AB Dor in '94: I. HST/GHRS Observations of the Quiescent Chromosphere of an Active Star

Abstract

We analyze HST/GHRS spectra of AB Doradus, the prototypical ultra-rapidly rotating K dwarf. We observed chromospheric (Mg II) and transition region (C II, Si IV, C IV, and N V) lines periodically throughout the stellar rotation period, and provide a low dispersion stellar atlas of 78 emission lines. The quiescent line profiles of the chromospheric and transition region lines show narrow cores superposed on very broad wings. The broad wings of the Mg II k & h lines and of the transition region lines can be explained by emission from gas co-rotating with the star and extending out to near the Keplerian co-rotation radius (2.8 stellar radii). While this is not a unique solution, it is consistent with previous studies of H-alpha emission that are naturally explained by large co-rotating prominences. We find no evidence for rotational modulation of the emission line fluxes. The density diagnostics suggest that the transition region is formed at constant pressure, with an electron density 2-3 E12 /cm^3 at a temperature of 30,000 K. The electron pressure is about 100 times larger than that for the quiet Sun. The emission measure distribution shows a minimum between log(T) = 5 and 5.5. The Mg II line exhibits three interstellar absorption components along the 15 pc line of sight. We identify the lowest velocity component with the G cloud, but the other components are not identified with any interstellar clouds previously detected from other lines of sight.

Full text

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arXiv:astro-ph/0012487v1 22 Dec 2000
Accepted by AJ, to appear in April 2001
AB Dor in ’94: I. HST/GHRS Observations of the Quiescent Chromosphere of
an Active Star
J.C. Brandt1,2
Laboratory for Atmospheric and Space Physics, Campus Box 392, University of Colorado,
Boulder, CO 80309-0392
jbrandt@lodestar.phys.unm.edu
S.R. Heap1
Laboratory for Astronomy and Solar Physics, Code 681 NASA/Goddard Space Flight Center,
Greenbelt, MD 20771
hrsheap@stars.gsfc.nasa.gov
F.M. Walter1
State University of New York, Physics and Astronomy, Stony Brook, NY 11794-3800
fwalter@astro.sunysb.edu
E.A. Beaver1
Center for Astrophysics and Space Sciences C-0111, University of California, San Diego, La
Jolla, CA 92093-0111
ebeaver@ucsd.edu
A. Boggess1
2420 Balsam Dr. Boulder CO 80304
boggess@lyrae.colorado.edu
K.G. Carpenter1
Laboratory for Astronomy and Solar Physics, Code 681 NASA/Goddard Space Flight Center,
Greenbelt, MD 20771
carpenter@stars.gsfc.nasa.gov
D.C. Ebbets1
Ball Aerospace & Technologies Corp., P. O. Box 1062, AR1, Boulder, CO 80306

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debbets@ball.com
J.B. Hutchings1
Dominion Astrophysical Observatory, 5071 West Saanich Road, Victoria, BC, Canada V8X 4M6
john.hutchings@hia.nrc.ca
M. Jura1
Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1562
jura@clotho.astro.ucla.edu
D.S. Leckrone1
Laboratory for Astronomy and Solar Physics, Code 681 NASA/Goddard Space Flight Center,
Greenbelt, MD 20771
hrsleckrone@hrs.gsfc.nasa.gov
J.L. Linsky1
JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO
80309-0440
jlinsky@jila.colorado.edu
S.P. Maran1
Space Sciences Directorate, Code 600, NASA/Goddard Space Flight Center, Greenbelt MD 20771
hrsmaran@eclaire.gsfc.nasa.gov
B.D. Savage1
Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI
53706
bdsavage@facstaff.wisc.edu
A.M. Smith1
Laboratory for Astronomy and Solar Physics, Code 681 NASA/Goddard Space Flight Center,
Greenbelt, MD 20771
hrssmith@hrs.gsfc.nasa.gov
L.M. Trafton1

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MacDonald Observatory and Astronomy Department, University of Texas, Austin, TX 78712
lmt@astro.as.utexas.edu
R.J. Weymann1
Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena CA
91101
rjw@ociw.edu
D. Norman
State University of New York, Physics and Astronomy, Stony Brook, NY 11794-3800
dnorman@sbast3.ess.sunysb.edu
S. Redfield
JILA, University of Colorado, Boulder, CO 80309-0440
sredfiel@casa.colorado.edu
ABSTRACT
We analyze HST/GHRS spectra of AB Doradus, the prototypical ultra-rapidly ro-
tating K dwarf. We observed chromospheric (Mg II) and transition region (C II, Si IV,
C IV, and N V) lines periodically throughout the stellar rotation period, and provide
a low dispersion stellar atlas of 78 emission lines. The quiescent line profiles of the
chromospheric and transition region lines show narrow cores superposed on very broad
wings. The broad wings of the Mg II k & h lines and of the transition region lines can be
explained by emission from gas co-rotating with the star and extending out to near the
Keplerian co-rotation radius (2.8 stellar radii). While this is not a unique solution, it
is consistent with previous studies of Hα emission that are naturally explained by large
co-rotating prominences. We find no evidence for rotational modulation of the emission
line fluxes. The density diagnostics suggest that the transition region is formed at con-
stant pressure, with an electron density 2–3 × 1012cm−3at a temperature of 3×104K.
The electron pressure is about 100 times larger than that for the quiet Sun. The emis-
sion measure distribution shows a minimum between log(T) = 5 and 5.5. The Mg II
1GHRS Investigation Definition Team
2Now at Department of Physics and Astronomy, Institute for Astrophysics, University of New Mexico, Albu-
querque, NM 87131

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line exhibits three interstellar absorption components along the 15 pc line of sight. We
identify the lowest velocity component with the G cloud, but the other components are
not identified with any interstellar clouds previously detected from other lines of sight.
Subject headings: stars: activity — stars: chromospheres — stars: individual(AB Dor)
— ISM: kinematics and dynamics
1.Introduction
We study magnetically-active solar-like stars because the characteristics of such stars may
provide insights into the nature of solar-like magnetic activity. While we accept the solar paradigm,
that stellar activity is a consequence of magnetic fields, it is not at all clear that any simple scaling
of solar coronal structures can account for the level of activity seen in the most active stars. Indeed,
the very assumption of solar-like structures may bias our interpretation of the observations. Walter
& Byrne (1997), and Walter (1999) proposed a non-solar paradigm for active stars, based in part
on observations of AB Doradus. In this picture, a quasi-dipolar global magnetic field may dominate
the large-scale activity, while the hotter coronal gas is confined by solar-like magnetic structures
to a small scale height. An active stellar atmosphere would then consist of a compact, solar-
like chromosphere/transition region/corona and an extended, co-rotating envelope of large volume.
Ayres et al. (1998) posit a similar picture for the magnetospheres of active Hertzsprung gap giants.
Ultraviolet spectra sample the chromosphere and transition region, at temperatures from about
104K through 2×105K. While emission in the cooler lines may result in part from heating by acoustic
fluxes, the transition region line emission is almost certainly from hot plasma trapped in magnetic
loops. Time-resolved spectra obtained over a stellar rotation can yield important constraints on
magnetic filling factors, global asymmetries, and atmospheric scale heights. Velocity-resolved line
profiles can be inverted to yield spatial resolution on these stars, and images of their surfaces. Our
goal, through detailed observations of highly active stars, is to test our understanding of stellar
coronae and chromospheres, of the morphology of the magnetic field, and of its relation to the
observed activity.
1.1.AB Doradus
AB Doradus (HD 36705; K0-2 IV–V), the brightest (V=6.7) of the ultra-rapid rotators (Prot=0.51479 d;
v sin i=91 km s−1), is the quintessential active young single star. In addition to large flares (e.g.,
Robinson & Cameron 1986), large starspots (Anders et al 1992), and large coronal and chromo-
spheric fluxes (Pakull 1981; Vilhu, Gustafsson, & Walter 1991), AB Dor also possesses co-rotating
material at 2−5 stellar radii (Cameron & Robinson 1989), in the form of cool prominences or Hα
clouds (Cameron et al. 1990). Its brightness and activity levels make it an ideal target to study
the extremum of stellar magnetic activity. Among the recent studies are those by Rucinski et al.

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(1995b); Mewe et al. (1996); K¨ urster et al. (1997); Schmitt, Cutispoto, & Krautter (1998) Vilhu
et al. (1998), and Ake et al. (2000).
The parallax (Guirado et al. 1997) places the star at 15 pc, slightly above the zero-age main
sequence (Cameron & Foing 1997), with a probable age close to that of the Pleiades. AB Dor is a
member of a multiple star system. The dMe4 star Rossiter 137B (AB Dor B) is a common proper
motion companion at an angular distance of 10 arcsec (Innis et al. 1985; Innis, Thompson, &
Coates 1986; Vilhu et al. 1989). Guirado et al. (1997) detected a low mass astrometric companion,
AB Dor C, with an inferred separation of a few tenths of an arcsec. R 137B is outside the GHRS
aperture; AB Dor C falls within the aperture, but is unlikely to make any significant contribution
to the observed flux.
1.2. The November 1994 Campaign
We observed AB Dor with the Goddard High Resolution Spectrograph (GHRS; Brandt et
al. 1994; Heap et al. 1995; Robinson et al. 1998) aboard the Hubble Space Telescope, as part
of a multiwavelength campaign in November 1994 (Walter et al. 1995). Our goal was to obtain
simultaneous spectroscopic and photometric observations at X-rays, UV, optical, and radio wave-
lengths over at least one stellar rotation period, in order to correlate the coronal, chromospheric,
and optical behaviors, and to come up with a 3-dimensional picture of the atmosphere of a very
active star.
The optical spectroscopy and photometry, and the Doppler images, have been reported by
Cameron et al. (1999). AB Dor behaved normally (for AB Dor). There was a prominent pho-
tometric wave, indicating a highly asymmetric starspot distribution. We observed no large flares
(i.e., flares with durations in excess of about an hour). Cameron et al. (1999) report a number of
strong absorption events due to cool material (extended prominences) co-rotating at high altitude.
The Doppler image (Cameron et al. 1999) shows a dark feature at high latitudes, with some low
latitude spottedness. Vilhu et al. (1998) report a continuous GHRS observation of the C IV line
which immediately preceeded our observations.
We will report on the variability of the chromosphere and corona in a subsequent paper (Walter
et al., in preparation). Here we present an analysis of the quiescent chromosphere of AB Dor as
viewed over one stellar rotation period by the GHRS instrument on the HST.
2.The Observing Program and Data Reductions
We observed AB Dor (program 5181) with the HST/GHRS on 1994 November 14-15 while the
target was in the continuous viewing zone. The observations began near the end of stellar rotation