arXiv subjects

T. R. Geballe

Publications and source records attributed to T. R. Geballe.

At least 109 records · Page 6Linked to original sources

arXiv2005

Infrared spectroscopy of Nova Cassiopeiae 1993 (V705 Cas). IV. A closer look at the dust

Nova Cassiopeiae 1993 (V705 Cas) was an archetypical dust-forming nova. It displayed a deep minimum in the visual light curve, and spectroscopic evidence for carbon, hydrocarbon and silicate dust. We report the results of fitting the infrared spectral energy distribution with the DUSTY code, which we use to determine the properties and geometry of the emitting dust. The emission is well described as originating in a thin shell whose dust has a carbon:silicate ratio of ~2:1 by number (1.26:1 by mass) and a relatively flat size distribution. The 9.7micron and 18micron silicate features are consistent with freshly-condensed dust and, while the lower limit to the grain size distribution is not well constrained, the largest grains have dimensions \~0.06micron; unless the grains in V705 Cas were anomalously small, the sizes of grains produced in nova eruptions may previously have been overestimated in novae with optically thick dust shells. Laboratory work by Grishko & Duley may provide clues to the apparently unique nature of nova UIR features.

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arXiv2005

Search for H3+ in HD141569A

A search for H3+ line emission, reported to have been detected toward the young star HD 141569A and possibly originating in a clump of planet-forming gas orbiting the star, has yielded negative results. Observations made at the United Kingdom Infrared Telescope and at the Subaru Telescope during 2001-2005 covered 11 major transitions of H3+ from 3.42 to 3.99 um. No H3+ emission lines were detected; one marginal detection at 3.9855 um in June 2002 was not confirmed in later spectra. The upper limits to the line strengths are significantly lower than the previously reported detections. Supplemental slit-scanning spectroscopy using adaptive optics was performed within 0.38 of HD 141569A to search for extended emission from H3+, but no emission was detected. We compare our upper limit to the luminosity in H3+ from HD 141569A with that possible from a gas giant protoplanet and also from a jovian mass exoplanet in close orbit about its central star.

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arXiv2005

Infrared Spectroscopy of U Equulei's Warm Circumstellar Gas

Medium and high resolution spectroscopy of U Equulei from 1 to 4 microns during 1997-2003 has revealed information about its unusual circumstellar envelope, observed previously at optical and radio wavelengths. Strong absorption bands of H2O and of CO dominate the 1-4um spectrum. The gas has a mean temperature of 600 K and 12C/13C =< 10. The CO 2-0 line profiles and velocities imply no net ejection or infall and indicate either rapid radial gas motions being seen along a narrow continuum beam, or absorption by orbiting gas that is nearly coincident with a highly extended continuum source. The gas could be located in a disk-like structure. The observed high column densities of warm CO and H2 normally would be associated with sufficient dust to completely obscure the star at optical wavelengths. The observations thus indicate either a highly abnormal gas-to-dust ratio, consistent with the earlier optical observation of abundant refractory metal oxides in the circumstellar gas, or peculiar geometry and/or illumination.

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arXiv2005

An Extremely Large Excess of 18O in the Hydrogen-Deficient Carbon Star, HD 137613

We report the discovery of a uniquely large excess of 18O in the hydrogen-deficient carbon (HdC) star, HD 137613, based on a spectrum of the first overtone bands of CO at 2.3-2.4 micron in which three strong absorption bands of 12C18O are clearly present. Bands of 12C16O also are present but no bands of 13C16O or 12C17O are seen. We estimate an isotopic ratio 16O/18O ~1. The Solar value of this ratio is ~500. Neither He-core burning nor He-shell flash burning can produce the isotopic ratios of oxygen and carbon observed in HD 137613. However, a remarkable similarity exists between the observed abundances and those found in the outer layers of the broad He-shell of early-AGB stars, soon after the end of He-core burning. It is not known how the outer envelope down to the He-shell could be lost but some mechanism of enhanced mass loss must be involved. HD 137613 may be a post-early-AGB star with the outer layers of the former He-burning shell as its photosphere. The unusual elemental abundances of the HdC stars resemble those of the R Coronae Borealis (RCB) stars, but HdC stars do not produce clouds of dust that produce declines in brightness. None of the other RCB or HdC stars observed shows significant 18O.

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arXiv2005

Infall and SiO emission in V838 Mon

We present moderate and high resolution infrared spectroscopy of the peculiar eruptive variable V838 Mon, which underwent a series of remarkable outbursts in early 2002. During the period covered by our observations, 2002 December-2003 December, the near-infrared spectrum continued to show many of the characteristics of a very cool supergiant. However, throughout this period the spectrum also revealed strong and variable SiO first overtone emission, and Paschen Beta emission. The 2003 December spectrum contained a series of Ti I lines with inverse P Cygni profiles. This is clear evidence that some material is falling inward towards the star.

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arXiv2005

Massive Stars in the SGR 1806-20 Cluster

We report the discovery of additional hot and massive stars in the cluster surrounding the soft gamma repeater SGR 1806-20, based upon UKIRT and Keck near-infrared spectroscopy. Of the newly identified stars, three are Wolf-Rayet stars of types WC8, WN6, and WN7, and a fourth star is an OB supergiant. These three stars, along with four previously discovered, imply a cluster age of \~3.0-4.5 Myr, based on the presence of WC stars and the absence of red supergiants. Assuming coevality, this age suggests that the progenitor of SGR 1806-20 had an initial mass greater than ~50 Msun. This is consistent with the suggestion that SGRs are post-supernovae end states of massive progenitors, and may suggest that only massive stars evolve into magnetars that produce SGRs. It also suggests that very massive stars can evolve into neutron stars, not just black holes, as recently predicted by theory. The cluster age also provides constraints on the very high mass object, LBV 1806-20.

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arXiv2004

The sub-millimetre evolution of V4334 Sgr (Sakurai's Object)

We report the results of monitoring of V4334 Sgr (Sakurai's Object) at 450 microns and 850 microns with SCUBA on the James Clerk Maxwell Telescope. The flux density at both wavelengths has increased dramatically since 2001, and is consistent with continued cooling of the dust shell in which Sakurai's Object is still enshrouded, and which still dominates the near-infrared emission. Assuming that the dust shell is optically thin at sub-millimetre wavelengths and optically thick in the near-infrared, the sub-millimetre data imply a mass-loss rate during 2003 of ~3.4(+/0.2)E-5 for a gas-to-dust ratio of 75. This is consistent with the evidence from 1-5micron observations that the mass-loss is steadily increasing.

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arXiv2004

L' and M' Photometry of Ultracool Dwarfs

We have compiled L' and M' photometry of 63 single and binary M, L,and T dwarfs obtained at UKIRT using the MKO filter set. This compilation includes new L' data for 8 L dwarfs and 13 T dwarfs and new M' data for 7 L dwarfs, 5 T dwarfs, and Gl 229A. We compute L_bol, BC_K, and T_eff for 42 dwarfs whose flux-calibrated JHK spectra, L' photometry, and trigonometric parallaxes are available, and we estimate these quantities for 9 other dwarfs whose parallaxes and flux-calibrated spectra have been obtained. BC_K is a well-behaved function of near-IR spectral type with a dispersion of ~0.1 mag for types M6-T5; it is significantly more scattered for types T5-T9. T_eff declines steeply and monotonically for types M6-L7 and T4-T9, but is nearly constant at ~1450 K for types L7-T4 with assumed ages of ~3 Gyr. This constant T_eff is evidenced by nearly unchanging values of L'-M' between types L6 and T3. We compare the observed K, L', and M' luminosities of L and T dwarfs in our sample with those predicted by model atmospheres with varying surface gravities and sedimentation efficiencies. The models indicate that the L3-T4.5 dwarfs generally have higher gravities (log g = 5.0-5.5) than the T6-T9 dwarfs (log g = 4.5-5.0). The predicted M' luminosities of T dwarfs are 1.5-2.5 times larger than those derived empirically for the T dwarfs in our sample. This discrepancy is attributed to absorption at 4.5-4.9 um by CO, which is not expected under the condition of chemical equilibrium assumed in the models. We determine that the L3 dwarf Kelu-1 and the T0 dwarf SDSS J0423-0414 are probable binary systems. We compute log(L_bol/L_sun) = -5.73 +/- 0.05 and T_eff = 600-750 K for the T9 dwarf 2MASS J0415-0935, which supplants Gl 570D as the least luminous and coolest brown dwarf presently known.

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arXiv2004

Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: the Effects of Temperature, Clouds, and Gravity

We present JHK photometry and spectroscopy for a sample of newly identified L and T dwarfs from the Sloan Digital Sky Survey (SDSS) and known dwarfs from the SDSS and the Two Micron All Sky Survey (2MASS). Spectral classification has been carried out using four infrared indices; we identify 9 new L8-9.5 dwarfs and 14 new T dwarfs from SDSS, including the latest SDSS dwarf yet found, the T7 SDSS J1758+4633. We classify 2MASS J0415-0935 as T9, the latest and coolest dwarf found to date. We combine these results with previously published data to produce a sample of 59 L dwarfs and 42 T dwarfs. We compare the near-IR colors and absolute magnitudes of dwarfs near the L-T transition with predictions made by models of the distribution and evolution of photospheric condensates. There is scatter in the spectral indices for L dwarfs, suggesting that these indices are probing different levels of the atmosphere. The near-IR colors of the L dwarfs also scatter, likely due to variations in the altitudes, spatial distributions and thicknesses of the condensate clouds. We identify a group of late L dwarfs that are blue for their spectral type and that have enhanced FeH, H2O and KI absorption, possibly due to an unusually small amount of condensates. The scatter seen in the H-K color for late T dwarfs can be reproduced by models with a range in surface gravity. The variation is probably due to the effect on the K-band flux of pressure-induced H2 opacity. The correlation of H-K color with gravity is supported by the strengths of the J-band KI doublet. Gravity is closely related to mass for field T dwarfs and the gravities implied by the H-K colors indicate that the T dwarfs in our sample have masses in the range 15-75 M_Jupiter, except for SDSS J1110+0116 which possibly has a mass of 10-15 M_Jupiter.

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arXiv2004

Warm high velocity CO in the wind of Sakurai's Object (= V4334 Sgr)

We present UKIRT UIST spectra of Sakurai's Object (=V4334 Sgr) showing CO fundamental band absorption features around 4.7 microns. The line-centres are at heliocentric radial velocity of -170+/-30 km/s. The number and relative strengths of the lines indicate a CO gas temperature of 400+/-100 K and CO column density of 7(+3/-2)x10^17 per square cm. The gas was moving away from the central star at an average speed of 290+/-30 km/s in 2003 September. The lines appeared sometime between mid 1999 (well after the opaque dust shell formed) and mid 2000 and may have been somewhat more blue--shifted initially than they are now. The observed CO velocity and temperature indicate the continued presence of a fast wind in the object, previously seen in the He I 1.083 micron line beginning just prior to massive dust formation, and more recently in atomic and ionized lines. The dust continuum is consistent with a temperature of 350+/-30 K, indicating continued cooling of the shell. The similar CO temperature suggests that the bulk of the CO absorption occurs just outside of the dust continuum surface.

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arXiv2004

CO bands in V4334 Sgr (Sakurai's Object): the 12C/13C ratio

We present the results of our analysis of a high resolution (R ~ 30000) infrared spectrum of V4334 Sgr (Sakurai's Object) around the first overtone CO bands, obtained in 1998 July. The 12CO and 13CO bands are well-resolved, and we compute synthetic hydrogen-deficient model atmosphere spectra to determine the 12C/13C ratio. We find 12C/13C ~ 4 +/-1, consistent with the interpretation of V4334 Sgr as an object that has undergone a very late thermal pulse.

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arXiv2004

Preliminary Parallaxes of 40 L and T Dwarfs from the U.S. Naval Observatory Infrared Astrometry Program

We present preliminary trigonometric parallaxes and proper motions for 22 L dwarfs and 18 T dwarfs measured using the ASTROCAM infrared imager. Relative to absolute parallax corrections are made by employing 2MASS and/or SDSS photometry for reference frame stars. We combine USNO infrared and optical parallaxes with the best available CIT system photometry to determine M_J, M_H, and M_K values for 37 L dwarfs between spectral types L0 to L8 and 19 T dwarfs between spectral types T0.5 and T8 and present selected absolute magnitude versus spectral type and color diagrams, based on these results. Luminosities and temperatures are estimated for these objects. Of special interest are the distances of several objects which are at or near the L-T dwarf boundary so that this important transition can be better understood. The previously reported early-mid T dwarf luminosity excess is clearly confirmed and found to be present at J, H, and K. The large number of objects that populate this luminosity excess region indicates that it cannot be due entirely to selection effects. The T dwarf sequence is extended to M_J~16.9 by 2MASS J041519-0935 which, at d = 5.74 pc, is found to be the least luminous [log(L/L_sun)=-5.58] and coldest (T_eff~760 K) brown dwarf known. Combining results from this paper with earlier USNO CCD results we find that, in contrast to the L dwarfs, there are no examples of low velocity (V_tan < 20 km/s) T dwarfs. We briefly discuss future directions for the USNO infrared astrometry program.

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arXiv2003

The peculiar circumstellar environment of NGC2024-IRS2

We re-examine the nature of NGC2024-IRS2 in light of the recent discovery of the late O-type star, IRS2b, located 5 arcsec from IRS2. Using L-band spectroscopy, we set a lower limit of Av = 27.0 mag on the visual extinction towards IRS2. Arguments based on the nature of the circumstellar material, favor an Av of 31.5 mag. IRS2 is associated with the UCHII region G206.543-16.347 and the infrared source IRAS 05393-0156. We show that much of the mid-infrared emission towards IRS2, as well as the far infrared emission peaking at ~ 100 micron, do not originate in the direct surroundings of IRS2, but instead from an extended molecular cloud. Using new K-, L- and L'-band spectroscopy and a comprehensive set of infrared and radio continuum measurements from the literature, we apply diagnostics based on the radio slope, the strength of the infrared hydrogen recombination lines, and the presence of CO band-heads to constrain the nature and spatial distribution of the circumstellar material of IRS2. Using simple gaseous and/or dust models of prescribed geometry, we find strong indications that the infrared flux originating in the circumstellar material of IRS2 is dominated by emission from a dense gaseous disk with a radius of about 0.6 AU. At radio wavelengths the flux density distribution is best described by a stellar wind recombining at a radius of about 100 AU. Although NGC2024/IRS2 shares many similarities with BN-like objects, we do not find evidence for the presence of a dust shell surrounding this object. Therefore, IRS2 is likely more evolved.

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arXiv2003

Winds in R Coronae Borealis Stars

We present new spectroscopic observations of the He I $λ$10830 line in R Coronae Borealis (RCB) stars which provide the first strong evidence that most, if not all, RCB stars have winds. It has long been suggested that when dust forms around an RCB star, radiation pressure accelerates the dust away from the star, dragging the gas along with it. The new spectra show that nine of the ten stars observed have P-Cygni or asymmetric blue-shifted profiles in the He I $λ$10830 line. In all cases, the He I line indicates a mass outflow - with a range of intensity and velocity. Around the RCB stars, it is likely that this state is populated by collisional excitation rather than photoionization/recombination. The line profiles have been modeled with an SEI code to derive the optical depth and the velocity field of the helium gas. The results show that the typical RCB wind has a steep acceleration with a terminal velocity of \Vinf = 200-350 \kms and a column density of N $\sim10^{12}$ cm$^{-2}$ in the He I $λ$10830 line. There is a possible relationship between the lightcurve of an RCB star and its He I $λ$10830 profile. Stars which have gone hundreds of days with no dust-formation episodes tend to have weaker He I features. The unusual RCB star, V854 Cen, does not follow this trend, showing little or no He I absorption despite high mass-loss activity. The He I $λ$10830 line in R CrB itself, which has been observed at four epochs between 1978 and 2001, seems to show a P-Cygni or asymmetric blue-shifted profile at all times whether it is in decline or at maximum light.

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arXiv2003

Interaction between the Northeastern Boundary of Sgr A East and Giant Molecular Clouds

We have detected the v=1-0 S(1) (2.1218 um) and v=2-1 S(1) (2.2477 um) lines of H_2 in the Galactic centre, in a 90 x 27 arcsec region between the northeastern boundary of the non-thermal source, Sgr A East, and the giant molecular cloud (GMC) M-0.02-0.07. The detected H_2 v=1-0 S(1) emission has an intensity of 1.6 - 21 x 10^-18 W/m^2/arcsec^2 and is present over most of the region. Along with the high intensity, the broad line widths (FWHM = 40 - 70 km/s) and the 2-1 S(1) / 1-0 S(1) line ratios (0.3 - 0.5) can be best explained by a combination of C-type shocks and fluorescence. The detection of shocked H_2 is clear evidence that Sgr A East is driving material into the surrounding adjacent cool molecular gas. The H_2 emission lines have two velocity components at about +50 km/s and 0 km/s, which are also present in the NH_3(3,3) emission mapped by McGary, Coil, and Ho (2001, ApJ, 559, 326). This two-velocity structure can be explained if Sgr A East is driving C-type shocks into both the GMC M-0.02-0.07 and the northern ridge of McGary, Coil, and Ho (2001).

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arXiv2003

Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many interstellar molecules. In dense clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density, and temperature. On the other hand, observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. However, diffuse cloud models have been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons, the electron fraction, and the cosmic-ray ionisation rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards zeta Persei) where the electron fraction is already known. Taken together, these results allow us to derive the value of the third uncertain model parameter: we find that the cosmic-ray ionisation rate in this sightline is forty times faster than previously assumed. If such a high cosmic-ray flux is indeed ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.

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arXiv2002

Absorption Line Survey of H3+ toward the Galactic Center Sources I. GCS 3-2 and GC IRS3

We present high-resolution (R = 20000) spectroscopy of H3+ absorption toward the luminous Galactic center sources GCS 3-2 and GC IRS 3. With the efficient wavelength coverage afforded by Subaru IRCS, six absorption lines of H3+ have been detected in each source from 3.5 to 4.0 um, three of which are new. In particular the 3.543 um absorption line of the R(3, 3)^l transition arising from the metastable (J, K) = (3, 3) state has been tentatively detected for the first time in the interstellar medium, where previous observations of H3+ had been limited to absorption lines from the lowest levels: (J, K) = (1, 0) of ortho-H3+ and (1, 1) of para-H3+. The H3+ absorption toward the Galactic center takes place in dense and diffuse clouds along the line of sight as well as the molecular complex close to the Galactic nucleus. At least four kinematic components are found in the H3+ absorption lines. We suggest identifications of the velocity components with those of HI, CO, and H2CO previously reported from radio and infrared observations. H3+ components with velocities that match those of weak and sharp CO and H2CO lines are attributed to diffuse clouds. Our observation has revealed a striking difference between the absorption profiles of H3+ and CO, demonstrating that the spectroscopy of H3+ provides information complementary to that obtained from CO spectroscopy. The tentative detection of the R(3, 3)^l line and the non-detection of spectral lines from other J > 1 levels provide observational evidence for the metastability of the (3, 3) level, which is theoretically expected. This suggests that other metastable J = K levels with higher J may also be populated.

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arXiv2002

Models of infrared spectra of Sakurai's Object (V4334 Sgr) in 1997

Theoretical spectral energy distributions computed for a grid of hydrogen-deficient and carbon-rich model atmospheres have been compared with the observed infrared (1--2.5 $μ$m) spectra of V4334 Sgr (Sakurai's Object) on 1997 April 21 and July 13. The comparison yields an effective temperature of \Tef = 5500 $\pm$ 200 K for the April date and \Tef = 5250 $\pm$ 200 K for July. The observed spectra are well fitted by Asplund et al. (1999) abundances, except that the carbon abundance is higher by 0.3 dex. Hot dust produces significant excess continuum at the long wavelength ends of the 1997 spectra. \keywords{Stars: individual: V4334 Sgr (Sakurai's Object) -- Stars: AGB and post-AGB evolution -- Stars: model atmospheres -- Stars: energy distributions -- Stars: effective temperatures}

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