Search Publications
The Accuracy of the Born and Ray Approximations in Time-Distance Helioseismology
Kosovichev, A. G.; Birch, A. C.; Price, G. H. +1 more
Time-distance helioseismology measures the time for acoustic wave packets to travel, through the solar interior, from one location on the solar surface to another. Interpretation of travel times requires an understanding of their dependence on subsurface inhomogeneities. Traditionally, time-distance measurements have been modeled in the ray approx…
Microwave Detection of Umbral Oscillation in NOAA Active Region 8156: Diagnostics of Temperature Minimum in Sunspot
Shibasaki, Kiyoto
A radio brightness oscillation of 3 minutes at 17 GHz was detected in a compact radio source associated with a sunspot umbra in the NOAA active region 8156. We interpret the radio brightness oscillation by the density and temperature fluctuations due to upward-traveling acoustic waves through the third harmonic gyroresonance layer (2000 G). SOHO/S…
Microstructure of the Local Interstellar Cloud and the Identification of the Hyades Cloud
Redfield, Seth; Linsky, Jeffrey L.
We analyze high-resolution UV spectra of the Mg II h and k lines for 18 members of the Hyades to study inhomogeneity along these proximate lines of sight. The observations were taken by the Space Telescope Imaging Spectrograph (STIS) instrument on board the Hubble Space Telescope (HST). Three distinct velocity components are observed. All 18 lines…
Constraints on Galaxy Formation from Stars in the Far Outer Disk of M31
Ferguson, Annette M. N.; Johnson, Rachel A.
Numerical simulations of galaxy formation within the cold dark matter (CDM) hierarchical clustering framework are unable to produce large disk galaxies without invoking some form of feedback to suppress gas cooling and collapse until a redshift of unity or below. An important observational consequence of delaying the epoch of disk formation until …
Star Counts across the Red Giant Branch Bump and Below
Zoccali, M.; Bono, G.; Piotto, G. +1 more
We present a new observable-Rbump-which is the ratio between the star counts across the red giant branch (RGB) bump and fainter RGB stars to investigate the occurrence of a deep-mixing phenomenon during these evolutionary phases. The comparison between predicted and empirical Rbump-values, based on a large and homogeneous set…
A Sample of Clusters of Extragalactic Ultracompact H II Regions
Kobulnicky, Henry A.; Massey, Philip; Johnson, Kelsey E. +1 more
We report on the detection of optically thick free-free radio sources in the galaxies M33, NGC 253, and NGC 6946 using data in the literature. We interpret these sources as being young embedded star birth regions that are likely to be clusters of ultracompact H II regions. All 35 of the sources presented in this article have positive radio spectra…
Excitation of the Ganymede Ultraviolet Aurora
Wolven, Brian C.; Feldman, Paul D.; Strobel, Darrell F. +3 more
We analyze the ultraviolet aurorae observed on Ganymede by means of the Hubble Space Telescope and compare them to similar phenomena on Earth. We find that the tenuous nature of Ganymede's atmosphere precludes excitation of the aurora by high-energy electrons and requires a local acceleration mechanism. We propose the following as plausible mechan…
Detection of an X-Ray Jet in 3C 371 with Chandra
Maraschi, L.; Cheung, C. C.; Tavecchio, F. +4 more
We report the detection at X-rays of the radio/optical jet of 3C 371 from a short (10 ks) Chandra exposure in 2000 March. We also present a new MERLIN observation at 1.4 GHz together with a reanalysis of the archival Hubble Space Telescope Wide Field Planetary Camera 2 F555W image. Despite the limited signal-to-noise ratio of the Chandra data, the…
Searching for Ammonia in Grain Mantles toward Massive Young Stellar Objects
Whittet, D. C. B.; Gibb, E. L.; Chiar, J. E.
We present a study of the mid-infrared spectral regions of 17 objects observed with the Short-Wavelength Spectrometer on the Infrared Space Observatory (ISO SWS) in a search for the presence of the elusive NH3 solid-state absorption features at 2.96 and 9.0 µm. We compare the 3 µm profile with laboratory spectra of pure wate…
Early Chandra X-Ray Observations of η Carinae
Karovska, M.; Butt, Y. M.; Schlegel, E. M. +3 more
Subarcsecond-resolution Chandra observations of η Carinae reveal a 40''×70'' ring or partial shell of X-ray emission surrounding an unresolved bright central source. The spectrum of the central source is strongly absorbed and can be fitted with a high-temperature thermal continuum and emission lines. The surrounding shell is …