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Schommer, R. A.

Publications and source records attributed to Schommer, R. A..

11 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

The Type Ia supernova 1989B in NGC 3627 (M66)

We report extensive optical photometry and spectroscopy of the Type Ia supernova 1989B. Maximum light in B occurred approximately seven days after discovery on JD 2447565.3 +/- 1.0 (1989 February 7.8 +/- 1.0) at a magnitude of 12.34 +/- 0.05. The UBV light curves of this supernova were very similar to those of other well observed Type Ia events such as SN 1981B and SN 1980N. From a comparison of the UBVRIJHK photometry, we derive an extinction for SN 1989B of E(B-V) = 0.37 +/- 0.03 mags relative to the unobscured Type Ia SN 1980N. The properties of the dust responsible for the reddening of SN 1989B appear to have been similar to those of normal dust in the Milky Way. In particular, we find no evidence for an unusually low value of the ratio of the total to selective absorption. We derive a distance modulus of delta mu(sub 0) = -1.62 +/- 0.03 mag relative to the Type Ia SN 1980N. We present optical spectra which provide essentially continuous coverage of the spectral evolution of SN 1989B over the first month following B maximum. These data show the transition from the maximum-light spectrum, in which lines of elements such as Ca, Si, S, Mg, and O are most prominent, to the Fe-dominated spectrum observed a few weeks after maximum. This transition occurred quite smoothly over a two-week period following B maximum. Comparison of the spectra of SN 1989B with data for two other well observed Type Ia supernovae -- 1981B and 1986G -- reveals subtle differences in the relative strengths of the S II and Si II absorption lines at maximum light. However, these differences disappeared within a week or so after maximum with the onset of the Fe-dominated phase.

Wells, Lisa A.↗
NASA NTRS1993

The velocity field of clusters of galaxies within 100 megaparsecs. II - Northern clusters

Distances and peculiar velocities for galaxies in eight clusters and groups have been determined by means of the near-infrared Tully-Fisher relation. With the possible exception of a group halfway between us and the Hercules Cluster, we observe peculiar velocities of the same order as the measuring errors of about 400 km/s. The present sample is drawn from the northern Galactic hemisphere and delineates a quiet region in the Hubble flow. This contrasts with the large-scale flows seen in the Hydra-Centaurus and Perseus-Pisces regions. We compare the observed peculiar velocities with predictions based upon the gravity field inferred from the IRAS redshift survey. The differences between the observed and predicted peculiar motions are generally small, except near dense structures, where the observed motions exceed the predictions by significant amounts. Kinematic models of the velocity field are also compared with the data. We cannot distinguish between parameterized models with a great attractor or models with a bulk flow.

Mould, J. R.↗
NASA NTRS1993

SN 1991bg - A type Ia supernova with a difference

While SN 1991bg is an unusual type Ia SN in such a feature as the brief duration of the photospheric phase, which ended only two weeks after maximum, it shares with other Ia SNs strong Si II and Ca II lines near maximum light. In addition, the light and color curve slopes are almost identical with the templates at late times. The spectral evolution of SN 1991bg is also unique but not unrecognizable; nevertheless, the peculiarities associated with this event complicate the fundamental question as to whether the Ia SNs make good standard candles.

Leibundgut, Bruno↗
NASA NTRS1983

Photometry of resolved galaxies. II - Sextans A

Luminosity functions and color-magnitude diagrams are given for 652 stars in the Sextans A dwarf irregular galaxy, and they are compared with other Local Group galaxies. The brightest red and blue stars presently determined are in agreement with those of Sandage and Carlson (1982), and a group of red stars is identified which may be analogous to the bright carbon star population recently found in the Magellanic Clouds. While the similarity of Sextans A's luminosity function with those of the LMC and the Milky Way is in keeping with a universal luminosity function, stellar mass function and birthrate resemble those of the SMC. Per unit gas mass, Sextans A has a birthrate almost identical to the SMC, suggesting that the mean age of stars in Sextans A is significantly less than of those in the Magellanic Clouds.

Hoessel, J. G.↗
NASA NTRS1982

0351+026 - A QSO spawned by interacting galaxies

The results of a detailed investigation into the nature of the H I-rich quasi-stellar object 0351+026 are presented. An improved H I profile reveals the presence of two distinct components. A high-quality CCD image clearly shows the presence of two galaxies. The brighter galaxy hosts the QSO while the less luminous companion has a prominent, apparently stellar nucleus. Moreover, this image also shows streamers of nebulosity emanating from the two galaxies and has an overall morphology which is highly reminiscent of that seen in other strongly interacting galaxies. Optical spectroscopy of the system shows that the forbidden O III emission lies between the two nuclei and there is strong evidence of stellar absorption lines in both the host and companion galaxies. Overall, the data suggest that this system represents a violent encounter between two low-luminosity H I-rich galaxies. The possibility exists that this collision has triggered the QSO activity in the brighter galaxy.

Bothun, G. D.↗
NASA NTRS1982

X-ray observations of Abell 2218 and implications for the Sunyaev-Zel'dovich effect

The Einstein Observatory was used to obtain a high-resolution X-ray image and an X-ray flux measurement of the galaxy cluster Abell 2218. The combined system of intracluster medium and galaxies is modeled as a gas in hydrostatic equilibrium in a common gravitational potential with an isothermal galaxy component. This model is then fitted in the least-squares sense to the X-ray surface brightness distribution and to the optically determined galaxy surface density profile to determine model parameters. The gas is found to be nominally isothermal, moderately dense, and very hot (10-30 keV); these conditions are consistent with the Sunyaev-Zel'dovich effect, i.e., the diminution in the cosmic microwave background radiation intensity in the direction of this cluster. It is noted that the considerable imprecision inherent in the method of distance measurement based on the combination of microwave measurements with X-ray and optical data makes it impossible to make a definitive statement about the distance to Abell 2218.

Boynton, P. E.↗
NASA NTRS1982

Detection of neutral hydrogen emission and optical nebulosity in the low redshift QSO 0351+026

Spectroscopy, photometry, and imaging of the X-ray source 0351+026 are consistent with an active nucleus with the spectrum of a QSO or type I Seyfert, embedded in a low luminosity host galaxy of colors similar to M31. The H I observations reveal a tremendous amount of neutral gas associated with the system, M(H I)/L(B) = 15 in solar units, and the velocity width of the feature is 1500 km/s. Both parameters substantially exceed those seen in other galaxies and interacting pairs. The unique H I properties of the system combined with the presence of an active nucleus with QSO-like features is a tantalizing, but poorly understood, combination. At slightly larger redshift and thus inferior angular scale, this object would be indistinguishable from a QSO in its optical and X-ray characteristics.

Bothun, G. D.↗
NASA NTRS1981

The X-ray and optical characteristics of the cataclysmic variables V794 Aquilae

Observations of V794 Aql have revealed features that are consistent with its inclusion in the class of cataclysmic variables. The features include: (1) a 0.1-4.5 keV luminosity of 5 x 10 to the 30th ergs/sec for a d-value of 100 pc, with a hardness ratio of 11; (2) blue colors; (3) 0.5 mag variability on 40-min time scales, and 3.0 mag variability on the time scale of one day; and (4) a weak He II 4686 A, which argues against classification of V794 Aql as an AM Her-type variable. It is predicted that V794 Aql has a period shorter than six hours, with a strong possibility for eclipses, and is a strong UV emission source.

Szkody, P.↗
NASA NTRS1979

A spectroscopic flare of YZ Canis Minoris

Observations of a spectroscopic flare in YZ Canis Minoris are reported. Emission lines of Mg I, Fe I, and Fe II were found in the spectral region 5090-5350 A. The intensity of the emission lines declined only slightly over a span of 2 hours. No significant radial-velocity difference was found between the emission and absorption lines. The turbulent velocity dispersion was less than 20 km/s. Upper limits of 7500 and 15,000 gauss can be placed on the longitudinal and transverse magnetic fields. The equal intensities of the Mg I b lines imply that the emitting region was optically thick. The intensity ratios did not change with time.

Mochnacki, S. W.↗