X-ray properties of the young stellar and substellar objects in the IC 348 cluster - the CHANDRA View

Thomas Preibisch & Hans Zinnecker

Astronomical Journal, 123, 1613-1628

Abstract

We explore the X-ray properties of the young stellar and substellar objects in the open cluster IC 348 as seen in our deep CHANDRA X-Ray Observatory ACIS image. First, we give identifications of all X-ray sources and determine upper limits for the X-ray luminosities of the undetected cluster members. Then, we analyse the X-ray spectra of the young stellar objects, deriving plasma temperatures between ~0.7 keV and ~3 keV for the T Tauri stars in IC 348, and higher temperatures between ~3 keV and ~7 keV for flaring sources and two embedded young stellar objects. We find several large X-ray flares, in some of which a clear hardening of the X-ray spectra during the flare peak is seen.

Next, we use the exceptional optical, infrared, and X-ray data set of this cluster to study various correlations and their implications, and to discuss new answers to some long-standing questions related to X-ray emission from young (sub-) stellar objects. The X-ray luminosities of the young low-mass stars are strongly correlated to the stellar bolometric luminosities (L_X ~ 10^-4 x L_bol). Also, a good correlation between X-ray luminosity and stellar mass is found (L_X propto M^2). For the weak line T Tauri stars, we find a tight correlation between X-ray activity and chromospheric activity (L_X propto L_Halpha^0.8), supporting the hypothesis that the chromosphere is heated by X-rays from the overlying corona.

The observed X-ray properties of the brown dwarfs (and brown dwarf candidates) are very similar to those of late-type stars; we explain this behaviour as the consequence of the fact that very young substellar objects are still warm enough to maintain partially ionized atmospheres which are capable of sustaining electrical currents, while in the cooler neutral atmospheres of L and T dwarfs such currents are shut off (hence no X-ray emission).

Finally, we explore the difference between the X-ray luminosity functions of classical and weak-line T Tauri stars. We find that the classical T Tauri stars in IC 348 seem to be on average less X-ray luminous than the weak-line T Tauri stars. However, we suggest that this apparent difference is caused by a selection effect: there is a strong detection bias against those weak-line T Tauri stars which are optically faint and hence X-ray faint; the population of classical T Tauri stars, on the other hand, is essentially completely known due to its very prominent Halpha emission. This conclusion is corroborated by another new result: When using a photometrically selected, magnitude limited, complete sample of T Tauri stars and taking the K-L infrared excess as a tracer of circumstellar material, we find no evidence in IC 348 for a difference in X-ray properties of young stars with and without circumstellar matter; i.e. classical and ``naked'' T Tauri stars.


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ROSAT and CHANDRA images of IC 348 can be found here.



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