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UBV photometry of J V (Amalthea) near greatest western elongation shows this satellite to be about one magnitude fainter than previously believed. It is observed to be very red, having a B-V color index near + 1.5 mag. Unlike the Galilean satellites, J V appears to be a low-albedo object.
Observations of the B-gamma line at 2.17 microns are presented for the central few parsecs of the galactic-center region. The surface-brightness distribution of the line with 3.3- and 6.6-arcsec resolution has been sampled in 25 locations. The results show that the ionized gas is concentrated within the central parsec of the galactic-center region; the fine-scale structure seen at 10 microns does not appear to be present in the ionized gas. For the central parsec (20 arcsec) the equivalent width of the B-gamma line is, within large uncertainties, independent of the 2-micron continuum flux; for radial distances of the order of 30 arcsec the line equivalent width drops significantly. A comparison of the B-gamma line strength with radio observations shows that the 2.2-micron extinction is consistent with the 2.7 mag determined from the reddening of late-type stars. Observations of the CO band at 2.3 microns in the central 30 arcsec are also presented; they strengthen the interpretation that the 2-micron flux arises from late-type giant stars.
Multichannel-spectrometer scans of 56 of the brightest galaxies in faint clusters have been examined for evidence of color evolution. The dispersion introduced into the sample by some 25% of the galaxies which have unusual spectra is shown to be sufficient to obscure evolutionary variations. On omitting these galaxies it is found that over a time equivalent to a redshift of 0.46, B-V decreases by no more than 0.04 mag. A comparative analysis of the spectral energy distributions suggests that their differences may be due to a combination of the dispersion in metallicity among the members of the sample and to anomalous reddening of individual galaxies.
Infrared observations of the compact sources and background in the galactic center are used to derive the 1.25-12.5-micron interstellar extinction law. The depth of the interstellar 10-micron silicate absorption feature is derived from observations of the M supergiant IRS 7. The ratio of visual to silicate extinction is found to be 8 + or - 3. From the colors of individual sources and the background, it is concluded that there are no more than 6 mag of visual extinction within the central 3 pc of the galactic center.
We have discovered a highly variable star very close to the center of the SAS 3 error circle for the flaring X-ray source Aquila X-1 (3U 1908+00). Photographic photometry over the last 4 years shows covariability; the optical counterpart brightens from B greater than 20 to B approximately 17 mag during times of X-ray outburst, thus establishing the identification. Spectrophotometry of this star during its quiescent state shows spectral features of a K0 star with no obvious peculiarities; its distance must be at least 1.7 kpc.
This paper reports new photoelectric photometry of the nucleus of P/d'Arrest (1976e) during August 4, 5, and 6, 1976 (UT), just before perihelion passage. The comet appeared as a bright stellar nucleus with a faint surrounding coma. The average visual magnitude from the nuclear region (16-arcsec diaphragm) was 11.60. The visual brightness varies with an amplitude of 0.15 plus or minus 0.02 mag. There is evidence for a period of 5.17 plus or minus 0.01 hr. We interpret such a periodic variation as due to the rotation of the nucleus. The colors of the nuclear region are very similar to those of a solar-type star, and they do not vary significantly with aspect angle.
This paper presents interstellar H I column densities, mean H I space densities, total neutral hydrogen column densities, and total mean hydrogen space densities toward 100 stars surveyed in the L-alpha region with the U2 detector of the Copernicus satellite. The data for 47 stars are compared with OAO 2 results, and ratios of gas to E(B-V) color excess are computed for 30 'intercloud' stars, 45 'cloud' stars, and the combined sample of 75 stars. A definitive value of 5.8 x 10 to the 21st power atoms/sq cm per mag is obtained for the mean ratio of total neutral hydrogen to color excess, indicating that this ratio seems to be a constant, independent of location in the sky, except possibly toward stars with anomalous extinction. The anomalous ratio determined for Rho Oph is examined, and the galactic distribution of neutral hydrogen is evaluated. The following values are estimated for matter in the galactic plane within 500 pc of the sun: a total mean hydrogen space density of 1.15 atoms/cu cm, a mean H2 space density of 0.143 molecule/cu cm (possibly a lower limit), and a mean H I space density of 0.86 atom/cu cm (a possible overestimate).
The spectrum of a fast (63 km/s) sporadic meteor for which altitude and orbit data are available is reported. The -6 mag meteor occurred on January 5, 1976, over southern New Mexico. Fifty features have been identified in the spectrum. The spectrum is essentially identical to that of Perseid meteors of similar brightness. It is shown that heavy-particle collisions do not dominate bright meteor spectra lacking time resolution. Low-energy electron excitation of Ca II is strongly indicated in the wake to such an extent as to strongly affect spectra of this type lacking time resolution.
Observations of Algol (Beta Persei) at 10 microns are presented which are of relevance to the system's behavior around the time of its secondary minimum. No evidence is found for a significant excess at 10 microns. The data show a smooth decline by 0.3 mag which starts at about phase 0.42, while the limited data obtained at phases greater than 0.50 are consistent with a symmetrical curve about phase 0.50.
Results are reported for UBVR photometric observations of the optical candidate for the pulsating X-ray source 4U 1626-67. The data are found to be consistent with a constant V magnitude of approximately 19.00 and marginally consistent with constant B-V and U-B colors, but two averaged measures obtained about 2 hr apart suggest that the B-V and U-B colors are variable and that this variability may be entirely in U and B. Additional evidence for variability of the optical candidate is examined, and it is shown that the optical object is probably variable with a B amplitude of no more than about 0.5 mag and time scales as short as less than the order of 1 min. A power spectrum analysis reveals no significant peaks at or near the X-ray pulse period of 7.68 sec or its harmonics. It is concluded that although the optical data do not provide any additional evidence that 4U 1626-67 is a binary, a short-period binary system with the optical emission strongly dominated by an accretion disk is probably favored.
Faint blue stars are suggested as the optical counterparts of the X-ray bursters 4U (MXB) 1636-53 and 4U (MXB) 1735-44 as well as the 7-sec X-ray pulsar 4U 1626-67. The candidate stars have large UV excesses and were discovered well within the 20- and 30-arcsec error radii determined with the rotating modulation collimator aboard SAS 3. Photographic observations and photoelectric photometry of the candidate stars are discussed. It is shown that the candidates for 4U 1626-67 and 4U 1636-53 lie further from the reddened-supergiant or main sequence than any known X-ray source counterparts and have colors that are inconsistent with those of a reddened normal star, while the candidate for 4U 1735-44 has the colors of both Sco X-1 and a late O or BO star suffering about 1 mag of extinction.
Infrared (JHK) photometry is presented for a sample of E and S0 galaxies. The sample comprises 80 field galaxies, 35 galaxies in the Virgo cluster, and 22 in the Coma cluster. The galaxies span a range of 6 mag in absolute magnitude. Multiaperture UBVR photometry for 107 field galaxies is also presented. The infrared data are combined with these optical data or with published optical data to give V-K colors. This paper describes the observational techniques and lists the data.
Copernicus ultraviolet scans of the peculiar mass-losing star P Cyg are described. From the L-alpha profile and diffuse band strengths reported in the literature, a value of interstellar reddening E(B V) of approximately 0.35 mag is derived, leading to the conclusion that the star is intrinsically reddened. This value for the color excess leads to an estimated distance for P Cyg of 0.6-1.8 kpc, on the basis of which a revised visual absolute magnitude of -7.6 to -5.2 is obtained. The wind from P Cyg is quite unlike that for other early B supergiants, displaying a low terminal velocity and low ionization. This difference is connected with the great extension of its photosphere and with the fact that the acceleration of the flow begins below the photosphere. It is suggested that the wind in P Cyg results from dynamical instabilities quite distinct from the mechanism which initiates the winds in other OB stars.
It is established that BL Lacertae objects constitute a class of X-ray emitter by precisely locating X-ray sources identified with Mrk 421, PKS 0548-322, PKS 2155-304, and 2A 1219+305. Irregular variations of the order of 1 mag, characteristic of BL Lacertae objects, are reported for the last two objects. The measurement of an extremely steep X-ray spectrum from Mrk 421, in contrast to previous flat X-ray spectra above 3 keV, suggests that different structural states of an accretion disk are being observed.
Results are reported for extensive spectrophotometric observations, over the wavelength range from 3800 to 7800 A, of the peculiar 14th-mag emission-line object Stephenson-Sanduleak (SS) 433, which has been identified with a variable nonthermal radio source and is positionally coincident with the X-ray source A1909+04. It is found that the spectrum is dominated by intense variable Balmer and He I emission, that three of the He I lines have very prominent P Cygni profiles, and that virtually all the emission features undergo changes in equivalent width on a time scale of days. Anomalous emission features are described which consist of three previously unreported, very strong, broad emission lines at unfamiliar wavelengths in the green, red, and IR, and which vary nightly in intensity, profile, and wavelength. SS 433 is inferred to be a luminous object with an absolute visual magnitude of -3.1 or brighter.
This paper directly examines the claims of Segal (1976) that the (m,z) Hubble diagram is fitted best by a square law z = Kr-squared rather than by the traditional Hubble law z = Hr in the low-redshift range, z no more than about 0.01, corresponding to galaxies brighter than 14th mag. Segal attempts to fit a distance relation to the (m,z) scatter diagram in which each individual galaxy is plotted. The exact relation between the mean redshift for all galaxies in a small magnitude interval and the apparent magnitude is calculated. This relation is independent of luminosity function and peculiar velocity distribution about the general expansion, and is not affected by sample incompleteness as a function of apparent magnitude or the clustering of galaxies in the sample. Segal's method is affected by all of these and requires a highly sophisticated statistical analysis to deal with the non-Gaussian pointwise scatter. The present analysis favors the Hubble law and conclusively rules out the square law for the small redshift region.
Interstellar atomic carbon has been sampled over 33 different paths with visual extinctions ranging from about 0.15 to 1.5 mag. The trend of C I vs different forms of hydrogen is found to be steep and somewhat erratic. It is concluded that the observed behavior can be explained by strong differences in the responses of C I, H I, and H2 equilibria to physical conditions, together with marked variations of these conditions within different interstellar clouds.
The discovery of an intense high-latitude soft X-ray source (designated H2156-304) detected with the low-energy detectors of the HEAO A-2 experiment is reported. The error box of the source includes a 14th mag starlike object that may be coincident with the radio source PKS 2155-304 and has been suggested as a BL Lac candidate. Intensity variations of H2156-304 on time scales of about 1 sec to 1 day are discussed, along with a probable flare from the source. The derived energy spectrum of the source is shown to be well fitted by either a two-temperature thermal model with temperatures of 16 million and 1.6 million K or a single power-law model with a spectral index of 2.4 + or - 0.3. Hydrogen column densities of approximately 2.5 x 10 to the 20th and 2.0 x 10 to the 20th H atoms/sq cm are obtained for the two-temperature and power-law models, respectively. The emission mechanism of H2156-304 is considered, and the characteristics of this source are compared with those of three known soft X-ray-emitting BL Lac objects.