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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
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:
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:
- magnitude brighter than V=11
- rotation period less than 3.5 days
- vsini greater than 40 km/s
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
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).
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