by the discoverers. Spectroscopic features characteristic of low surface gravity objects, such as lack of alkali absorption and a triangular H-band continuum, are indeed prominent in our spectrum of GQ Lup B. The peculiar shape of the H-band continuum and the difference between the two spectral type estimates is well explained in the context of the diminishing strength of H2 collision induced absorption with decreasing surface gravity, as recently proposed for young ultra-cool dwarfs by Kirkpatrick and collaborators. Using our updated spectroscopic classification of GQ Lup B and a re-evaluation of the age and heliocentric distance of the primary, we perform a comparative analysis of the available sub-stellar evolutionary models to estimate the mass of the companion. We find that the mass of GQ Lup B is 0.010-0.040 Msun. Hence, it is unlikely to be a wide-orbit counterpart to the known radial-velocity extrasolar planets, whose masses are < 0.015 Msun. Instead, GQ Lup A/B is probably a member of a growing family of very low mass ratio widely separated binaries discovered through high-contrast imaging.

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arXiv2005

High Spectral Resolution Observations of the Massive Stars in the Galactic Center

We present high-resolution near-infrared spectra, obtained with the NIRSPEC spectrograph on the W. M. Keck II Telescope, of a collection of hot, massive stars within the central 25 arcseconds of the Galactic center. We have identified a total of twenty-one emission-line stars, seven of which are new radial velocity detections with five of those being classified as He I emission-line stars for the first time. These stars fall into two categories based on their spectral properties: 1) those with narrow 2.112, 2.113 micron He I doublet absorption lines, and 2) those with broad 2.058 micron He I emission lines. These data have the highest spectral resolution ever obtained for these sources and, as a result, both components of the absorption doublet are separately resolved for the first time. We use these spectral features to measure radial velocities. The majority of the measured radial velocities have relative errors of 20 kms, smaller than those previously obtained with proper-motion or radial velocity measurements for similar stellar samples in the Galactic center. The radial velocities estimated from the He I absorption doublet are more robust than those previously estimated from the 2.058 micron emission line, since they do not suffer from confusion due to emission from the surrounding ISM. Using this velocity information, we agree that the stars are orbiting in a somewhat coherent manner but are not as defined into a disk or disks as previously thought. Finally, multi-epoch radial velocity measurements for IRS 16NE show a change in its velocity presumably due to an unseen stellar companion.

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arXiv2005

The First Spatially Resolved Mid-IR Spectra of NGC 1068 Obtained at Diffraction-limited Resolution with LWS at Keck I Telescope

We present spatially resolved mid-IR spectra of NGC 1068 with a diffraction-limited resolution of 0.25\arcsec using the Long Wavelength Spectrometer (LWS) at the Keck I telescope. The mid-infrared image of NGC 1068 is extended along the N-S direction. Previous imaging studies have shown the extended regions are located inside the ionization cones indicating that the mid-infrared emission arises perhaps from the inner regions of the narrow-line clouds instead of the proposed dusty torus itself. The spatially resolved mid-IR spectra were obtained at two different slit position angles, +8.0 and -13.0 degrees across the elongated regions in the mid-IR. From these spectra, we found only weak silicate absorption toward the northern extended regions but strong in the nucleus and the southern extended regions. This is consistent with a model of a slightly inclined cold obscuring torus which covers much of the southern regions but is behind the northern extension. While a detailed analysis of the spectra requires a radiative transfer model, the lack of silicate emission from the northern extended regions prompts us to consider a dual dust population model as one of the possible explanations in which a different dust population exists in the ionization cones compared to that in the dusty torus. Dust inside the ionization cones may lack small silicate grains giving rise to only a featureless continuum in the northern extended regions while dust in the dusty torus has plenty of small silicate grains to produce the strong silicate absorption lines towards the nucleus and the southern extended regions.

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arXiv2002

Massive Stars in the Arches Cluster

We present and use new spectra and narrow-band images, along with previously published broad-band images, of stars in the Arches cluster to extract photometry, astrometry, equivalent width, and velocity information. The data are interpreted with a wind/atmosphere code to determine stellar temperatures, luminosities, mass-loss rates, and abundances. We have doubled the number of known emission-line stars, and we have also made the first spectroscopic identification of the main sequence for any population in the Galactic Center. We conclude that the most massive stars are bona-fide Wolf-Rayet (WR) stars and are some of the most massive stars known, having M_{initial} > 100 Msun, and prodigious winds, Mdot > 10^{-5} Msun yr^{-1}, that are enriched with helium and nitrogen; with these identifications, the Arches cluster contains about 5% of all known WR stars in the Galaxy. We find an upper limit to the velocity dispersion of 22 kms^{-1}, implying an upper limit to the cluster mass of 7(10^4) Msun within a radius of 0.23 pc; we also estimate the bulk heliocentric velocity of the cluster to be v_{cluster,odot} approximately +95 kms^{-1}.

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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.

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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.

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arXiv2000

Hot Stars and Cool Clouds: The Photodissociation Region M16

We present high-resolution spectroscopy and images of a photodissociation region (PDR) in M16 obtained during commissioning of NIRSPEC on the Keck II telescope. PDRs play a significant role in regulating star formation, and M16 offers the opportunity to examine the physical processes of a PDR in detail. We simultaneously observe both the molecular and ionized phases of the PDR and resolve the spatial and kinematic differences between them. The most prominent regions of the PDR are viewed edge-on. Fluorescent emission from nearby stars is the primary excitation source, although collisions also preferentially populate the lowest vibrational levels of H2. Variations in density-sensitive emission line ratios demonstrate that the molecular cloud is clumpy, with an average density n = 3x10^5 cm^(-3). We measure the kinetic temperature of the molecular region directly and find T_H2 = 930 K. The observed density, temperature, and UV flux imply a photoelectric heating efficiency of 4%. In the ionized region, n_i=5x10^3 cm^(-3) and T_HII = 9500 K. In the brightest regions of the PDR, the recombination line widths include a non-thermal component, which we attribute to viewing geometry.

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arXiv2000

Discovery of an Obscured Broad Line Region in the High Redshift Radio Galaxy MRC 2025-218

This paper presents infrared spectra taken with the newly commissioned NIRSPEC spectrograph on the Keck II Telescope of the High Redshift Radio Galaxy MRC 2025-218 (z=2.63) These observations represent the deepest infrared spectra of a radio galaxy to date and have allowed for the detection of Hbeta, [OIII] (4959/5007), [OI] (6300), Halpha, [NII] (6548/6583) and [SII] (6716/6713). The Halpha emission is very broad (FWHM = 9300 km/s) and luminous (2.6x10^44 ergs/s) and it is very comparable to the line widths and strengths of radio loud quasars at the same redshift. This strongly supports AGN unification models linking radio galaxies and quasars, although we discuss some of the outstanding differences. The [OIII] (5007) line is extremely strong and has extended emission with large relative velocities to the nucleus. We also derive that if the extended emission is due to star formation, each knot has a star formation rate comparable to a Lyman Break Galaxy at the same redshift.

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arXiv2000

2 micron Spectroscopy within 0.3 arcseconds of SgrA*

We present moderate (R~$\approx$~2,700) and high resolution (R~$\approx$~22,000) 2.0$-$2.4 \micron\ spectroscopy of the central 0.1 square arcseconds of the Galaxy obtained with NIRSPEC, the facility near-infrared spectrometer for the Keck II telescope. The composite spectra do not have any features attributable to the brightest stars in the central cluster, i.e.\ after background subtraction, W$_{\rm ^{12}CO(2-0)}$~$<$~2~Å. This stringent limit leads us to conclude that the majority, if not all, of the stars are hotter than typical red giants. Coupled with previously reported photometry, we conclude that the sources are likely OB main sequence stars. In addition, the continuum slope in the composite spectrum is bluer than that of a red giant and is similar to that of the nearby hot star, IRS16NW. It is unlikely that they are late-type giants stripped of their outer envelopes because such sources would be much fainter than those observed. Given their inferred youth ($τ_{\rm age}$~$<$~20~\Myr), we suggest the possibility that the stars have formed within 0.1 pc of the supermassive black hole. We find a newly-identified broad-line component (V$_{\rm FWHM}$ $\approx$ 1,000 \kms) to the 2.2178 \micron\ [\ion{Fe}{3}] line located within a few arcseconds of Sgr~A$^*$. A similar component is not seen in the Br-$γ$ emission.

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arXiv1998

Infrared Spectroscopy of GX 1+4/V2116 Oph: Evidence for a Fast Red Giant Wind?

We present infrared spectroscopy of the low-mass X-ray binary GX 1+4/V2116 Oph. This symbiotic binary consists of a 2-min accretion-powered pulsar and an M5 III red giant. A strong He I 1.083 micron emission line with a pronounced P Cygni profile was observed. From the blue edge of this feature, we infer an outflow velocity of 250(50) km/s. This is an order of magnitude faster than a typical red giant wind, and we suggest that radiation from the accretion disk or the neutron star may contribute to the acceleration of the outflow. We infer a wind mass loss rate of around 10^-6 Msun/yr. Accretion from such a strong stellar wind provides a plausible alternative to Roche lobe overflow for supplying the accretion disk which powers the X-ray source. The H I Paschen beta and He I 1.083 micron lines showed no evidence for the dramatic changes previously reported in some optical lines, and no evidence for pulsations at the 2-min pulsar period.

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arXiv1997

The External Shear Acting on Gravitational Lens B 1422+231

In a number of multiply imaged quasar systems, a significant contribution to the lensing potential is provided by groups and clusters of galaxies associated with the primary lens. As part of an ongoing effort to gather observational data on these systems, we present spectroscopy and near-infrared and optical photometry of galaxies in the field of the quadruple lens system B 1422+231. The spectra show that the primary lens and five nearby galaxies belong to a compact group at z = 0.338. The median projected radius of this group is 35 h^{-1} kpc and its velocity dispersion is 550 km/s. A straightforward application of the virial theorem yields a group mass of 1.4 x 10^{13} h^{-1} M(sun), which provides sufficient external shear to produce the observed image configuration. This data rules out a class of models and improves the system's prospects for a measurement of the Hubble constant.

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