Engineering topics

Balachandran, Suchitra

Publications and source records attributed to Balachandran, Suchitra.

5 records

NASA NTRS1995

An X-Ray Survey of the Open Cluster NGC 6475 (M7) with ROSAT

A ROSAT x-ray survey, with complimentary optical photometry, of the open cluster NGC 6475 has enabled the detection of approx. 50 late-F to K0 and approx. 70 K/M dwarf new candidate members, providing the first reliable detection of low-mass stars in this low. galactic latitude, 220 Myr old cluster. The x-ray observations reported here have a typical limiting sensitivity of L(sub x) approx. equal to 10(exp 29) erg/s. The detection frequency of early type cluster members is consistent with the hypothesis that the x-ray emitting early type stars are binary systems with an unseen, low-mass secondary producing the x rays. The ratio between x-ray and bolometric luminosity among NGC 6475 members saturates at a spectral-type/color which is intermediate between that in much younger and in much older clusters, consistent with rotational spindown of solar-type stars upon their arrival on the ZAMS. The upper envelope of x-ray luminosity as a function of spectral type is comparable to that of the Pleiades, with the observed spread in x-ray luminosity among low-mass members being likely due to the presence of binaries and relatively rapid rotators. However, the list of x-ray selected candidate members is likely biased against low-mass, slowly rotating single stars. While some preliminary spectroscopic information is given in an appendix, further spectroscopic observations of the new candidate members will aid in interpreting the coronal activity among solar-type NGC 6475 members and their relation to similar stars in older and younger open clusters.

Prosser, Charles F.↗

NASA NTRS1993

The evolution of the lithium abundances of solar-type stars. III - The Pleiades

New measurements of lithium in more than 100 Pleiades F, G, and K dwarfs are reported. Abundances are determined from spectrum synthesis fits to the data as well as from use of new covers of growth from the Li 6708-A feature. It is argued that most Late-F and early-G dwarfs in the Pleiades are consistent with the tight N(Li) vs mass relation seen in the Hyades in the same mass range. Most Li-rich stars have abundances at or near the primordial level for Population I, and none exceed that level by a significant amount. At any given color the stars that rotate fast have the most Li and have the strongest chromospheric activity. Ways in which an apparent spread in N(Li) could arise from an intrinsically tight n(Li)-mass relation are considered, and it is concluded that the spread is probably real and is not an artifact of line formation conditions or inhomogeneous atmospheres on the stars.

Soderblom, David R.↗

NASA NTRS1993

Lithium and rapid rotation in chromospherically active single giants

The rotational velocities presently obtained via spectroscopic observations of a group of moderately rapidly rotating, chomospherically active single giants indicate that Gray's (1989) rotostat hypothesis requires modification. Their rapid rotation appears to be due to high Li abundances, and results in increased chromospheric activity. A scenario is projected in which the surface convection zone reaches the rapidly rotating core just as a star begins its first ascent of the giant branch, and dredges both high angular momentum material and freshly synthesized Li to the surface.

Fekel, Francis C.↗

NASA NTRS1991

A spectroscopic investigation of the eclipsing binary Epsilon Aurigae

The objectives were to examine, in detail, the spectra of the eclipsing binary Epsilon Aurigae taken with the IUE satellite telescope during the 1982 to 1984 eclipse. All of the low resolution spectra were analyzed and UV light curves are presented. The primary findings are as follows: (1) a constant eclipse depth from 1600 A to longer wavelengths and a sharp drop in the eclipse depth from 1600 to 1200 A; (2) the absence of large amplitude fluctuations in the UV as expected from a Cepheid primary; and (3) equal ingress and egress times in contradiction to that interpreted from visible light curves. The effects of these findings on the eclipse geometry are being studied.

Balachandran, Suchitra↗

NASA NTRS1988

Lithium in lower-main-sequence stars of the Alpha Persei cluster

Lithium abundances are presented for main-sequence stars of spectral types F, G, and K in the young open cluster Alpha Per. For 46 cluster members, a correlation between Li abundance and projected rotational velocity v sin i is found: all of the Li-poor stars are slow rotators. Two explanations are proposed to account for the correlation: (1) that the Li depletion is introduced following a rapid spin-down phase experienced by young low-mass stars, and that this episode of Li depletion may be the dominant one determining the spread of Li abundances among young low-mass main-sequence stars, and (2) that star formation has occurred over a finite period such that the older stars have undergone a spin-down and depletion of Li by a means that may or may not depend on rotation. The Li abundance in the warm and rapidly rotating stars appears to be undepleted, as is predicted by recent models of pre-main-sequence stars. The depletion observed in the cool stars exceeds the level predicted by these models.

Balachandran, Suchitra↗