arXiv subjects

S. H. Saar

Publications and source records attributed to S. H. Saar.

29 records · Page 2Linked to original sources

arXiv2012

Differential Emission Measure Analysis of Multiple Structural Components of Coronal Mass Ejections in the Inner Corona

In this paper, we study the temperature and density properties of multiple structural components of coronal mass ejections (CMEs) using differential emission measure (DEM) analysis. The DEM analysis is based on the six-passband EUV observations of solar corona from the Atmospheric Imaging Assembly onboard the \emph{Solar Dynamic Observatory}. The structural components studied include the hot channel in the core region (presumably the magnetic flux rope of the CME), the bright loop-like leading front (LF), and coronal dimming in the wake of the CME. We find that the presumed flux rope has the highest average temperature ($>$8 MK) and density ($\sim$1.0 $\times10^{9}$ cm$^{-3}$), resulting in an enhanced emission measure (EM) over a broad temperature range (3 $\leq$ T(MK) $\leq$ 20). On the other hand, the CME LF has a relatively cool temperature ($\sim$2 MK) and a narrow temperature distribution similar to the pre-eruption coronal temperature (1 $\leq$ T(MK) $\leq$ 3). The density in the LF, however, is increased by 2% to 32% compared with that of the pre-eruption corona, depending on the event and location. In coronal dimmings, the temperature is more broadly distributed (1 $\leq$ T(MK) $\leq$ 4), but the density decreases by $\sim$35% to $\sim$40%. These observational results show that: (1) CME core regions are significantly heated, presumably through magnetic reconnection, (2) CME LFs are a consequence of compression of ambient plasma caused by the expansion of the CME core region, and (3) the dimmings are largely caused by the plasma rarefaction associated with the eruption.

astro-ph.SR↗

arXiv2009

Interactions of the magnetospheres of stars and close-in giant planets

Since the first discovery of an extrasolar planetary system more than a decade ago, hundreds more have been discovered. Surprisingly, many of these systems harbor Jupiter-class gas giants located close to the central star, at distances of 0.1 AU or less. Observations of chromospheric 'hot spots' that rotate in phase with the planetary orbit, and elevated stellar X-ray luminosities,suggest that these close-in planets significantly affect the structure of the outer atmosphere of the star through interactions between the stellar magnetic field and the planetary magnetosphere. Here we carry out the first detailed three-dimensional MagnetoHydroHynamics (MHD) simulation containing the two magnetic bodies and explore the consequences of such interactions on the steady-state coronal structure. The simulations reproduce the observable features of 1) increase in the total X-ray luminosity, 2) appearance of coronal hot spots, and 3) phase shift of these spots with respect to the direction of the planet. The proximate cause of these is an increase in the density of coronal plasma in the direction of the planet, which prevents the corona from expanding and leaking away this plasma via a stellar wind. The simulations produce significant low temperature heating. By including dynamical effects, such as the planetary orbital motion, the simulation should better reproduce the observed coronal heating.

astro-ph.SR↗

arXiv2009

Hinode X-Ray Telescope Detection of Hot Emission from Quiescent Active Regions: A Nanoflare Signature?

The X-Ray Telescope (XRT) on the Japanese/USA/UK {\it Hinode (Solar-B)} spacecraft has detected emission from a quiescent active region core that is consistent with nanoflare heating. The fluxes from 10 broadband X-ray filters and filter combinations were used to constructed Differential Emission Measure (DEM) curves. In addition to the expected active region peak at Log T = 6.3-6.5, we find a high-temperature component with significant emission measure at Log T $>$ 7.0. This emission measure is weak compared to the main peak -- the DEM is down by almost three orders of magnitude -- which accounts of the fact that it has not been observed with earlier instruments. It is also consistent with spectra of quiescent active regions: no Fe XIX lines are observed in a CHIANTI synthetic spectrum generated using the XRT DEM distribution. The DEM result is successfully reproduced with a simple two-component nanoflare model.

astro-ph.SR↗

arXiv2008

First Observation of Planet-Induced X-ray Emission: The System HD 179949

We present the first observation of planet-induced stellar X-ray activity, identified for the HD 179949 system, using Chandra / ACIS-S. The HD 179949 system consists of a close-in giant planet orbiting an F9V star. Previous ground-based observations already showed enhancements in Ca II K in phase with the planetary orbit. We find an ~30% increase in the X-ray flux over quiescent levels coincident with the phase of the Ca II enhancements. There is also a trend for the emission to be hotter at increased fluxes, confirmed by modeling, showing the enhancement at ~1 keV compared to ~0.4 keV for the background star.

astro-ph↗

arXiv2007

The Orbital Period of the Wolf-Rayet Binary IC 10 X-1; Dynamic Evidence that the Compact Object is a Black Hole

IC 10 X-1 is a bright (Lx=10^38 ergs/s) variable X-ray source in the local group starburst galaxy IC 10. The most plausible optical counterpart is a luminous Wolf-Rayet star, making IC 10 X-1 a rare example of a Wolf-Rayet X-ray binary. In this paper, we report on the detection of an X-ray orbital period for IC 10 X-1of 34.4 hours. This result, combined with a re-examination of optical spectra, allow us to determine a mass function for the system f(m)=7.8 Msun and a probable mass for the compact object of 24-36 Msun. If this analysis is correct, the compact object is the most massive known stellar black black hole. We further show that the observed period is inconsistent with Roche lobe overflow, suggesting that the binary is detached and the black hole is accreting the wind of the Wolf-Rayet star. The observed mass loss rate of [MAC92] 17-A is sufficient to power the X-ray luminosity of IC 10 X-1.

astro-ph↗

arXiv2006

Confirmation of the Planet Hypothesis for the Long-period Radial Velocity Variations of Beta Geminorum

We present precise stellar radial velocity measurements for the K giant star Beta Gem spanning over 25 years. These data show that the long period low amplitude radial velocity variations found by Hatzes & Cochran (1993) are long-lived and coherent. An examination of the Ca II K emission, spectral line shapes from high resolution data (R = 210,000), and Hipparcos photometry show no significant variations of these quantities with the RV period. These data confirm the planetary companion hypothesis suggested by Hatzes & Cochran (1993). An orbital solution assuming a stellar mass of 1.7 M_sun yields a period, P = 589.6 days, a minimum mass of 2.3 M_Jupiter, and a semi-major axis, and a = 1.6 AU. The orbit is nearly circular (e = 0.02). Beta Gem is the seventh intermediate mass star shown to host a sub-stellar companion and suggests that planet-formation around stars much more massive than the sun may common.

astro-ph↗

arXiv2006

Spot patterns and differential rotation in the eclipsing pre-CV binary,V471 Tau

We present surface spot maps of the K2V primary star in the pre-cataclysmic variable binary system, V471 Tau. The spot maps show the presence of large high latitude spots located at the sub-white dwarf longitude region. By tracking the relative movement of spot groups over the course of four nights (eight rotation cycles), we measure the surface differential rotation rate of the system.Our results reveal that the star is rotating rigidly with a surface shear rate, dOmega = 1.6 +/- 6 mrad/d. The single active star AB Dor has a similar spectral type, rotation period, and activity level as the K star in V471 Tau but displays much stronger surface shear (46 < dOmega < 58 mrad/d). Our results suggest that tidal locking may inhibit differential rotation; this reduced shear, however, does not affect the overall magnetic activity levels in active K dwarfs.

astro-ph↗

arXiv2003

The low-level radial velocity variability in Barnard's star (=GJ 699). Secular acceleration, indications for convective redshift, and planet mass limits

We report results from 2 1/2 yr of high precision radial velocity (RV) monitoring of Barnard's star. The high RV measurement precision of the VLT-UT2+UVES of 2.65 m/s made the following findings possible. (1) The first detection of the change in the RV of a star caused by its space motion (RV secular acceleration). (2) An anti-correlation of the measured RV with the strength of the filling-in of the Halpha line by emission. (3) Very stringent mass upper limits to planetary companions. Using only data from the first 2 years, we obtain a best-fit value for the RV secular acceleration of 5.15+/-0.89 m/s/yr. This agrees with the predicted value of 4.50 m/s/yr based on the Hipparcos proper motion and parallax combined with the known absolute radial velocity of the star. When the RV data of the last half-year are added the best-fit slope is strongly reduced to 2.97+/-0.51 m/s/yr suggesting the presence of additional RV variability in the star. Part of it can be attributed to stellar activity as we demonstrate by correlating the residual RVs with an index that describes the filling-in of the Halpha line by emission. A correlation coefficient of -0.50 indicates that the appearance of active regions causes a blueshift of photospheric absorption lines. Assuming that active regions basically inhibit convection we discuss the possibility that the fundamental (inactive) convection pattern in this M4V star produces a convective redshift. We also determine upper limits to the projected mass msini and to the true mass m of hypothetical planetary companions in circular orbits. For the separation range 0.017-0.98 AU we exclude any planet with msini>0.12 Mjupiter and m>0.86 Mjupiter. Throughout the habitable zone, i.e. 0.034-0.082 AU, we exclude planets with msini>7.5 Mearth and m>3.1 Mneptune.

astro-ph↗

arXiv2001

Further Analysis of Stellar Magnetic Cycle Periods

We further investigate relationships between activity cycle periods in cool stars and rotation to include new cycle data, and explore different parameterizations of the problem. We find that relations between cycle and rotational frequencies (omega_cyc vs. Omega) and between their ratio and the inverse Rossby number (omega_cyc/Omega vs. Ro^{-1}) show many similarities, including three branches and similar rms scatter. We briefly discuss some implications for dynamo models.

astro-ph↗

arXiv2000

Extensive Serendipitous X-ray Coverage of a Flare Star with ROSAT

We report the serendipitous discovery of a flare star observed with the ROSAT X-ray observatory. From optical spectra, which show strong and variable emission lines of the hydrogen Balmer series and neutral helium, we classify this object as a M3.0Ve star, and estimate a distance of 52 pc from published photometry. Due to the star's close proximity (13.6') to the calibration source and RS CVn binary AR Lacertae, long term X-ray coverage is available in the ROSAT archive (~50 hours spanning 6.5 years). Two large flare events occurred early in the mission (6-7/1990), and the end of a third flare was detected in 6/1996. One flare, observed with the Position Sensitive Proportional Counter (PSPC), had a peak luminosity Lx=1.1x10^{30} erg/s, an e-folding rise time of 2.2 hours and a decay time of 7 hours. This decay time is one of the longest detected on a dMe star, providing evidence for the possibility of additional heating during the decay phase. A large HRI flare (peak Lx=2.9x10^{30} erg/s) is also studied. The "background" X-ray emission is also variable - evidence for low-level flaring or microflaring. We find that >=59% of the HRI counts and >=68% of the PSPC counts are due to flares. At least 41% of the HRI exposure time and 47% of the PSPC are affected by detectable flare enhancement.

astro-ph↗

arXiv1998

Optical and UV observations of a strong flare in the young, single K2-dwarf LQ Hya

We present high resolution optical echelle spectra and IUE observations during a strong flare on 1993 December 22 in the very active, young, rapidly rotating, single K2 dwarf LQ Hya. The initial impulsive phase of the flare, which started sometime between 2:42 UT and 4:07 UT, was characterized by strong optical continuum enhancement and blue-shifted emission lines with broad wings. The optical chromospheric lines reached their maximum intensity at $\approx$5:31 UT, by which time the blue-shift vanished and the optical continuum enhancement had sharply decreased. Thereafter, the line emission slowly decreased and the lines red-shift in a gradual phase that lasted at least two more hours. The Mg II lines behaved similarly. Quiescent C IV flux levels were not recovered until 21 hours later, though a data gap and a possible second flare make the interpretation uncertain. In addition to the typically flare-enhanced emission lines (e.g., H_alpha and H_beta), we observe He I D_3 going into emission, plus excess emission (after subtraction of the quiescent spectrum) in other He I and several strong neutral metal lines (e.g., Mg I b). Flare enhancement of the far UV continuum generally agrees with a Si I recombination model. We estimate the total flare energy, and discuss the broad components, asymmetries, and Doppler shifts seen in some of the emission lines.

astro-ph↗