X-ray variability of PMS stars - Toward an explanation of the different X-ray properties of CTTS and WTTS

Abstract

The intense X-ray emission from coronae and accretion shocks in young PMS stars is likely to play an important role in the evolution and dispersal of circumstellar disks. Several aspects of the physics of this X-ray emission remain mysterious, e.g., whether and how much accretion affects coronal emission. We studied the X-ray variability of ~1 Myr old low-mass PMS stars as a function of timescale, stellar rotation, and stellar characteristics, in order to gain insights on the working of PMS coronae, their X-ray emission, and the circumstellar environment in which they are immersed. We have exploited the ~850 ksec long Chandra observation of the Orion Nebula Cluster obtained by the COUP collaboration in Jan. 2003, and statistically analyzed the X-ray lightcurves of low-mass stars in several subsamples. In particular, we characterized the different X-ray behavior of stars with and without circumstellar accretion disks. Accreting stars (Classical T Tauri Stars, CTTSs) are found to be more variable than non accreting ones (Weak-lined T Tauri Stars, WTTSs) at all timescales and in all the X-ray energy bands considered. Variability is seen to increase with time-scale up to ~10 days, i.e. the longest probed. Signatures of rotational modulation are observed for both CTTSs and WTTSs, and most clearly for CTTSs in the soft X-ray band. Lower mass stars are more variable than higher mass ones. We propose that the difference in variability between CTTSs and WTTSs may be explained assuming that the X-ray emission of CTTS is affected by time-variable absorption due circumstellar structures, such as warps in the inner disk and/or accretion streams. This suggestion is appealing because, in the hypothesis that the coronae of CTTSs and WTTSs are similar, it may also explain why CTTSs have lower and more scattered X-ray emission levels with respect to WTTSs.

15 pages, 10 figures, accepted for publication in A&A

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• Further X-ray detections of Herbig stars
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• Supersaturation and activity-rotation relation in PMS stars: the young Cluster h Per
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• Using Multiwavelength Variability to Explore the Connection between X-ray Emission, the Far-Ultraviolet H2 Bump, and Accretion in T Tauri Stars
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• Young open clusters in the galactic star forming region NGC 6357
• Evolution of circumstellar discs in young star-forming regions
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• Exploring the possibility of Peter Pan discs across stellar mass
• Star Formation in Orion's L1630 Cloud: an Infrared and Multi-epoch X-ray Study
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• Revealing the Star-Disk-Jet Connection in GM Aur using Multiwavelength Variability
• Gas absorption and dust extinction towards the Orion Nebula Cluster
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• X-ray deficiency on strong accreting T Tauri stars - Comparing Orion with Taurus
• Testing the Potential for Radio Variability in Disks around T Tauri Stars with Observations and Chemical Modeling
• X-ray variability of cool stars: Magnetic activity and accretion
• Chandra Observations of Six Peter Pan Disks: Diversity of X-ray-driven Internal Photoevaporation Rates Doesn't Explain Their Rare Longevity
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• Interaction between Winds from Weak-lined T Tauri Stars with Exoplanetary Magnetospheres