Frederick Walter

I discuss EINSTEIN X-ray observations of late-type (G and K) stars. For the RS CVn systems and the evolved G stars, there is a linear relation between X-ray surface flux and stellar angular velocity. The situation is more complex among the G-K dwarfs: rapid rotators, with rotational periods less than ∼10d, also show a linear relation between X-ray surface flux and angular velocity, but the slow rotators fall some two orders of magnitude below that relation.

We have an ongoing program following the temporal evolution of classical novae using the SMARTS consortium suite of telescopes located at CTIO. These observations include UBVRI-JHK light curves using ANDICAM on the 1.3m, supplemented with optical photometric calibrations using the 1.0m. Spectroscopy in the optical at low to moderate dispersion is obtained with the 1.5m. Generally these data are augmented by observations made on other facilities, particularly the 3.5m telescope at Apache Point Observatory. Here we present work in progress covering the first three months temporal evolution of the classical ONeMg nova outburst of V1187 Sco. V1187 Sco was discovered by A. Takao on 2004 August 3.583 UT near maximum brightness (IAUC 8380). Rate of decline from maximum indicates V1187 Sco to be a very fast nova with t2 ˜ 7 days, and t3 ˜ 16 days, and by mid-October had faded by ˜ 5 mag at V. Emission lines are broad indicating velocities of ˜ 3500 km/s, which are maintained throughout the first three months. We will present the photometric and spectral development of V1187 Sco based on SMARTS data. This research has been supported in part by grants from NASA and NSF, with observing time enabled by the SMARTS consortium.

We present an HST/STIS E140M spectrum of the CTTS RU Lupi. The UV spectrum of RU Lupi is dominated by emission lines, including tracers of hot accreting gas and cool molecular gas. We also detect a strong continuum and wind absorption features. We analyze 90 fluorescent H2 emission lines, and use them to reconstruct the intrinsic Ly-alpha profile.

Through use of service observers, SMARTS offers the novel opportunity to obtain frequent observations of rapidly changing celestial phenomena on timescales from hours to years. This opens up a whole new temporal domain in stellar astrophysics for investigation. We shall summarize some of the exciting results we have been obtaining over the past two years. These include new insights into accretion in pre-main sequence stars, investigations of the accretion in polars, details of the spectrophotometric evolution of novae, spectrophotometric monitoring of a new FU Orionis object, and determination of stellar rotation periods. Undergraduate research with SMARTS is supported by an NSF grant to Stony Brook University.

The sigma Ori cluster is an unbound aggregate of a few hundred young, low-mass stars centered on the multiple system sigma Ori. This cluster is of great interest because it is at an age when roughly half of the stars have lost their protoplanetary disks, and the cluster has a very large population of brown dwarfs. One of the largest sources of uncertainty in the properties of the cluster is that the distance is not well known. The directly measured Hipparcos distance to sigma Ori AB is 350 (+120,-90) pc. On the other hand, the distance to the Orion OB1b subgroup (of which sigma Ori is thought to be a member), 473+/-40 pc, is far better determined, but it is an indirect estimate of the cluster's distance. Also, Orion OB1b may have a depth of 40 pc along our line of sight. We use main sequence fitting to 9 main sequence cluster members to estimate a best fit distance of 420+/-30 pc, assuming a metallicity of -0.16+/-0.11 or 444 pc assuming solar metallicity. A distance as close as 350 pc is inconsistent with the observed brightnesses of the cluster members. At the best fit distance, the age of the cluster is 2-3 Myrs.

99% of the mass of the circumstellar disks surrounding young stars is in cool gas, and most of that is molecular hydrogen (H_2). We are studying this H_2 gas by observing the permitted fluorescent transitions in the UV Lyman and Werner bands. To this end we have been awarded 40 orbits on the HST and 50 ksec of time on

The authors present results of ASCA X-ray observations of two pre-main-sequence stars: the weak emission line T Tauri star HD 142361 (G2 IV) and the Herbig Ae star HD 104237 (A4e). The solid-state imaging spectrometer (SIS) spectra for HD 142361 show a clear emission line from H-like Mg XII (1.47 keV) and spectral fits reveal a multi-temperature plasma with a hot component of at least 16 MK. The SIS spectra of HD 104237 show a complex temperature structure with the hottest plasma at temperatures in excess of ≡30 MK (2.5 keV). Thus, mechanisms that predict only soft X-ray emission (<=1 keV) can now be ruled out for Herbig Ae stars.

The HEAO 1 low-energy detectors have observed soft X-ray emission from nine sources which are classified as RS CVn systems. Spectra in the energy range 0.15-2.8 keV have been obtained of two of these, Alpha Aur (Capella) and UX Ari (HD 21242). The spectrum of Capella indicates intense line emission at 0.85 keV which is interpreted as emission from an