ALBERT

Description: Two UMa-type contact binaries, XY Leo and VW Cep, have been observed simultaneously with Exosat, the VLA, and, in the case of XY Leo, optically. The temporal coverage of each star was sufficient to monitor them throughout two orbital revolutions (P about 0.25 days), however, no orbital modulation of either the X-ray or 6-cm data was seen for either star. A large flare from VW Cep was detected, the first such simultaneous flare ever seen on a star other than the sun. Its behavior before, during, and after the flare is remarkably similar to that found in solar flares. For both stars, it is demonstrated that the 6-cm emission cannot be the result of bremsstrahlung radiation of the X-ray emitting plasma.

Description: The preliminary results are reported of a survey of every EINSTEIN image which overlaps any high-latitude molecular cloud in a search for X-ray emitting pre-main sequence stars. This survey, together with complementary KPNO and IRAS data, will allow the determination of how prevalent low mass star formation is in these clouds in general and, particularly, in the translucent molecular clouds.

Description: In order to discern whether the high-latitude molecular clouds are regions of ongoing star formation, we have used X-ray emission as a tracer of youthful stars. The entire Einstein database yields 18 images which overlap 10 of the clouds mapped partially or completely in the CO (1-0) transition, providing a total of approximately 6 deg squared of overlap. Five previously unidentified X-ray sources were detected: one has an optical counterpart which is a pre-main-sequence (PMS) star, and two have normal main-sequence stellar counterparts, while the other two are probably extragalactic sources. The PMS star is located in a high Galactic latitude Lynds dark cloud, so this result is not too suprising. The translucent clouds, though, have yet to reveal any evidence of star formation.

Description: We have analyzed archival Einstein Observatory images of a roughly 4.5 square degree region centered on the Orion Nebula. In all, 245 distinct X-ray sources have been detected in six High Resolution Imager (HRI) and 17 Imaging Proportional Counter (IPC) observations. An optical database of over 2700 stars has been assembled to search for candidate counterparts to the X-ray sources. Roughly half the X-ray sources are identified with a single Orion Nebula cluster member. The 10 main-sequence O6-B5 cluster stars detected in Orion have X-ray activity levels comparable to field O and B stars. X-ray emission has also been detected in the direction of four main-sequence late-B and early-A type stars. Since the mechanisms producing X-rays in late-type coronae and early-type winds cannot operate in the late-B and early-A type atmospheres, we argue that the observed X-rays, with L(sub X) approximately = 3 x 10(exp 30) ergs/s, are probably produced in the coronae of unseen late-type binary companions. Over 100 X-ray sources have been associated with late-type pre-main sequence stars. The upper envelope of X-ray activity rises sharply from mid-F to late-G, with L(sub x)/L(sub bol) in the range 10(exp -4) to 2 x 10(exp -3) for stars later than approximately G7. We have looked for variability of the late-type cluster members on timescales of a day to a year and find that 1/4 of the stars show significantly variable X-ray emission. A handful of the late-type stars have published rotational periods and spectroscopic rotational velocities; however, we see no correlation between X-ray activity and rotation. Thus, for this sample of pre-main-sequence stars, the large dispersion in X-ray activity does not appear to be caused by the dispersion in rotation, in contrast with results obtained for low-mass main-sequence stars in the Pleiades and pre-main-sequence stars in Taurus-Auriga.

Description: We have analyzed 56 Einstein Observatory Imaging Proportional Counter (IPC) observations within +/- 3 deg of the Galactic plane in order to determine the low-latitude soft X-ray background flux in the 0.56-1.73 keV band. Any detected X-ray point source which fell within our regions of study was removed from the image, enabling us to present maps of the background flux as a function of Galactic latitude along 18 meridians. These maps reveal considerable structure to the background in the Galactic plane on an angular scale of approximately 1 deg. Our results are compared with those of an earlier study of the 1 keV X-ray background along l = 25 deg by Kahn & Caillault. The double-peaked structure they found is not discernible in our results, possibly because of the presence of solar backscattered flux in their data. A model which takes into account contributions to the background by extragalactic and stellar sources, the distribution of both atomic and molecular absorbing material with the Galaxy, the energy dependence of the cross section for absorption of X-rays, and the energy dependence of the detector has been constructed and fitted to these new data to derive constraints on the scale height, temperature, and volume emissivity of the unaccounted-for X-ray-emitting material. The results of this model along l = 25 deg are roughly similar to those of the model of Kahn & Caillault along the same meridian.

Description: We present a catalog of infrared point sources from the IRAS Faint Source Survey at Galactic latitudes the absolute magnitude of b is greater than or equal to 30 deg. The aim of this paper is to provide a list of possible star-forming sites at high Galactic latitudes in order to address the question of whether or not the translucent molecular clouds (which are most easily identified at high latitudes) are capable of star formation. The primary list of sources has 12, 25, 60, and 100 micron fluxes within the range typical of pre-main-sequence or T Tauri stars. A secondary list has the same range of 12, 25, and 60 micron fluxes, but only upper limits at 100 microns. A total of 127 candidates from the first category and 65 candidates from the second category are identified and their positions and infrared spectral characteristics tabulated. Although the colors and fluxes of these sources are typical of T Tauri or pre-main-sequence stars and YSOs, extragalactic sources and planetary nebulae sometimes have similar colors. These lists provide a starting point for optical spectroscopy or other techniques to positively identify these objects. We can determine an upper limit to the star forming efficiency of high-latitude molecular clouds assuming all the candidates in our sample are pre-main sequence stars of one solar mass. The upper limit of a few tenths of 1% is less than the star-forming efficiency of local dark cloud complexes such as the Taurus-Auriga or rho Ophiuchus clouds.

Description: The star formation status of the translucent high-latitude molecular cloud, MBM 40, is explored through analysis of radio, infrared, optical, and X-ray data. With a peak visual extinction of 1 to 2 mag, MBM 40 is an example of a high-latitude cloud near the diffuse/translucent demarcation. However, unlike most translucent clouds, MBM 40 exhibits a compact morphology and a kinetic energy-to gravitational potential energy ratio near unity. Our radio data, encompassing the CO (J = 1-0), CS (J = 2-1), and H2CO 1(sub 11-1(sub 10), spectral line transitions, reveal that the cloud contains a ridge of molecular gas with n greater than or equal to 10(exp 3)/ cc. In addition, the molecular data, together with IRAS data, indicate that the mass of MBM 40 is approx. 40 solar mass. In light of the ever-increasing number of recently formed stars far from any dense molecular clouds or cores, we searched the environs of MBM 40 for any trace of recent star formation. We used the ROSAT All-Sky Survey X-ray data and a ROSAT PSPC pointed observation toward MBM 40 to identify 33 stellar candidates with properties consistent with pre-main-sequence (PMS) stars. Follow-up optical spectroscopy of the candidates with V less than 15.5 was conducted with the 1.5 m Fred Lawrence Whipple Observatory telescope in order to identify signatures of T Tauri or pre-main- sequence stars (such as the Li 6708 A resonance line). Since none of our optically observed candidates display standard PMS signatures, we conclude that MBM 40 displays no evidence of recent or ongoing star formation. The absence of high-density molecular cores in the cloud and the relatively low column density compared to star-forming interstellar clouds may be the principal reasons that MBM 40 is devoid of star formation. More detailed comparison between this cloud and other, higher extinction translucent and dark clouds may elucidate the necessary initial conditions for the onset of low-mass star formation.

Description: An extensive set of IUE observations of solar-type stars (spectral types F5-G5) in the Pleiades is presented. Spectra were obtained in January and August 1988 for both the transition region and chromospheric emission wavelength regions, respectively. Mg II fluxes were detected for two out of three Pleiades stars and C IV upper limits for two of these stars. Long-wavelength high-resolution spectra were also obtained for previously unobserved solar-type stars in the Hyades. With the inclusion of spectra of additional Hyades stars obtained from the IUE archives, surface fluxes and fractional luminosities for both clusters' solar-type stars are calculated; these values provide a better estimate for the Mg II saturation line for single stars.

Description: The detection of 12 X-ray-emitting main-sequence B6-A3 stars in the Orion Nebula is reported. The luminosity of these stars in the 0.2-3.5 keV band is considerably in excess of field stars of the same spectral type. Consideration is given to the possibilities that these stars may not actually be the source of the X-rays, but that the emission may be attributable to T Tauri or 'naked' T Tauri star companions, or that these hot stars are indeed the sources of emission and that current theories of X-ray emission from early-type stars must be amended.

Description: We have acquired light curves at a wavelength of 6 cm of the eclipsing binary V471 Tauri around the orbit, in order to determine the geometrical location of the radio emission in the binary. Each light curve shows a broad minimum near the time of optical eclipse, suggesting that the radio luminosity originates between the two stars. Other observations at X-ray, UV, and visual wavelengths are also supportive of the idea of a gas cloud more or less permanently located between the stars. This could be explained if the radio emission arises from the interaction of the magnetic fields of the secondary and the white dwarf near the line of centers.