arXiv subjects

Marc Gagne

Publications and source records attributed to Marc Gagne.

22 records · Page 2Linked to original sources

arXiv2002

Chandra Observations of the Pleiades Open Cluster: X-ray Emission from Late-B to Early-F Type Binaries

We present the analysis of a 38.4 ks and a 23.6 ks observation of the core of the Pleiades open cluster. The Advanced CCD Imaging Spectrometer on board the Chandra X-ray Observatory detected 99 X-ray sources in a 17'X17' region, including 18 of 23 Pleiades members. Five candidate Pleiades members have also been detected, confirming their cluster membership. Fifty-seven sources have no optical or near-infrared counterparts to limiting magnitudes V=22.5 and J=14.5. The unidentified X-ray sources are probably background AGN and not stars. The Chandra field of view contains seven intermediate mass cluster members. Five of these, HII 980 (B6 + G), HII 956 (A7 + F6), HII 1284 (A9 + K), HII 1338 (F3 + F6), and HII 1122 (F4 + K), are detected in this study. All but HII 1284 have high X-ray luminosity and soft X-ray spectra. HII 1284 has X-ray properties comparable to non-flaring K-type stars. Since all five stars are visual or spectroscopic binaries with X-ray properties similar to F-G stars, the late-type binary companions are probably producing the observed coronal X-ray emission. Strengthening this conclusion is the nondetection by Chandra of two A stars, HII 1362 (A7, no known companion) and HII 1375 (A0 + A SB) with X-ray luminosity upper limits 27-54 times smaller than HII 980 and HII 956, the B6-A7 stars with cooler companions. Despite the low number statistics, the Chandra data appear to confirm the expectation that late-B and A stars are not strong intrinsic X-ray sources. The ACIS spectra and hardness ratios suggest a gradual increase in coronal temperature with decreasing mass from F4 to K. M stars appear to have somewhat cooler coronae than active K stars.

astro-ph↗

arXiv2001

Mass Loss and Magnetospheres: X-rays from Hot Stars and Young Stellar Objects

High-resolution X-ray spectra of high-mass stars and low-mass T-Tauri stars obtained during the first year of the Chandra mission are providing important clues about the mechanisms which produce X-rays on very young stars. For zeta Puppis (O4 If) and zeta Ori (O9.5 I), the broad, blue-shifted line profiles, line ratios, and derived temperature distribution suggest that the X-rays are produced throughout the wind via instability-driven wind shocks. For some less luminous OB stars, like theta^1 Ori C (O7 V) and tau Sco (B0 V), the line profiles are symmetric and narrower. The presence of time-variable emission and very high-temperature lines in theta^1 Ori C and tau Sco suggest that magnetically confined wind shocks may be at work. The grating spectrum of the classical T-Tauri star TW Hya is remarkable because the forbidden-line emission of He-like Ne IX and O VII is very weak, implying that the X-ray emitting region is very dense, n = 6E+12 cgs, or that the X-rays are produced very close to the ultraviolet hotspot at the base of an accretion funnel. ACIS light curves and spectra of flares and low-mass and high-mass young stellar objects in Orion and rho Ophiuchus further suggest that extreme magnetic activity is a general property of many very young stars.

astro-ph↗

arXiv1998

The Active Corona of HD 35850 (F8 V)

We present Extreme Ultraviolet Explorer spectroscopy and photometry of the nearby F8 V star HD 35850 (HR 1817). The EUVE spectra reveal 28 emission lines from Fe IX and Fe XV to Fe XXIV. The Fe XXI 102, 129 A ratio yields an upper limit for the coronal electron density, log n < 11.6 per cc. The EUVE SW spectrum shows a small but clearly detectable continuum. The line-to-continuum ratio indicates approximately solar Fe abundances, 0.8 < Z < 1.6. The resulting emission-measure distribution is characterized by two temperature components at log T of 6.8 and 7.4. The EUVE spectra have been compared with non-simultaneous ASCA SIS spectra of HD 35850. The SIS spectrum shows the same temperature distribution as the EUVE DEM analysis. However, the SIS spectral firs suggest sub-solar abundances, 0.34 < Z < 0.81. Although some of the discrepancy may be the result of incomplete X-ray line lists, we cannot explain the disagreement between the EUVE line-to-continuum ratio and the ASCA-derived Fe abundance. Given its youth (t ~ 100 Myr), its rapid rotation (v sin i ~ 50 km/s), and its high X-ray activity (Lx ~ 1.5E+30 ergs/s), HD 35850 may represent an activity extremum for single, main-sequence F-type stars. The variability and EM distribution can be reconstructed using the continuous flaring model of Guedel provided that the flare distribution has a power-law index of 1.8. Similar results obtained for other young solar analogs suggest that continuous flaring is a viable coronal heating mechanism on rapidly rotating, late-type, main-sequence stars.

astro-ph↗

arXiv1997

Periodic X-ray Emission from the O7 V Star theta^1 Orionis C

We report the discovery of large-amplitude, periodic X-ray emission from the O7 V star theta^1 Orionis C, the central star of the Orion Nebula. Ten ROSAT HRI snapshots of the Trapezium cluster taken over the course of 21 days show that the count rate of theta^1 Ori C varies from 0.26 to 0.41 counts/s with a clear 15-day period. The soft X-ray variations have the same phase and period as H-alpha and He II 4686 variations reported by Stahl et al., and are in anti-phase with the C IV and Si IV ultraviolet absorption features. We consider five mechanisms which might explain the amplitude, phase, and periodicity of the X-ray variations: (1) colliding-wind emission with an unseen binary companion, (2) coronal emission from an unseen late-type pre-main--sequence star, (3) periodic density fluctuations, (4) absorption of magnetospheric X-rays in a corotating wind, and (5) magnetosphere eclipses. The ROSAT data rule out the first three scenarios, but cannot rule out either of the latter two which require the presence of an extended magnetosphere, consistent with the suggestion of Stahl et al. that theta^1 Ori C is an oblique magnetic rotator. As such, theta^1 Ori C may be the best example of a high-mass analog to the chemically peculiar, magnetic Bp stars.

astro-ph↗