11 research outputs found
Why do some spotted stars become bluer as they become fainter?
Chromospherically active, spotted stars generally become redder as well as
fainter when large starspots rotate into view on the stellar disc. However, the
RS CVn system UX Ari (a triple-lined system), becomes bluer as it gets fainter.
One possible explanation is that hot, bright facular regions accompany the
cool, dark photospheric spots of the active component. The bluer flux of the
hotter, inactive component does not appear to be sufficient to explain the
observed behaviour. We have begun a search for additional chromospherically
active stars with a similar relation between colour and brightness, to
investigate whether these relations can be explained in the same way. Our
results for V711 Tau are presented here, and we conclude that the faculae
explanation holds also in this case.Comment: 3 pages, 1 figure, proceedings from the 13th Cambridge Workshop on
cool stars, stellar systems, and the Sun, Hamburg, Germany, July 2004,
ESA-SP, in pres
A Survey of IUE Spectra of the Active Binary System UX Arietis
To investigate the ultraviolet (UV) activity of the bright, non-eclipsing,
double-lined spectroscopic binary system UX Ari, IUE spectra (194 images) were
taken from the IUE archive. The spectra, obtained during the period 1978-1996,
show emission lines originating in the chromosphere and transition region. The
long-wavelength low dispersion spectra were examined for ultraviolet excess by
comparing the UV continuum level of UX Ari with the levels of Cet and
Cep in the spectral range 2100 - 3200 . The individual MgII
h and k emission-line fluxes of component stars show that the contributions to
the activity of the system for G5 V and K0 IV are about 20% and 80%,
respectively. Apart from the flare event observed on 1979 Jan 1, there are some
flux enhancements in the years 1987, 1991 and 1994 which may suggest existence
of a periodicity of about 7-9 years. Examination of the ultraviolet excess in
the system showed that there is some UV excess in UX Ari, which varies from 1%
up to 24% with the exception of two images which showed no UV excess. The
results revealed that there was an agreement on the source of emission regions
which could be attributed to the magnetic activity. The contribution of G5 V
and K0 IV components to the MgII activity of the system suggests a need to take
into consideration the spot distribution not only on the surface of K0 IV but
also on the surface of G5 V component of UX Ari.Comment: 15 pages, 10 figure
Long-term chromospheric activity of non-eclipsing RS CVn-type stars
Context. The IUE database provides a large number of UV high and
low-resolution spectra of RS CVn-type stars from 1978 to 1996. In particular,
many of these stars were monitored continuously during several seasons by IUE.
Aims. Our main purpose is to study the short and long-term chromospheric
activity of the RS CVn systems most observed by IUE: HD 22468 (V711 Tau, HR
1099, K1IV+G5V), HD 21242 (UX Ari, K0IV+G5V) and HD 224085 (II Peg, K2IV).
Methods. We first obtain the Mount Wilson index S from the IUE high and
low-resolution spectra. Secondly, we analyse with the Lomb-Scargle periodogram
the mean annual index S and the amplitude of its rotational modulation.
Results. For HD 22468 (V711 Tau, HR 1099), we found a possible chromospheric
cycle with a period of 18 years and a shorter cycle with a period of 3 years,
which could be associated to a chromospheric "flip-flop" cycle. The data of HD
224085 (II Peg) also suggest a chromospheric cycle of 21 years and a flip-flop
cycle of 9 years. Finally, we obtained a possible chromospheric cycle of 7
years for HD 21242 (UX Ari).Comment: accepted for publication in Astronomy and Astrophysic
Limits of ultra-high-precision optical astrometry: Stellar surface structures
We investigate the astrometric effects of stellar surface structures as a
practical limitation to ultra-high-precision astrometry, e.g. in the context of
exoplanet searches, and to quantify the expected effects in different regions
of the HR-diagram. Stellar surface structures are likely to produce
fluctuations in the integrated flux and radial velocity of the star, as well as
a variation of the observed photocentre, i.e. astrometric jitter, and closure
phase. We use theoretical considerations supported by Monte Carlo simulations
to derive statistical relations between the corresponding astrometric,
photometric, and radial-velocity effects. For most stellar types the
astrometric jitter due to stellar surface structures is expected to be of order
10 micro-AU or greater. This is more than the astrometric displacement
typically caused by an Earth-size exoplanet in the habitable zone, which is
about 1-4 micro-AU for long-lived main-sequence stars. Only for stars with
extremely low photometric variability (<0.5 mmag) and low magnetic activity,
comparable to that of the Sun, will the astrometric jitter be of order 1
micro-AU, suffcient to allow the astrometric detection of an Earth-sized planet
in the habitable zone. While stellar surface structure may thus seriously
impair the astrometric detection of small exoplanets, it has in general
negligible impact on the detection of large (Jupiter-size) planets and on the
determination of stellar parallax and proper motion. From the starspot model we
also conclude that the commonly used spot filling factor is not the most
relevant parameter for quantifying the spottiness in terms of the resulting
astrometric, photometric and radial-velocity variations.Comment: 12 pages, 4 figures, submitted to A&
Unravelling the nature of HD 81032 - a new RS CVn Binary
BVR photometric and quasi-simultaneous optical spectroscopic observations of
the star HD 81032 have been carried out during the years 2000 - 2004. A
photometric period of d has been detected for this star. A
large group of spots with a migration period of years is
inferred from the first three years of the data. H and Ca II H and K
emissions from the star indicate high chromospheric activity. The available
photometry in the BVRIJHK bands is consistent with spectral type of K0 IV
previously found for this star. We have also examined the spectral energy
distribution of HD 81032 for the presence of an infrared colour excess using
the 2MASS JHK and IRAS photometry, but found no significant excess in any band
abovethe normal values expected for a star with this spectral type. We have
also analyzed the X-ray emission properties of this star using data obtained by
the ROSAT X-ray observatory during its All-Sky Survey phase. An X-ray flare of
about 12 hours duration was detected during the two days of X-ray coverage
obtained for this star. Its X-ray spectrum, while only containing 345 counts,
is inconsistent with a single-temperature component solar-abundance coronal
plasma model, but implies either the presence of two or more plasma components,
non-solar abundances, or a combination of both of these properties. All of the
above properties of HD 81032 suggest that it is a newly identified, evolved RS
CVn binary.Comment: 18 pages, 10 figures, 3 tables, Accepted for the publication in JAp
Spots, plages, and flares on lambda Andromedae and II Pegasi
We present the results of a contemporaneous photometric and spectroscopic
monitoring of lambda And and II Peg aimed at investigating the behavior of
surface inhomogeneities in the atmospheres of these active stars which have
nearly the same temperature but different gravity. The light curves and the
modulation of the surface temperature, as recovered from LDRs, are used to map
the photospheric spots, while the H-alpha emission has been used as an
indicator of chromospheric inhomogeneities. The spot temperatures and sizes
were derived from a spot model applied to the contemporaneous light and
temperature curves. We find larger and cooler spots on II Peg (T_sp ~ 3600 K)
compared to lambda And (T_sp ~ 3900 K); this could be the result of both the
different gravity and the higher activity level of the former. Moreover, we
find a clear anti-correlation between the H-alpha emission and the photospheric
diagnostics. We have also detected a modulation of the intensity of the HeI D_3
line with the star rotation. A rough reconstruction of the 3D structure of
their atmospheres has been also performed by applying a spot/plage model to the
light and temperature curves and to the H-alpha flux modulation. A close
spatial association of photospheric and chromospheric active regions has been
found in both stars. Larger and cooler spots have been found on II Peg, the
system with the active component of higher gravity and higher activity level.
The area ratio of plages to spots seems to decrease when the spots get bigger.
Moreover, with the present and literature data, a correlation between the
temperature difference Delta_T = T_ph - T_sp and the surface gravity has been
also suggested. In addition, a strong flare affecting the H-alpha, the HeI D_3,
and the cores of NaI D_1,2 lines has been observed on II Peg.Comment: Accepted for publication in A&A (7 November 2007) 12 Pages, 7 Figure
Recovering facular areas through Doppler imaging
Similar to the Sun, stellar active regions appear to consist of dark
spots in combination with surrounding bright facular regions.
The present study uses the Doppler imaging technique to recover the
stellar surface structures from simulated spectral observations
obtained from a given theoretical stellar surface.
The objective of the study is to investigate how the recovered
surface structure depends on observing parameters such as rotational
phase coverage, choice of spectral lines, signal-to-noise ratio, and
projected equatorial rotational velocity.
The inversions demonstrate that those stellar surface active
regions with high temperature contrast in both longitudinal and
latitudinal directions can be satisfactorily reproduced using
Doppler imaging.
Observations must, however, fulfill such strict conditions that
a complete reconstruction is somewhat unrealistic.
Nevertheless, in a more realistic example, the hot facular areas
result in quite distinct and easily recognizable features on the
stellar surface map, even though they are not completely
reconstructed
Separation of high resolution spectra of the multiple star system
This paper presents a technique for separation of individual
components in composite stellar spectra. The
technique is based on a comparison of residual depths of the
component spectral lines to residual depths of the corresponding
spectral lines of a single star of the same spectral class. We apply
the technique to high resolution spectra of the RS CV
BV photometry of UX Ari in the period 1987â2002
We present 14.3 years of previously unpublished photometric observations of UX Ari. The observations were obtained between 1987.9 and 2002.2 using the T3 0.4-metre Automatic Photoelectric Telescope at Fairborn Observatory and consist of 1228 B band and 1213 V band measurements. The comparison star was 62 Ari. We have analyzed the new data together with previously published photometric observations. The V magnitude shows variations with dominant periods of about 12 and 25 years, where the longest period seems to correspond to an activity cycle. The previously reported anticorrelation between the colour and V magnitude variations is confirmed