Please note: this page is still under construction.If you have any questions, feel free to call me at (410) 338-4948
or contact me via e-mail at welty@stsci.edu.
Please note: this page is still under construction.
If you have any questions, feel free to call me at (410) 338-4948
or contact me via e-mail at
welty@stsci.edu.
I work in the Commanding (Instruction Management) group at the Space Telescope Science Institute. Most of my time goes into working on software for the Advanced Camera for Surveys (ACS, pronounced "ace"), which is to be installed in the HST during the 1999 servicing mission. In my former life my research focussed on pre-main sequence stars and evolved active stars. I still try to take some time to finish old projects and keep up with what's going on in those areas.
are called T Tauri stars (TTS),
while the more massive PMS stars are called Herbig Ae/Be stars.
The TTS class is typically split into two groups:
those that do and do not have significant circumstellar accretion disks.
Accretion activity generally produces
strong emission lines in the stellar spectra.
The TTS with disks are called "classical" T Tauri stars (CTTS)
because early
H
emission line surveys identified the first ones.
Those without disks are usually called weak-lined T Tauri stars (WTTS).
It should be noted that there is not a one-to-one correspondence
between the C/WTTS designations, based on the
H line strength,
and the presence or absence of significant disk material.
Line strength is affected by more than just accretion activity.
The TTS are sometimes grouped according to other phenomenology. These groups are usually named for the prototype object. Examples are YY Ori stars and FU Ori objects (or FUors). The FUors (the subject of my Ph.D. dissertation; see also Welty et al. 1992) are the result of an instability in a CTTS disk that causes the mass accretion rate to increase dramatically. Consequently, the disk luminosity increases, with rise times from months to several years followed by a slow (decades) decline. At its peak, the disk luminosity dominates at visible wavelengths.
My recent and ongoing PMS star research activities at Penn State, mostly in collaboration with Larry Ramsey, are described in the following sections:
Relevant journal articles are also available for viewing.
Selection criteria for the WTTS were:
One of the interesting results is the discovery of periodic radial velocity variations in 4 of these targets at the few km/s level with periods equal to the photometrically determined rotation periods. These variations are due to non-uniform photospheric temperature distributions, i.e., starspots. None were detected in HP Tau/G2, probably because its lines are so broad and shallow and the signal-to-noise ratio was not as good as for the others. Variations that are not obviously periodic were detected in UX Tau A. The spot-induced variation is seen in both members of the close V773 Tau pair after subtraction of orbits I determined from the data. The figure below shows these variations for V410 Tau during the 1992 and 1993 observing runs (from Welty & Ramsey 1995).
During the 1993 observations of V410 Tau there was evidence for 3 major flares.
One of those events was well observed on the 5th night of the run.
The first figure below shows the behavior of
H
during the run. The sinusoidal component reflects rotational modulation
of a non-uniform chromosphere. The flare is obvious.
The second figure shows flare components of the
H
and He D3
line profiles. Time increases upward.
The narrow He line appears only during the flares.
Its radial velocity variation was used to locate the flare
with respect to recently published Doppler images of the star
(Strassmeier, Welty, & Rice 1994; Hatzes 1995).
These results will be published soon
(Welty & Ramsey 1997).
Selection criteria for CTTS were more relaxed than the WTTS criteria above because no available targets meet the latter. Nevertheless, selection was guided by the same concerns. The CTTS observed are T Tau, RY Tau, DF Tau, DG Tau, RW Aur. GW Ori was observed during a similar run in 1992 November.
Radial velocity variations are apparent in a few of those targets. For T Tau and RY Tau, those variations appeared to be periodic, suggesting companions of brown dwarf mass very close to the stars. I obtained spectra of T Tau during the fall of 1995 with the solar-stellar spectrograph at the McMath-Pierce telescope of the National Solar Observatory to test that idea. Preliminary results indicate that the radial velocity variations are not periodic. REU student Jennifer Kozak has worked with me on these data.
I am also working with Steve Strom on far red spectra we obtained with the 4-m echelle spectrograph at KPNO in 1989 to search for molecular carbon features. Among the results are: a lone detection of C2 in DR Tau; radial velocities in general agreement with cloud velocities; a radial velocity difference between the members of the DK Tau pair that indicates the two stars probably do not comprise a binary system.
I obtained many spectra of AB Aur during my fall FOE runs at KPNO (see the WTTS section above). The 1992 data were obtained in part to support the MUSICOS 1992 campaign. Preliminary MUSICOS results appear in Catala et al. 1994, and more complete analysis will soon appear in Böhm et al. (1996), which also includes some of my data from 1991, 1993, and 1994.
The series of FOE spectra, providing simultaneous coverage of many important spectral lines, should be valuable for more detailed diagnosis of the circumstellar environment of AB Aur. During the summer of 1996 REU student Amanda Kirby has worked on this project. I hope to have results available by the end of the year. A less extensive database of FOE spectra of HD 200775 will also be analyzed.
Analysis of far-red 4-m echelle spectra of several Herbig Ae/Be stars, obtained during the CTTS molecular carbon run (see above), is also in progress. Several Paschen series lines and other lines will be used to test infall models.
The FOE data have already produced one new WTTS orbit solution.
V773 Tau (=HDE 283447) was known to have a K-band companion
at a projected separation of about 20 AU.
The FOE observations revealed double-lined structure in the optically
bright member and were sufficient to determine orbital elements for its
51 day orbit. The radial velocity data are shown below.
See
Welty (1995).
Beginning of Pre-Main Sequence Stars
Go to Welty's home page
FK Com is an apparently single G8 III star
with vsini = 162.5 km/s.
The angular momentum implied requires that it have a close unseen companion
or that it be the product of a binary merger.
Evidence tends to favor the latter possibility.
Penn State's archive of active star spectra contains
a set of 51 FOE spectra of FK Com obtained during an 8 night run on the
KPNO
coudé feed telescope.
In those data I found a periodic variation
of absorption linewidth that suggested non-radial pulsations.
Other less extensive datasets revealed
different patterns of linewidth variation.
In 1994 May I obtained 403 new FOE spectra during an 8 night run
on the KPNO 2.1-m telescope.
Larry Ramsey and I wanted to get higher quality data that would also
provide better time sampling to study our pulsation hypothesis further.
REU student
Peter Ratzlaff
worked with me on these data.
Preliminary results indicate that no linewidth variations were present
during the 1994 observing run.
Nevertheless, these data will provide a wealth of other information about
FK Com.
RS CVn systems are binary stars at least one member of which is a late-type (GKM) post-main sequence star, and whose orbital periods are generally less than three weeks. Penn State has an extensive archive of active star spectra, including many RS CVn systems. Dozens of journal articles and a few Ph.D. dissertations have not exhausted the potential of these data. In particular, studies of long-term behavior have not yet been carried out. Series of spectra of possible mass-transfer systems are also available. Such projects would be ideal for a summer REU student.