We present new X-ray, EUV, and radio observations of the coronal emission from the RS CVn binarie... more We present new X-ray, EUV, and radio observations of the coronal emission from the RS CVn binaries HRI099 (V711 Tau) and a 2 CrB. RS CVn systems possess coronae that display extreme activity levels and frequent flaring. Our observations provide multiwavelength records of coronal variability and flaring over more than two binary orbits for each system. While the EUV and X-ray spectra show the flare response of the thermal plasma, the radio data give the corresponding information for the nonthermal electron population. The HRI099 data contain one of the most energetic flare outbursts observed from HRI099 that lasts for over 3 days. Coronal flaring is common in these systems and, in fact, is the normal condition. a 2 CrB was observed with the XTE and ASCA X-ray satellites and the VLA radio array on 1997 March 11-13. The radio observations show large variations at 3, 6 and 20 em; including a flare rise seen on March 11 and a complex flare outburst that lasted for most of March 12. Polarised 20 em bursts were seen associated with flare rises; those asssociated with the flares at March 11.7 and 12.3 were left circularly polarised, while that at March 12.6 was right circularly polarised. A weak excess of right CP was seen during the March 12.5 subflare at 6 cm. The rapid radio decay at 3 and 6 cm suggests that the flare region was expanding rapidly. Outside the flares CP was absent at all three frequencies. At least three flares were detected by XTE, with two of them having radio counterparts.
We argue that the low-velocity absorption features seen in the Mg n resonance lines of seven conf... more We argue that the low-velocity absorption features seen in the Mg n resonance lines of seven confirmed and three probable hybrid-chromosphere stars are interstellar rather than circumstellar in origin. From a comparison of radial velocities based on all available spectra in the International Ultraviolet Explorer (I UE) archives with estimates of the interstellar velocity along each line of sight, we find a good correlation between the observed position of the low-velocity component and the predicted interstellar feature. We also show that previous arguments in favor of the circumstellar origin of the low-velocity absorption features are either incorrect or implausible. Our conclusion may modify previously proposed models of hybrid star winds which have assumed a pnon that both Mg ii absorption components are circumstellar.
Monthly Notices of the Royal Astronomical Society, 2000
We have carried out BVR photometric and Ha spectroscopic observations of the star HD 61396 during... more We have carried out BVR photometric and Ha spectroscopic observations of the star HD 61396 during 1998 March 20 to 1999 April 3. We have discovered regular optical photometric variability from this star, with an inferred period of 31X95^0X10 dY and an amplitude of ,0.18 mag. A possible period of 35X34^0X12 dY as determined with Hipparcos, cannot be completely ruled out, however. Modelling of its photometric light curve with two circular spots indicates that 5±21 per cent of the stellar surface is covered by dark starspots which are ,830 K cooler than the surrounding photosphere, and produce the observed rotational modulation of the optical flux. Optical spectroscopy reveals a variable Ha emission feature, indicating that it is an unusually active star. In addition, we have analysed archival X-ray data of HD 61396, obtained from serendipitous observations with the ROSAT X-ray observatory, and we also discuss the radio properties of this star, based on both published Green Bank and unpublished VLA observations. The strong photometric variability and Ha emission, the relatively hard X-ray spectrum, and the high X-ray and radio luminosities imply that HD 61396 is most likely to be a member of the RS CVn class of evolved active binary stars. Its X-ray and radio luminosities place it among the five most luminous active binaries detected so far.
The Swift team reports the detection of three bright hard X-ray flares from the RS CVn binary sys... more The Swift team reports the detection of three bright hard X-ray flares from the RS CVn binary system V711 Tau (also known as HR 1099). The flares occurred on Nov 29, 2006 and were seen by the Burst Alert Telescope (BAT) on Swift in 64-second exposures starting at 04:30:39, 04:31:43, and 04:33:51 UTC. The intensity of the flares were 0.097, 0.071 and 0.088 +/- 0.013 counts/sec/cm^2, respectively, in the 15-50 keV band.
ABSTRACT The Swift team reports the detection of a superflare from the dMe flare star EV Lac. The... more ABSTRACT The Swift team reports the detection of a superflare from the dMe flare star EV Lac. The Burst Alert Telescope (BAT) triggered on EV Lac during a pre-planned slew at 2008-04-25T05:13:57 = T0(BAT), and resulted in an automatic trigger slew to the source. The hard X-ray source was already declining in flux as it entered the BAT field of view during the pre- planned slew, with a peak intensity in the BAT 15-50 keV band of around 660 mCrab, or 0.15 counts/cm2/s.
On 2014 April 23, the Swift satellite responded to a hard X-ray transient detected by its Burst A... more On 2014 April 23, the Swift satellite responded to a hard X-ray transient detected by its Burst Alert Telescope, which turned out to be a stellar flare from a nearby, young M dwarf binary DGCVn. We utilize observations at X-ray, UV, optical, and radio wavelengths to infer the properties of two large flares. The X-ray spectrum of the primary outburst can be described over the 0.3-100 keV bandpass by either a single very high-temperature plasma or a nonthermal thick-target bremsstrahlung model, and we rule out the nonthermal model based on energetic grounds. The temperatures were the highest seen spectroscopically in a stellar flare, at T X of 290 MK. The first event was followed by a comparably energetic event almost a day later. We constrain the photospheric area involved in each of the two flares to be >10 20 cm 2 , and find evidence from flux ratios in the second event of contributions to the white light flare emission in addition to the usual hot, T∼10 4 K blackbody emission seen in the impulsive phase of flares. The radiated energy in X-rays and white light reveal these events to be the two most energetic X-ray flares observed from an M dwarf, with X-ray radiated energies in the 0.3-10 keV bandpass of 4×10 35 and 9×10 35 erg, and optical flare energies at E V of 2.8×10 34 and 5.2×10 34 erg, respectively. The results presented here should be integrated into updated modeling of the astrophysical impact of large stellar flares on close-in exoplanetary atmospheres.
Elemental abundance effects in active coronae have eluded our understanding for almost three deca... more Elemental abundance effects in active coronae have eluded our understanding for almost three decades, since the discovery of the first ionization potential (FIP) effect on the sun. The goal of this paper is to monitor the same coronal structures over a time interval of six days and resolve active regions on a stellar corona through rotational modulation. We report on four iso-phase X-ray spectroscopic observations of the RS CVn binary EI Eri with XMM-Newton, carried out approximately every two days, to match the rotation period of EI Eri. We present an analysis of the thermal and chemical structure of the EI Eri corona as it evolves over the six days. Although the corona is rather steady in its temperature distribution, the emission measure and FIP bias both vary and seem to be correlated. An active region, predating the beginning of the campaign, repeatedly enters into our view at the same phase as it rotates from beyond the stellar limb. As a result, the abundances tend slightly, but consistently, to increase for high FIP elements (an inverse FIP effect) with phase. We estimate the abundance increase of high FIP elements in the active region to be of about 75% over the coronal mean. This observed fractionation of elements in an active region on time scales of days provides circumstantial clues regarding the element enrichment mechanism of non-flaring stellar coronae.
We have obtained 21 ~one-hour observations of the prototypical spotted star, BY Dra, with the VLA... more We have obtained 21 ~one-hour observations of the prototypical spotted star, BY Dra, with the VLA. These data are sufficiently well spaced in time to allow for complete rotational and orbital phase coverage; the resultant radio light curves are compared to their optical counterpart. We also plot the emission versus time without phase-folding, in order to search for longer periods of activity. Splitting the data into smaller (three minute) time bins allows for the determination of the normalized amplitude distribution of the flux; we compare this with known stellar optical and X-ray flare distributions.
We present a multiwavelength study of three chromospherically active stars, namely FR Cnc (= BD +... more We present a multiwavelength study of three chromospherically active stars, namely FR Cnc (= BD +161753), HD 95559 and LO Peg (=BD +224409), including newly obtained optical photometry, low-resolution optical spectroscopy for FR Cnc, as well as archival IR and X-ray observations. The BVR photometry carried out during the years 2001 2004 has found significant photometric variability to be present in all three stars. For FR Cnc, a photometric period 0.8267± 0.0004 d has been established. The strong variation in the phase and amplitude of the FR Cnc light curves when folded on this period implies the presence of evolving and migrating spots or spot groups on its surface. Two independent spots with migration periods of 0.97 and 0.93 years respectively are inferred. The photometry of HD 95559 suggests the formation of a spot (group) during the interval of our observations. We infer the existence of two independent spots or groups in the photosphere of LO Peg, one of which has a migration...
BVR photometric and quasi-simultaneous optical spectroscopic observations of the star HD 81032 ha... more 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 18.802 ± 0.07 d has been detected for this star. A large group of spots with a migration period of 7.43 ± 0.07 years is inferred from the first three years of the data. Hα 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 above the 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.
We have made extensive observations of the time-variability of solar active regions in the far-UV... more We have made extensive observations of the time-variability of solar active regions in the far-UV using the ultraviolet spectrometer on SMM. We describe the three different modes of solar variability (impulsive events, bursts and oscillations) that are evident in our data and discuss their physical implications.
We describe high-sensitivity VLA observations of rapidly varying radio emission (‘flares’) from t... more We describe high-sensitivity VLA observations of rapidly varying radio emission (‘flares’) from two stars of very different types, one of which (λ And) is a Long-Period RS CVn system, and the other (HR 5942) is a magnetic Bp star. In both cases, however, the physical mechanism producing the radio emission is most likely to be gyrosynchrotron radiation from mildly relativistic, power-law electrons.
Study of X-ray coronae from late-type stars with moderate resolution X-ray spectroscopy with ASCA... more Study of X-ray coronae from late-type stars with moderate resolution X-ray spectroscopy with ASCA, has led to the characterization of temperatures and measurements of elemental abundances in their coronae. Several RS CVn and Algol-type binary systems, and single late-type stars have been observed. We present here the results obtained from X-ray spectroscopy of recently observed single F-G-K-M type dwarfs. The sample observed with ASCA contains αCen (Mewe et al. 1997), π1UMa (Drake et al. 1994), YY Gem, Speedy Mic, GJ 890 (Singh et al. 1997), EK Dra, HN Peg, κ1Cet (Guedel et al. 1997), AB Dor (Mewe et al. 1996) and HD 35850 (Tagliaferri et al. 1997).
We present Chandra and XMM-Newton X-ray spectra of the rapidly rotating K giant YY Mensae (d = 29... more We present Chandra and XMM-Newton X-ray spectra of the rapidly rotating K giant YY Mensae (d = 290 pc). YY Men is a member of a loosely-defined class of rapidly rotating single cool giant stars (``FK Com stars''), whose outstanding property is a projected equatorial velocity measured up to 110 km/s, in contrast with the expected maximum of 6 km/s
We present Chandra and XMM-Newton high-resolution X-ray spectra of the rapidly rotating giant YY ... more We present Chandra and XMM-Newton high-resolution X-ray spectra of the rapidly rotating giant YY Mensae. It belongs to the class of ``FK Com stars'', i.e. single cool giant stars with projected equatorial velocities measured up to 110 km/s. Chandra and XMM-Newton provide excellent high-resolution X-ray spectra to study the properties of this extreme coronal source. The X-ray spectrum is dominated
We present new X-ray, EUV, and radio observations of the coronal emission from the RS CVn binarie... more We present new X-ray, EUV, and radio observations of the coronal emission from the RS CVn binaries HRI099 (V711 Tau) and a 2 CrB. RS CVn systems possess coronae that display extreme activity levels and frequent flaring. Our observations provide multiwavelength records of coronal variability and flaring over more than two binary orbits for each system. While the EUV and X-ray spectra show the flare response of the thermal plasma, the radio data give the corresponding information for the nonthermal electron population. The HRI099 data contain one of the most energetic flare outbursts observed from HRI099 that lasts for over 3 days. Coronal flaring is common in these systems and, in fact, is the normal condition. a 2 CrB was observed with the XTE and ASCA X-ray satellites and the VLA radio array on 1997 March 11-13. The radio observations show large variations at 3, 6 and 20 em; including a flare rise seen on March 11 and a complex flare outburst that lasted for most of March 12. Polarised 20 em bursts were seen associated with flare rises; those asssociated with the flares at March 11.7 and 12.3 were left circularly polarised, while that at March 12.6 was right circularly polarised. A weak excess of right CP was seen during the March 12.5 subflare at 6 cm. The rapid radio decay at 3 and 6 cm suggests that the flare region was expanding rapidly. Outside the flares CP was absent at all three frequencies. At least three flares were detected by XTE, with two of them having radio counterparts.
We argue that the low-velocity absorption features seen in the Mg n resonance lines of seven conf... more We argue that the low-velocity absorption features seen in the Mg n resonance lines of seven confirmed and three probable hybrid-chromosphere stars are interstellar rather than circumstellar in origin. From a comparison of radial velocities based on all available spectra in the International Ultraviolet Explorer (I UE) archives with estimates of the interstellar velocity along each line of sight, we find a good correlation between the observed position of the low-velocity component and the predicted interstellar feature. We also show that previous arguments in favor of the circumstellar origin of the low-velocity absorption features are either incorrect or implausible. Our conclusion may modify previously proposed models of hybrid star winds which have assumed a pnon that both Mg ii absorption components are circumstellar.
Monthly Notices of the Royal Astronomical Society, 2000
We have carried out BVR photometric and Ha spectroscopic observations of the star HD 61396 during... more We have carried out BVR photometric and Ha spectroscopic observations of the star HD 61396 during 1998 March 20 to 1999 April 3. We have discovered regular optical photometric variability from this star, with an inferred period of 31X95^0X10 dY and an amplitude of ,0.18 mag. A possible period of 35X34^0X12 dY as determined with Hipparcos, cannot be completely ruled out, however. Modelling of its photometric light curve with two circular spots indicates that 5±21 per cent of the stellar surface is covered by dark starspots which are ,830 K cooler than the surrounding photosphere, and produce the observed rotational modulation of the optical flux. Optical spectroscopy reveals a variable Ha emission feature, indicating that it is an unusually active star. In addition, we have analysed archival X-ray data of HD 61396, obtained from serendipitous observations with the ROSAT X-ray observatory, and we also discuss the radio properties of this star, based on both published Green Bank and unpublished VLA observations. The strong photometric variability and Ha emission, the relatively hard X-ray spectrum, and the high X-ray and radio luminosities imply that HD 61396 is most likely to be a member of the RS CVn class of evolved active binary stars. Its X-ray and radio luminosities place it among the five most luminous active binaries detected so far.
The Swift team reports the detection of three bright hard X-ray flares from the RS CVn binary sys... more The Swift team reports the detection of three bright hard X-ray flares from the RS CVn binary system V711 Tau (also known as HR 1099). The flares occurred on Nov 29, 2006 and were seen by the Burst Alert Telescope (BAT) on Swift in 64-second exposures starting at 04:30:39, 04:31:43, and 04:33:51 UTC. The intensity of the flares were 0.097, 0.071 and 0.088 +/- 0.013 counts/sec/cm^2, respectively, in the 15-50 keV band.
ABSTRACT The Swift team reports the detection of a superflare from the dMe flare star EV Lac. The... more ABSTRACT The Swift team reports the detection of a superflare from the dMe flare star EV Lac. The Burst Alert Telescope (BAT) triggered on EV Lac during a pre-planned slew at 2008-04-25T05:13:57 = T0(BAT), and resulted in an automatic trigger slew to the source. The hard X-ray source was already declining in flux as it entered the BAT field of view during the pre- planned slew, with a peak intensity in the BAT 15-50 keV band of around 660 mCrab, or 0.15 counts/cm2/s.
On 2014 April 23, the Swift satellite responded to a hard X-ray transient detected by its Burst A... more On 2014 April 23, the Swift satellite responded to a hard X-ray transient detected by its Burst Alert Telescope, which turned out to be a stellar flare from a nearby, young M dwarf binary DGCVn. We utilize observations at X-ray, UV, optical, and radio wavelengths to infer the properties of two large flares. The X-ray spectrum of the primary outburst can be described over the 0.3-100 keV bandpass by either a single very high-temperature plasma or a nonthermal thick-target bremsstrahlung model, and we rule out the nonthermal model based on energetic grounds. The temperatures were the highest seen spectroscopically in a stellar flare, at T X of 290 MK. The first event was followed by a comparably energetic event almost a day later. We constrain the photospheric area involved in each of the two flares to be >10 20 cm 2 , and find evidence from flux ratios in the second event of contributions to the white light flare emission in addition to the usual hot, T∼10 4 K blackbody emission seen in the impulsive phase of flares. The radiated energy in X-rays and white light reveal these events to be the two most energetic X-ray flares observed from an M dwarf, with X-ray radiated energies in the 0.3-10 keV bandpass of 4×10 35 and 9×10 35 erg, and optical flare energies at E V of 2.8×10 34 and 5.2×10 34 erg, respectively. The results presented here should be integrated into updated modeling of the astrophysical impact of large stellar flares on close-in exoplanetary atmospheres.
Elemental abundance effects in active coronae have eluded our understanding for almost three deca... more Elemental abundance effects in active coronae have eluded our understanding for almost three decades, since the discovery of the first ionization potential (FIP) effect on the sun. The goal of this paper is to monitor the same coronal structures over a time interval of six days and resolve active regions on a stellar corona through rotational modulation. We report on four iso-phase X-ray spectroscopic observations of the RS CVn binary EI Eri with XMM-Newton, carried out approximately every two days, to match the rotation period of EI Eri. We present an analysis of the thermal and chemical structure of the EI Eri corona as it evolves over the six days. Although the corona is rather steady in its temperature distribution, the emission measure and FIP bias both vary and seem to be correlated. An active region, predating the beginning of the campaign, repeatedly enters into our view at the same phase as it rotates from beyond the stellar limb. As a result, the abundances tend slightly, but consistently, to increase for high FIP elements (an inverse FIP effect) with phase. We estimate the abundance increase of high FIP elements in the active region to be of about 75% over the coronal mean. This observed fractionation of elements in an active region on time scales of days provides circumstantial clues regarding the element enrichment mechanism of non-flaring stellar coronae.
We have obtained 21 ~one-hour observations of the prototypical spotted star, BY Dra, with the VLA... more We have obtained 21 ~one-hour observations of the prototypical spotted star, BY Dra, with the VLA. These data are sufficiently well spaced in time to allow for complete rotational and orbital phase coverage; the resultant radio light curves are compared to their optical counterpart. We also plot the emission versus time without phase-folding, in order to search for longer periods of activity. Splitting the data into smaller (three minute) time bins allows for the determination of the normalized amplitude distribution of the flux; we compare this with known stellar optical and X-ray flare distributions.
We present a multiwavelength study of three chromospherically active stars, namely FR Cnc (= BD +... more We present a multiwavelength study of three chromospherically active stars, namely FR Cnc (= BD +161753), HD 95559 and LO Peg (=BD +224409), including newly obtained optical photometry, low-resolution optical spectroscopy for FR Cnc, as well as archival IR and X-ray observations. The BVR photometry carried out during the years 2001 2004 has found significant photometric variability to be present in all three stars. For FR Cnc, a photometric period 0.8267± 0.0004 d has been established. The strong variation in the phase and amplitude of the FR Cnc light curves when folded on this period implies the presence of evolving and migrating spots or spot groups on its surface. Two independent spots with migration periods of 0.97 and 0.93 years respectively are inferred. The photometry of HD 95559 suggests the formation of a spot (group) during the interval of our observations. We infer the existence of two independent spots or groups in the photosphere of LO Peg, one of which has a migration...
BVR photometric and quasi-simultaneous optical spectroscopic observations of the star HD 81032 ha... more 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 18.802 ± 0.07 d has been detected for this star. A large group of spots with a migration period of 7.43 ± 0.07 years is inferred from the first three years of the data. Hα 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 above the 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.
We have made extensive observations of the time-variability of solar active regions in the far-UV... more We have made extensive observations of the time-variability of solar active regions in the far-UV using the ultraviolet spectrometer on SMM. We describe the three different modes of solar variability (impulsive events, bursts and oscillations) that are evident in our data and discuss their physical implications.
We describe high-sensitivity VLA observations of rapidly varying radio emission (‘flares’) from t... more We describe high-sensitivity VLA observations of rapidly varying radio emission (‘flares’) from two stars of very different types, one of which (λ And) is a Long-Period RS CVn system, and the other (HR 5942) is a magnetic Bp star. In both cases, however, the physical mechanism producing the radio emission is most likely to be gyrosynchrotron radiation from mildly relativistic, power-law electrons.
Study of X-ray coronae from late-type stars with moderate resolution X-ray spectroscopy with ASCA... more Study of X-ray coronae from late-type stars with moderate resolution X-ray spectroscopy with ASCA, has led to the characterization of temperatures and measurements of elemental abundances in their coronae. Several RS CVn and Algol-type binary systems, and single late-type stars have been observed. We present here the results obtained from X-ray spectroscopy of recently observed single F-G-K-M type dwarfs. The sample observed with ASCA contains αCen (Mewe et al. 1997), π1UMa (Drake et al. 1994), YY Gem, Speedy Mic, GJ 890 (Singh et al. 1997), EK Dra, HN Peg, κ1Cet (Guedel et al. 1997), AB Dor (Mewe et al. 1996) and HD 35850 (Tagliaferri et al. 1997).
We present Chandra and XMM-Newton X-ray spectra of the rapidly rotating K giant YY Mensae (d = 29... more We present Chandra and XMM-Newton X-ray spectra of the rapidly rotating K giant YY Mensae (d = 290 pc). YY Men is a member of a loosely-defined class of rapidly rotating single cool giant stars (``FK Com stars''), whose outstanding property is a projected equatorial velocity measured up to 110 km/s, in contrast with the expected maximum of 6 km/s
We present Chandra and XMM-Newton high-resolution X-ray spectra of the rapidly rotating giant YY ... more We present Chandra and XMM-Newton high-resolution X-ray spectra of the rapidly rotating giant YY Mensae. It belongs to the class of ``FK Com stars'', i.e. single cool giant stars with projected equatorial velocities measured up to 110 km/s. Chandra and XMM-Newton provide excellent high-resolution X-ray spectra to study the properties of this extreme coronal source. The X-ray spectrum is dominated
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Papers by Stephen Drake