Differential rotation and star-spot evolution on HK Aqr in 2001 and 2002
Mon. Not. R. Astron. Soc. 352, 589–599 (2004)
doi:10.1111/j.1365-2966.2004.07949.x
Differential rotation and star-spot evolution on HK Aqr in 2001 and 2002
J. R. Barnes,1 D. J. James2,3 and A. Collier Cameron1
1 School
of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS
d’Astrophysique, Observatoire de Grenoble, BP 53, F-38041 Grenoble, Cedex 9, France
3 Physics and Astronomy Department, Vanderbilt University, 1807 Station B, Nashville, TN 37235, USA
2 Laboratoire
Accepted 2004 April 22. Received 2004 April 6; in original form 2004 January 15
ABSTRACT
Key words: line: profiles – methods: data analysis – techniques: miscellaneous – stars:
activity – stars: individual: HK Aqr (Gl 890) – stars: late-type.
1 INTRODUCTION
In recent years the photospheric spot distributions on a number of
late-type rapid rotators have been derived through the use of indirect imaging techniques. Nevertheless, the rapidly rotating late-K to
M dwarfs have largely remained beyond the abilities of the technique
due to their rarity and low luminosities. Conventional Doppler imaging using only a single line or a small portion of spectrum has been
impossible because spectra with sufficient signal-to-noise (S/N) and
with the necessary short exposure times cannot be obtained with
4-m class telescopes with current instrumentation. This problem
has been partially overcome for imaging codes which model spot
filling factor rather than temperature. The technique of least-squares
deconvolution, first applied by Donati & Collier Cameron (1997)
has subsequently been adopted by Barnes et al. (1998) to obtain
high-resolution images of faint cluster G dwarfs and the K dwarfs,
PZ Tel and Speedy Mic (Barnes et al. 2000, 2001a).
HK Aqr (m v = 10.85) is one of the fastest known M-dwarf rapid
rotators (v sin i = 69 km s−1 ), and is among the most well-studied
stars at this spectral type (dM1.5e). The rotation period of 0.4307
d was determined by Young et al. (1990) from V-band photometric
variations. These variations were shown by Bopp et al. (1988) to be
as much as 0.15 mag at any given epoch. A thorough study of the Hα
line (which appears in emission) by Byrne, Eibe & Rolleston (1996)
revealed the presence of absorption transients which were attributed
to a system of prominences below, or at the corotation radius. Using
the data set of Byrne et al. (1996) and Barnes & Collier Cameron
(2001, hereafter BC01) presented the first images of HK Aqr, which
revealed the presence of star-spots at low and intermediate latitudes.
The suggestion that high-latitude spot activity is responsible for the
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2004 RAS
light-curve morphology (Young et al. 1990) was found not necessarily to be correct. It was shown that the light-curve shape derived
from the spectroscopically derived surface images (showing only
low-intermediate latitudes covered with star-spots) was consistent
with previous light-curve variations.
In this paper we investigate evolution of the star-spot morphology
of HK Aqr by presenting new images derived from spectroscopic
data taken at two sites during 2001 and 2002. We compare these
results with previous images derived for this star and we investigate
the amplitude of the latitude-dependent surface rotation.
2 O B S E RVAT I O N S A N D DATA R E D U C T I O N
The observations of HK Aqr were made during two seasons of
observing in 2001 and 2002 using the Anglo-Australian Telescope
(AAT) with the University College London Echelle Spectrograph
(UCLES), and the Blanco 4-m Cerro Tolo Inter-American (CTIO)
telescope. Observations at the AAT were taken on 2001 September
03 and 04 with the UCLES 31.6 g mm−1 grating and a slit width of
1.2 arcsec. A useful wavelength range of 4360–6904 Å was recorded
on the 2K × 4K EEV2 CCD with 2× binning in the dispersion
direction and 3× binning in the cross-dispersion direction. One night
of observations was made at the Blanco 4-m telescope on 2001
September 07. The 31.61 g mm−1 echelle spectrograph is Cassegrain
mounted and observations were made using a 1.2-arcsec slit width.
The 2K × 2K TEK CCD at the Blanco 4-m telescope yielded a
useful wavelength range of 4243–7292 Å and was binned by a factor
of 2× in the cross-dispersion direction. We also observed HK Aqr
at the Blanco 4-m telescope on 2002 July 30 and 31 and August 01
and 02 with a wavelength range of 4136–7214 Å, and a 1.2-arcsec
slit width, as in 2001. A summary of the observations is given in
Table 1.
We present images of the rapidly rotating (P rot = 0.431 d) M1.5Ve dwarf HK Aqr, from data
obtained during 2001 and 2002. Star-spots are found distributed at a range of latitudes during
both observing seasons, but unlike other solar-type rapid rotators of higher mass, HK Aqr does
not show a strong polar cap at either epoch. In addition to the surface brightness images, we
make use of the four-night time-base over which the 2002 observations were made, in order to
derive an estimate of the latitudinal dependent rotation on HK Aqr. We find that the equatorlap-pole time lies in the range −1449 to +448 d. Although very small, we are therefore unable
to determine whether the degree of differential rotation is in the solar or anti-solar sense.
590
J. R. Barnes, D. J. James and A. Collier Cameron
Table 1. Journal of observations for HK Aqr at the AAT and CTIO 4-m telescopes.
Object
AAT
2001 Jul 03
HR 7121
HK Aqr
HK Aqr
Gl 876
Gl 825
HK Aqr
Moon
HK Aqr
Gl 825
HD 23169
2001 Jul 04
HK Aqr
2002 Jul 30
Gl 825
HR 6970
HR 8183
HR 7447
HK Aqr
2002 Jul 31
HR 5384
HR 6349
HR 6143
HK Aqr
2002 Aug 01
HR 5384
HR 6349
Gl 555
HR 6143
HK Aqr
2002 Aug 02
HR 5384
HR 6349
HR 6143
HK Aqr
start
UT
end
Exp time
(s)
No. of frames
Comments
09:47
10:21
11:22
15:29
15:41
15:55
16:41
16:46
17:32
17:40
09:47
10:28
15:19
15:39
15:46
16:40
16:44
16:53
17:35
17:45
10
400
400
600
300
400
1,3,5
400
150
300
1
1
15
1
1
6
3
6
1
1
17:55
19:03
400
9
23:29
23:50
06:21
06:47
23:44
23:50
06:41
09:22
900
8
1200
400
1
1
1
1
M1V template
B2V standard
M4V template
02:56
03:06
03:18
03:29
03:55
02:54
03:07
03:21
03:29
10:08
500
80
150
25
500,450,550
1
1
1
1
37
M1–2V template
G8III RV std
K0III RV std
B5 III telluric std
23:26
23:41
23:55
03:23
23:29
23:44
23:55
11:00
200
200
22
400
1
1
1
44
G1V RV std
F8.5IV–V RV std
B2 III–IV telluric std
23:20
23:32
23:49
00:28
03:40
23:23
23:35
00:20
00:28
10:37
150
200
900
25
425
1
1
2
1
41
G1V RV std
F8.5IV–V RV std
M4 std
B2 III–IV telluric std
23:13
23:26
23:36
04:41
23:15
23:29
23:36
10:10
120
150
18
450
1
1
1
31
G1V RV std
F8.5IV–V RV std
B2 III–IV telluric std
Pixel-to-pixel variations were removed using flat-field exposures
taken with an internal tungsten reference lamp. The worst cosmic
ray events were removed at the pre-extraction stage using the FIGARO
routine BCLEAN. Scattered light was modelled by fitting polynomials of degree 7 to the sets of inter-order pixels at each X-position in
each frame, and the spectra were optimally extracted (Horn (...truncated)
