Scientific Interests of Alan D. Welty


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.


Pre-Main Sequence Stars


Introduction

Pre-main sequence (PMS) stars are stars still in the process of formation, not yet burning hydrogen in their cores. Many are still accumulating mass from the surrounding environment. They occupy a portion of the Hertzsprung-Russell diagram above and to the right of the main sequence. Astronomers classify PMS stars into several types. Those less than about 2.5 M-sun 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-alpha 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-alpha 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:

Weak-lined T Tauri Stars - Classical T Tauri Stars - FU Ori objects
Herbig Ae/Be stars - PMS Binaries

Relevant journal articles are also available for viewing.


Weak-lined T Tauri stars

I have used the Penn State Fiber Optic Echelle spectrograph (FOE) and Kitt Peak 2.1-m telescope to obtain series of spectra of several weak-lined T Tauri stars to study their stellar activity and binarity. Each of three observing runs (in 1992 November, 1993 December, and 1994 December) was 7 nights long. The number of spectra obtained for each target depended on target priority, brightness, and rotation period, and was subject to limitation due to bad weather conditions. A few targets were observed in more than one run.

Selection criteria for the WTTS were:

These criteria permit adequate signal-to-noise ratio to be obtained in exposure times short enough that phase smearing is unimportant, ensure that photospheric lines are adequately resolved by the FOE (R = 12000; 25 km/s), and allow observation over at least two stellar rotations. Observation over multiple rotations permits discrimination between effects of rotational modulation and transient events. The WTTS observed are V410 Tau, V773 Tau, HDE 283572, HP Tau/G2, UX Tau A, and SU Aur.

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-alpha 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-alpha 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).


Classical T Tauri stars

I have used the Penn State Fiber Optic Echelle spectrograph (FOE) and Kitt Peak 2.1-m telescope to obtain series of spectra of several classical T Tauri stars to study their stellar activity, accretion, and binarity. Their observing run in 1994 December lasted 7 nights. The number of spectra obtained for each target depended on target priority and brightness, and was subject to limitation due to bad weather conditions.

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.


FU Orionis Objects

I obtained several spectra of FU Ori and Z CMa during the winter FOE runs at KPNO (see the WTTS section above). In two KPNO 4-m echelle spectra obtained a year apart, I found that absorption line widths were decreasing with time in V1057 Cyg, as expected in the Hartmann & Kenyon accretion disk model. The FOE data will be used to test this for FU Ori and Z CMa, and to search for short timescale changes in line strengths, profiles, widths, and radial velocities. Changes on timescales of a few days could be indicative of departures from axisymmetry in those systems, giving clues about disk structure and the accretion phenomenon.


Herbig Ae/Be Stars

These are the intermediate mass counterparts of the T Tauri stars. Although most of these stars have significant IR excesses and are associated with bright reflection nebulosity (one of the original criteria for membership in the class), there is generally no strong evidence for circumstellar disks; circumstellar envelopes can explain the IR excesses in most cases.

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.


Pre-Main Sequence Binaries

The T Tauri star FOE spectra I collected (see the WTTS section above) form the beginning of a dataset to be used to search for radial velocity variations indicative of companions to those stars. This project will continue in the years ahead using data to be obtained with the Hobby*Eberly Telescope (HET).

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).


Recent PMS Star Publications

Böhm, T., Catala, C., T., Donati, J.-F., Welty, A.,
Baudrand, J., Butler, J., Carter, B., Collier-Cameron, A., Czarny, J., Foing, B., Ghosh, K., Hao, J., Houdebine, E., Huang, L., Jiang, S., Neff, J., Rees, D., Semel, M., Simon, T., Talavera, A., Zhai, D., & Zhao, F. 1996, A&A, 170, 431
Azimuthal Structures in the Wind and Chromosphere of the Herbig Ae Star AB Aur. Results from the MUSICOS 1992 Campaign
Catala, C., Böhm, T., Donati, J.-F., Simon, T., Welty, A., Houdebine, E.,
Huang, L., Jiang, S., Zhai, D., Neff, J., Foing, B., Ghosh, K., Butler, J., Collier-Cameron, A., Baudrand, J., Czarny, J., Zhao, F., Talavera, A., Kennelly, T., Walker, G., Carter, B., Rees, D., Semel, M., Cutispoto, G., & Rodonò, M. 1994, Solar Physics, 155, 185
Azimuthal Structures in the Wind and Chromosphere of the Herbig Ae Star AB Aur. Preliminary Results from the MUSICOS 1992 Campaign
Feigelson, E. D., Welty, A. D., Imhoff, C. L., Hall, J. C., Etzel, P. B.,
Phillips, R. B., & Lonsdale, C. L. 1994, ApJ, 432, 373
Multiwavelength Study of the Magnetically Active T Tauri Star HD 283447
Grasdalen, G. L., Sloan, G., Stout, N., Strom, S. E., & Welty, A. D.
1989, ApJ, 339, L37
Circumstellar Gas Associated with HL Tauri: Evidence for a Remnant Infalling Envelope
Hartigan, P., Kenyon, S. J., Hartmann, L., Strom, S. E., Edwards, S., Welty, A. D.,
& Stauffer, J. 1991, ApJ, 382, 617
Optical Excess Emission in T Tauri Stars
Patterer, R. J., Ramsey, L. W., Huenemoerder, D. P., & Welty, A. D.
1993, AJ, 105, 1519
Lithium Line Variations in Weak-Lined T Tauri Stars
Strassmeier, K. G., Welty, A. D., & Rice, J. B. 1994, A&A, 285, L17
A Doppler Image of the Weak T Tauri Star V410 Tau
Welty, A. D. 1995, AJ, 110, 776
Discovery of a Pre-Main-Sequence Spectroscopic Binary: V773 Tauri [view PostScript]
Welty, A. D., Barden, S. C., Huenemoerder, D. P., & Ramsey, L. W.
1992, AJ, 103, 1673
BF Orionis: Evidence for an Infalling Circumstellar Envelope
Welty, A. D. & Ramsey, L. W. 1995, AJ, 110, 336
The Activity of Weak-lined T Tauri Stars I: V410 Tauri [view PostScript]
Welty, A. D. & Ramsey, L. W. 1997, ApJ, submitted
Spectroscopic Observations of a Flare on V410 Tauri [view PostScript: text and table of submitted version]
Welty, A. D., Strom, S. E., Edwards, S., Kenyon, S. J., & Hartmann, L. W.
1992, ApJ, 397, 260
Optical Spectroscopy of Z Canis Majoris, V1057 Cygni, & FU Orionis: Accretion Disks & Signatures of Disk Winds
Welty, A. D., Strom, S. E., Strom, K. M., Hartmann, L. W., Kenyon, S. J.,
Grasdalen, G. L., & Stauffer, J. R. 1990, ApJ, 349, 328
Further Evidence for Differential Rotation in V1057 Cygni
Welty, A. D. & Wade, R. A. 1995, AJ, 109, 326
On the Nature of 17 Leporis [view PostScript]


Beginning of Pre-Main Sequence Stars
Go to Welty's home page


Evolved Active Stars

In my ``spare time'' I work with a large database of FOE spectra of several RS CVn systems and the peculiar rapidly rotating giant FK Com. I have also analyzed ROSAT PSPC observations of a few of these stars.

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.

Welty, A. D. & Ramsey, L. W. 1994, AJ, 108, 299
ROSAT Observations of FK Comae Berenices
Welty, A. D. & Ramsey, L. W. 1994, ApJ, 435, 848
The Shape of FK Comae Berenices: Evidence for a Recently Coalesced Binary
Welty, A. D. & Ramsey, L. W. 1995, AJ, 109, 2187
On the Role of Mass Transfer in X-Ray Emission of RS CVn Systems
Welty, A. D., Ramsey, L. W., Iyengar, M., Nations, H. L., & Buzasi, D. L.
1993, PASP, 105, 1427
A Long Term Study of Ha Line Variations in FK Comae Berenices


Web page by Alan D. Welty (welty@stsci.edu)
Last update: 1997 November 17