Active Late-type Stars Research Articles

ITAL]EXTREME ULTRAVIOLET EXPLORER[/ITAL][ITAL]Extreme Ultraviolet Explorer[/ITAL] Optical Identification Campaign. IV. A Northern Hemisphere Sample of Active Late-Type Stars and Typical EUV Sources

We present new optical identifications of previously unidentified faint extreme ultraviolet sources. Our total sample of 30 identified sources, of which 22 are new identifications, includes 24 late-type stars, three white dwarfs, two cataclysmic variables (CVs), and one active galactic nucleus. These sources are joint detections of the faint sources from the all-sky surveys of the Extreme Ultraviolet Explorer (EUVE) in the 58–174 A (0.071–0.214 keV) EUV band and of the ROSAT Position Sensitive Proportional Counter in the 5–120 A (0.1–2.4 keV) X-ray band. We obtained medium-resolution spectra of the possible optical counterparts with the Shane 3 m telescope at Lick Observatory using the Kast double spectrograph covering a bandpass of 3600–7500 A. Our sample of active late-type stars is dominated by K and M stars showing strong Balmer and Ca II emission lines. The white dwarfs are fairly typical for those detected in the EUVE survey with Teff and log g ranging from 35 to 53 kK and 7.6 to 8.7, respectively. We found strong H and He emission lines typical of cataclysmic variables (CVs) for EUVE J0854+390 and EUVE J1802+180. EUVE J0854+390 is a newly identified cataclysmic variable showing radial velocity shifts to the red as large as ≈400 km s-1. We associate EUVE J1802+180 with the previously identified CV, V884 Her (RX J1802.1+1804). Including the present work (22 new identifications), EUVE optical identification campaigns have identified ≈28% of the presently cataloged NOID sources.

Measurements of the general magnetic fields of active late-type stars

Measurements of the general magnetic fields of active, late-type stars obtained with the 2.6-m Sha\(\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{l} \)n telescope of the Crimean Astrophysical Observatory are reported. Statistically significant values of the magnetic fields of ∈ Eri and 61 Cyg A have been detected. The magnetic fields range from −10 to 21 G. The magnetic-field values for ∈ Eri and 61 Cyg A exceed the solar field by an order of magnitude; ∈ Eri is more active than the Sun, while 61 Cyg A has approximately the same activity level as the Sun. There is some evidence for variations in the general magnetic fields of these stars associated with their axial rotation.

ROSAT HRI observations of P Cyg and surrounding area

We present a detailed analysis of deep ROSAT HRI observations of the luminous blue variable P Cyg and its surrounding radio nebula. The HRI image provides a point source at the position of P Cyg. However, we show that this emission can be attributed to the ultraviolet leak of the ROSAT HRI. The X-ray flux upper limit derived from the HRI data of this star is discussed in the context of X-ray emission from hot stars. Furthermore, we present a search for diffuse X-ray emission possibly associated with the radio nebula surrounding P Cyg. We compare our results to model predictions and X-ray fluxes observed for shells around other hot stars. Additionally, we detect 10 X-ray sources in the field of view. All but one of these X-ray emitters have stellar counter parts in the Palomar Sky Survey. We suggest that they are active late-type stars possibly belonging to Cyg OB1.

A Search for Rotational Modulation of X‐Ray Centers on the Classical Be Star γ Cassiopeiae

In this paper we discuss X-ray observations of γ Cas obtained in 1998 November with the Rossi X-Ray Timing Explorer (RXTE). The data were obtained nearly continuously over 54 hr, which is about twice the expected rotational period. An earlier RXTE light curve obtained in 1996 March over a 27 hr period showed X-ray flux arising from short-duration shots (flares) superimposed on an undulating component that was anticorrelated with fluctuations of the UV continuum over a timescale of ~10 hr. The object of the present study was to (1) examine the long-term variations of the X-ray characteristics through comparisons with this earlier data and (2) to determine whether variations of the basal flux repeat during a second rotation period. A comparison of the results with the 1996 data set shows a number of similarities and differences in the X-ray behavior: (a) the mean X-ray level in 1998 was only 60% of the 1996 level, (b) the basal fluxes in 1998 vary over shorter timescales (less than 2 hr) than in 1996, (c) the shots were found to show a slightly softer (cooler) mean color than the basal component in 1998, although they were slightly hotter in 1996, (d) fluctuations in the colors of the shot and basal fluxes generally track one another in both data sets, (e) cyclical patterns of X-ray flux decrease with a period of about 7.5 hr occurred in both data sets, and (f) the frequency of shots with a given integrated energy was found to decrease exponentially with energy, although the rate of decrease in 1996 was slower than in 1998. There was only marginal evidence for a repetition during the second half of the time sequence of long-term basal flux variations seen during the first half of the observations. We suspect, however, that the large intrinsic variability of the X-ray source would have masked a true replication. We also present archival IUE data that shows the presence of UV continuum variations in 1982 with similar characteristics to those seen in 1996. This suggests that the regions responsible for the UV variability are very long lived. The data also provide the basis for a refined but still tentative rotational period of 1.12277 days. Assuming a flare paradigm and a very simple electron beam model, we examine the atmospheric heating expected for the shot events. We conclude that it is possible to explain how the measured shot temperature can be smaller than the temperature deduced for the basal X-ray emission. We also discover that if the beam model is correct then the electrons within the beam have relatively high energies (>200 keV) and are nearly monoenergetic. In three appendices we discuss arguments, first, against the idea that the X-ray emission from γ Cas arises from mass accretion onto a hypothetical white dwarf companion or from an active late-type star and, second, in favor of its origin from near the surface of γ Cas.

Stellar coronal activity

I review recent results on coronal activity in late-type stars stressing the similarities and differences with respect to solar coronal activity. The emphasis is on recent stellar observations by ROSAT, ASCA and SAX and on prospects for future observations by AXAF and XMM. Reviewed topics include X-ray emission in different parts of the HR diagram, cool stars in open clusters and star forming regions, and X-ray spectroscopy of coronal sources.

Properties of Hot White Dwarfs in Extreme‐Ultraviolet/Soft X‐Ray Surveys

Intermediate-dispersion spectroscopy (Δ ~ 6 A) of 38 ultrasoft X-ray sources (ROSAT PSPC/WFC and EUVE)—27 H-rich white dwarfs (DA), one magnetic white dwarf (DAp), eight active galactic nuclei, a new cataclysmic variable, and an active late-type star—is presented. Atmospheric (Teff, log g) and stellar (age, mass) parameters of the DA white dwarfs are determined, for the first time in the case of 12 objects. Adding the present sample to the EUV-selected sample of previous studies by Vennes et al., I define an enlarged population of 141 hot white dwarfs and redetermine the DA white dwarf mass distribution taking into account improved mass measurements for ultramassive white dwarfs. High-dispersion spectroscopy (Δ ~ 1.0-1.4 A) of the Hα line core in a representative collection of white dwarfs (the ultramassive DA GD 50, two hot DAs, and the low-mass DA EUVE J0512-006) indicate low projected rotation velocity vrot sin i ≤ 30-65 km s-1 and no perceptible radial velocity variations in the low-mass white dwarf EUVE J0512-006. Questions on the origin and evolution of hot white dwarf stars are examined in light of these results.

Automatic spectral classification of low-mass late-type stars: LI Jin-zeng & HU Jing-yao Beijing Astronomical Observatory, CAS, Beijing 100080

We discuss observations of open clusters and star forming regions obtained with the RGS and EPIC instruments on board XMM-Newton. These observations provide a powerful tool to investigate the temperature structure, emission measure distribution and elemental abundances of stars from the pre-main sequence phase to post-main sequence evolution. We report in particular on EPIC and RGS spectroscopy of hot and cool stars in the very young cluster around σ Orionis, of two stars in the Hyades (the main-sequence star VB 50 and the clump giant VB 71) and of two pre-main sequence stars in Taurus–Auriga (the classical T Tauri star SU Aur and the weak-lined T Tauri star HD 283572). The implications of these observations for models of magnetic activity in late-type stars, of wind emission in early-type stars, and of X-ray production in the early stages of stellar evolution are briefly discussed.

Doppler Imaging of Stars with Surface Inhomogeneities

AbstractI review recent progress in the field of stellar surface imaging, with particular reference to advanced methods for mapping surface-brightness distributions on magnetically active late-type stars. New signal enhancement techniques, utilising profile information from hundreds or thousands of photospheric lines simultaneously, allow images to be derived for stars several magnitudes fainter than was previously possible. For brighter stars, the same techniques make it possible to map features as small as two or three degrees in extent on the stellar surface. While this opens up whole new areas of research, such as the ability to use starspot tracking to study surface differential rotation patterns on single and binary stars, caution must be exercised in the treatment of “nuisance” parameters such as the stellar rotation rate, surface abundances and radial velocity. At the very high S:N levels we now use, the effects of systematic errors in these parameters are easier to identify, isolate and eliminate. This leads to the possibility of making precise radial velocity variations (at the few hundred m s−2 level or better) in late-type stars even with equatorial rotation speeds as high as 100 km s−1. This is particularly topical given the recent discovery that one of our prime imaging targets, the young southern K0 dwarf AB Doradus, has an astrometric companion in a highly eccentric orbit with an inferred mass close to the H-burning limit.

Metallicity and photospheric abundances in field K and M dwarfs

Elemental abundances in late-type stars are of interest in several ways: they determine the location of the stars in the HR diagram and therefore their ages, as well as the atmospheric structure in their middle and upper photospheres. Especially in the case of chromospherically active late-type stars the question arises to what degree the upper photosphere is influenced by the nearby chromosphere. Analysing S/N ∼ 200 and Δλ/λ ∼ 20 000 data, we found a mean metallicity index [M/H] = −0.2 for programme K and M field stars based on an analysis of spectra in the region 5500–9000 Å. We also found that the Ca i 6162-Å transition is a potential surface gravity indicator for K-type stars. For the chromospheric activity interval 4.4 < log FMgii < 6.6 we did not find any chromospheric activity impact on photospheric and upper photospheric transitions. With the derived metallicity, we confirmed the Li abundance from our previous paper and thus its dependence on the Mg ii chromospheric activity index. The nature of the spectrum for the active M-type star Gl 896A is explained by pure rotation of 14 km s−1. As far as the lithium—rotation relation is concerned, the spectrum of Gl 517 is rotationally broadened as well, by 12 km s−1, and the Li abundance is the second highest in our sample of stars. However, there is no link between very high Li abundance, 2.2 dex, in the K dwarf star Gl 5 and stellar rotation.

Ultraviolet Spectroscopy of AB Doradus with the [ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]Telescope[/ITAL]: Impulsive Flares and Bimodal Profiles of C [CSC]iv[/CSC] λ1549 in a Young Star

We observed AB Doradus, a young and active late-type star (K0–K2 IV–V, P = 0.514 days), with the Goddard High Resolution Spectrograph of the post-COSTAR Hubble Space Telescope with time and spectral resolutions of 27 s and 15 km s-1, respectively (1994 November 14.08–14.30 UT). The wavelength band (1531–1565 A) included the strong C IV doublet (1548.202 and 1550.774 A, formed in the transition region at 105 K), the chromospheric Si II 1533.432 A line, and the blend of Si I, C I, and Fe II lines at 1561 A. The mean quiescent C IV flux state was characterized by FC IV = (7.80 ± 0.34) × 105 ergs cm-2 s-1, close to the saturated value and 100 times the solar one. The line profile (after removing the rotational and instrumental profiles) is bimodal, consisting of two Gaussians, one narrow (FWHM = 70 km s-1) and the other broad (FWHM = 330 km s-1). This bimodality is probably due to two separate broadening mechanisms and velocity fields at the coronal base. It is possible that transition-region transient events (random multiple velocities), with large surface coverage, give rise to the broadening of the narrow component while true microflaring is responsible for the broad one, as suggested by Wood, Linsky, & Ayres. The transition region was observed to flare frequently with different timescales and magnitudes. The largest impulsive flare seen in the C IV λ1549 emission, at day 14.22, reached in less than 1 minute a peak differential emission measure N V (104.85–105.15 K) = 1051.2 cm-3 and returned exponentially in 5 minutes to the 7 times lower quiescent level. The 3 minute average line profile of the flare was blueshifted (-190 km s-1) and broadened (FWHM = 800 km s-1). This impulsive flare could have been due to a chromospheric heating and subsequent evaporation by an electron beam, accelerated (by reconnection) at the apex of a coronal loop.

Evidence of Prominences on Cool Late-type Stars

AbstractWe review the observed characteristics of co-rotating clouds of neutral hydrogen recently detected in rapidly rotating, chromospherically active late-type stars and commonly termed “stellar prominences”. Their observed properties are placed in an overall interpretive framework and compared to standard solar prominences.

Faint Sources in the EUVE Survey. II. Identification of Two White Dwarfs and Four Late-Type Active Stars.

view Abstract Citations (9) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Faint Sources in the EUVE Survey. II. Identification of Two White Dwarfs and Four Late-Type Active Stars. Vennes, Stephane ; Korpela, Eric ; Bowyer, Stuart Abstract We identify six extreme ultraviolet (EUV) and soft x-ray sources from the joint EUVE/ROSAT catalog as two hydrogen-rich white dwarf stars (DA) and four late-type active stars (two dMe and two dKe). Both white dwarfs are relatively nearby (100 pc) and located in the Galactic plane; the DA0 white dwarf EUVE J2124+284 is very hot (Teff=53 000 K) and has a relatively low mass (0.48 Msun), while the DA1 white dwarf EUVE J2055+164 (Teff=38 400K) has a higher mass (0.85 Msun) than average. The new dM2.5e EUVE J2030+798 is at a distance of only ≍20 pc, while the new (dM1e EUVE J2332-012 is much more distant (≍80 pc) and below the Galactic plane (b = - 58°). The two K stars (EUVE J2206+637 and EUVE J2223+253) show weak Ca II H and K emission and are the most probable optical counterparts to these two EUV sources. Measurements of the ROSAT x-ray hardness ratio (0.1-0.4 vs 0.5-2.0 keV) in three objects support the identifications. The findings show in particular that many hot white dwarfs in the solar neighborhood await discovery. Publication: The Astronomical Journal Pub Date: October 1997 DOI: 10.1086/118586 Bibcode: 1997AJ....114.1567V Keywords: SURVEYS; WHITE DWARFS; STARS: LATE-TYPE full text sources ADS | data products SIMBAD (6)

Faint Sources in theEUVESurvey: Identification of White Dwarfs, Active Late‐Type Stars, and Galactic Nuclei

We report the classification of 21 new extreme-ultraviolet sources from the recent catalog of Lampton et al. The optical spectra presented identify the objects as 14 active late-type stars (including two double active stars and a possible T Tauri star), three white dwarfs, and six active galactic nuclei (a Seyfert galaxy, the BL Lac object 1ES 1028+511 [=EUVE J1031+508], and four quasi-stellar objects). We have detected Ca II absorption lines in the BL Lac object and measured its redshift. Two of the white dwarfs are unusually massive (M > 1.1 M☉). Our sample of late-type stars includes five previously known high proper motion objects (EUVE J1004+503, J2244-332A,B, J1802+642, and J1131-346), of which one is the well-known flare star TX PsA (EUVE J2244-332B). We report an unusually high level of activity for the primary component of the TX PsA system (EUVE J2244-332A), which may indicate flare activity. The group of late-type stars is on average almost 3 mag fainter (⟨m⟩ ≈ 13) than the typical member of the Extreme Ultraviolet Explorer (EUVE) all-sky survey catalog. All Galactic and extragalactic objects were also detected in the ROSAT Position Sensitive Proportional Counter survey, and most are at the faint limit of the EUVE detectors. These new identifications substantially increase the total number of EUV-selected extragalactic sources.

Influence of red dwarf component activity on accretion in close binary systems

Abstract Active late type stars lose mass as due to the effect of magnetic instabilities in their corona connected to flares. The phenomenon is similar to what happens in the Sun, but with greates intensity, with the stellar wind reaching M ∼ 10−11 Mθ yr−1. In later spectral types the effect is suspected to be more discontinuous and dominated by coronal mass ejections (CMEs): transient events of gas expulsion due to large-scale reconnection of the magnetic field. CMEs from active red dwarfs have been detected in the last few years. In these transient events up to 1016 g of gas are expelled on time-scales of some minutes. Some observations suggest that such events can support a mass loss rate from the active star M ∼ 10−11 Mθ yr−1, similar to the above case. We investigate the possibility that interaction between the expelled gas and the accretion pattern around the compact object in Cataclysmic variables (CVs) and low-mass X-ray binaries (LMXBs) gives rise to observable energy release. We estimate that in...

Extreme Ultraviolet Explorer Optical Identification Campaign.III. A Southern Hemisphere Sample of Active Late-Type Stars and White Dwarfs

view Abstract Citations (9) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Extreme Ultraviolet Explorer Optical Identification Campaign.III. A Southern Hemisphere Sample of Active Late-Type Stars and White Dwarfs Craig, Nahide ; Christian, Damian J. ; Dupuis, Jean ; Roberts, Bryce A. Abstract We present optical identifications of 30 previously unidentified extreme ultraviolet (EUV) and soft x-ray sources coinciding with the position of faint sources detected during the all-sky surveys of the Extreme Ultraviolet Explorer ({\euve}) in the 58-174 Angstroms (0.071-0.214 keV) band and of the \rosat\ Position Sensitive Proportional Counter (PSPC) in the 5-120 Angstroms (0.1-2.4 keV x-ray) band. We present low-resolution spectra of the possible optical counterparts of the x-ray and \euve\ sources obtained with 1.5 m telescope at CTIO in 1995 May and December and 1996 February. The optical counterparts for 23 sources are identified as late-type (dKe and dMe) stars, and 4 are identified as new DA white dwarfs. All spectra of sources identified as late-type stars show the Balmer series and Ca H and K lines in emission, indicating the signature of chromospheric activity. For the white dwarfs, the joint detection by the \rosat\ PSPC and the \euve\ Lexan band indicates that they have a low-abundance of metals and that their EUV emission is mostly attenuated by the local interstellar medium (ISM). Publication: The Astronomical Journal Pub Date: July 1997 DOI: 10.1086/118469 Bibcode: 1997AJ....114..244C full text sources ADS | data products SIMBAD (78) MAST (1)

Evidence for a new class of extreme ultraviolet sources

Most of the sources detected in the extreme ultraviolet (EUV; 100 Ang to 600 Ang) by the Rosat WFC and EUVE all-sky surveys have been identified with active late-type stars and hot white dwarfs that are near enough to escape absorption by interstellar gas. However, about 15% of EUV sources are as of yet unidentified with any optical counterparts. We examine whether the unidentified EUV sources may consist of the same population of late-type stars and white dwarfs. We present B and R photometry of stars in the fields of seven of the unidentified EUV sources. We detect in the optical the entire main-sequence and white-dwarf population out to the greatest distances where they could still avoid absorption. We use colour-magnitude diagrams to demonstrate that, in most of the fields, none of the observed stars have the colours and magnitudes of late-type dwarfs at distances less than 100 pc. Similarly, none are white dwarfs within 500 pc that are hot enough to be EUV-emitters. The unidentified EUV sources we study are not detected in X-rays, while cataclysmic variables, X-ray binaries, and active galactic nuclei generally are. We conclude that some of the EUV sources may be a new class of nearby objects, that are either very faint at optical bands or which mimic the colours and magnitudes of distant late-type stars or cool white dwarfs. One candidate for optically faint objects is isolated old neutron stars, slowly accreting interstellar matter. Such neutron stars are expected to be abundant in the Galaxy, and have not been unambiguously detected.

EUV activity in late-type stars during the ROSAT WFC All-Sky Survey -- I. Techniques and initial results

We study 127 active late-type stars, including the well-known flare stars, in the extreme ultraviolet (EUV) region of the electromagnetic spectrum, using the ROSAT Wide Field Camera (WFC) survey observations. Our analysis includes photometry for all the stars in the sample, timing analysis of the observed photon events, and a statistical analysis for possible small-scale, low-level variability. 49 stars in our sample were detected in one or both EUV wavebands. A total of 35 flares were seen, from 23 stars, not all of them well-known flare stars. In addition, about half ofthe detections exhibited low-level variability. Further analysis reveals that the observed low-level variability was not due to rotational or orbital modulation, but rather was related to small-scale activity on the stars, possibly in the form of small­ amplitude, flare-like events which were too faint to be recognized as individual flares at the sensitivity of the WFC. We call this low-level activity 'milliflaring'. We also give estimated upper-limit count rates for the non-detections.

The Sun as an X-Ray Star: Overview of the Method

We present a method to study the solar-stellar connection, i.e., the close similarity of the physical phenomena occurring on the Sun and on late-type active stars, by taking advantage of Yohkoh/SXT X-ray images. From such images, we first generate distribution functions of the whole disk differential emission measure, and then synthesize from these spectra analogous to those collected by X-ray telescope instruments aimed at stars other than the Sun. Here we illustrate the application of this method to the ROSAT/PSPC and ASCA/SIS, and discuss test cases as well as future applications.

Mapping Cool Gas in the ISM with ROSAT

AbstractWe have investigated the distribution of cool gas within the local interstellar medium (&lt;150 pc) using data derived from both the ROSAT X-ray and EUV all-sky surveys. The EUV/X-ray spectra for a sample of 89 white dwarf stars and 200 active late-type stars have been analysed to give an estimate of the gas column density along each object’s line of sight. In order to extend the sky coverage we have also included in the analysis 190 late-type stars detected only in the X-ray band. The derived NHI measurements confirm the size of the local low density bubble as typically 50-100 pc and also show that in the general direction of Loop I there is a substantial excess of absorption at a range of distances from 10-150 pc.

Magnetic arcades in stellar coronae I. Cylindrical geometry

X-ray spectroscopy performed by different astronomical spacecrafts has shown that many active late-type stars possess coronae. For such reason, the magnetic structure of stellar coronae has raised considerable interest and, by analogy with the Sun, it is generally assumed that stellar coronae are structured by magnetic fields having the shape of arcades. Most of those coronal magnetic field configurations assume translational symmetry and are based in planar source surfaces. However, as soon as either the length or the width of such source surfaces become non negligible as compared to the stellar radius, the application of the cylindrical geometry seems to be more appropriate. Then, one way of obtaining coronal magnetic configurations is to deal with source domains extended over a cylindrical surface. In this paper we generate potential coronal arcades based on cylindrical source surfaces with non negligible length or width compared to the stellar radius. The flux function, the magnetic field components, the shape of magnetic field lines and other characteristic magnitudes have been obtained and analyzed for both cases.