Magnetic Activity on VLM Stars and Brown Dwarfs

As a consequence of the similar interior structure of VLM stars and brown dwarfs, no dramatic change of the activity is expected at the substellar borderline, and we refer to both types of objects as "ultracool dwarfs."

Moving toward smaller and smaller masses, progressively lower activity levels are reported for the VLM stars and brown dwarfs. Several reasons may be responsible: First, below a critical effective temperature, the atmosphere may become too neutral to provide substantial coupling between material in outer layers and the magnetic fields (presumably) generated in the interior. This hypothesis was brought forward to explain the absence or low level of chromospheric Ha emission seen in most field L dwarfs [30].

Second, if the scaling between Lx and Lbol observed for late-type stars extends into the regime of ultracool dwarfs, the actual X-ray luminosities are expected to be very low, and are very difficult to reach with existing instrumentation for VLM objects outside the immediate solar neighborhood.

Both issues should affect the detectability of especially older, and thus fainter and cooler brown dwarfs. Indeed, the first X-ray detection of a bona-fide brown dwarf was reported in 1998 from a ROSAT observation of the ~2Myr old Chal star forming region [32, see Fig. 10.13]. Further searches for X-ray emitting brown

IP"

Ha 6

Ha 6

Ha 7

ROSAT PSPC Cha I

Ha 7

Ha 1

Ha 8

Ha 1

Ha 8

Ha 12

Fig. 10.13 ROSAT PSPC image of the central part of the Cha I dark cloud with the objects ChaHa 1 to 13 marked as crosses. X-ray sources identified with any of those objects are marked by circles (centered on the X-ray positions). ChaHa 1 was the first X-ray detected bona fide brown dwarf (Fig. 1 from [32])

dwarfs with ROSAT have proven little successful, yielding mostly rather high upper limits to their X-ray luminosity.

The new and far more sensitive X-ray observatories Chandra and XMM-Newton have begun to provide meaningful constraints for X-ray emission from VLM stars and brown dwarfs. However, the bulk of bona-fide brown dwarfs in star forming regions has still remained below the detection threshold of currently available X-ray observations because of the considerable distance of all star forming clouds (e.g., ~140pc for the nearby Taurus and p Ophiuchus, and ~450pc for the Orion Nebula), imposing a limiting sensitivity of logLx [ergs-^28 even for the most advanced X-ray instruments. However, the sensitivity of the instruments onboard Chandra and XMM-Newton has proven sufficient for first studies of the X-ray spectra and lightcurves of the brighter ones among the young brown dwarfs. Comparing their coronal temperature, flare frequency, and fractional X-ray luminosity to those of higher-mass premain sequence stars indicates no dramatic differences, providing evidence that their emission originates from the same mechanism.

Recent investigations have also addressed the issue of coronal emission from evolved brown dwarfs with their far lower bolometric luminosities and thus presumably also far lower X-ray luminosities. Quite a number of evolved brown dwarfs are located in the solar neighborhood at distances on the order of 10 pc. In Fig. 10.14,

M6 M7 Spectral Type

Fig. 10.14 Fractional X-ray luminosity vs. spectral type for M and L field dwarfs. filled circles, M dwarfs detected during the RASS; asterisks, flares on ultracool dwarfs; squares, quiescent emission or upper limits for ultracool dwarfs; large asterisk and square, the most recent X-ray detection of a brown dwarf (Gl 569 Bab). For clarity VLM dwarfs with the same spectral type have a small offset with respect to each other on the horizontal axis (Fig. 4 from [47])

M6 M7 Spectral Type

Fig. 10.14 Fractional X-ray luminosity vs. spectral type for M and L field dwarfs. filled circles, M dwarfs detected during the RASS; asterisks, flares on ultracool dwarfs; squares, quiescent emission or upper limits for ultracool dwarfs; large asterisk and square, the most recent X-ray detection of a brown dwarf (Gl 569 Bab). For clarity VLM dwarfs with the same spectral type have a small offset with respect to each other on the horizontal axis (Fig. 4 from [47])

we summarize our current knowledge about the activity level in ultracool dwarfs, expressed in terms of the fractional X-ray luminosity vs. spectral type. The RASS has revealed a large number of X-ray sources among the earlier M stars, but beyond spectral type ~M6 the coronae have remained largely inaccessible to ROSAT. The coolest dwarf detected so far in X-rays is LP 944-20, an intermediate age M9.5 brown dwarf (t ~ 500Myrs). LP 944-20 was seen by Chandra during a flare, and XMM-Newton has provided the lowest upper limit for the quiescent flux of any field dwarf so far: Lx < 3.1 x 1023ergs_1 [29,40]. As yet, no X-ray detection of an L dwarf was reported.

One of the striking features of Fig. 10.14 is that almost all detections of evolved ultracool dwarfs are attributed to flares, and it has remained unclear whether VLM stars and brown dwarfs are capable of producing quiescent, persistent X-ray emission. Only very recently, quiescent X-ray emission was for the first time reported from a confirmed brown dwarf [47]. Gl 569 Bab is composed of two brown dwarfs in a very tight orbit around a main sequence star with which it forms a hierarchical triple system. In X-rays only Chandra can resolve the brown dwarf binary (at a separation of only 5") from the primary. Gl 569 Bab showed both a huge flare and persistent emission after the outburst (see Fig. 10.15).

Fig. 10.15 X-ray lightcurve of Gl 569 Bab observed with Chandra during a large flare. The subsequent weak emission represents the first detection of quiescent X-rays from an evolved brown dwarf (Fig. 2 from [47])

10.4 Premain Sequence Stars

During the process of star formation from the start of the gravitational collapse of a molecular cloud core to the arrival of a star on the zero-age main-sequence, young stars pass through several phases. Low-mass objects (M < 2M0) in evolutionary stages before the quasi-steady hydrogen burning phase are collectively referred to as Young stellar objects (YSOs). They are classified by their spectral energy distribution from infrared to millimeter wavelengths, which are less affected by extinction than the optical passband.

In their youngest phases, YSOs are completely hidden behind their circumstellar material in the optical (Class Iprotostars; age ~105 yr), and even earlier also in the near-infrared (Class 0protostars; age ~ 104 yr). Once the forming star emerges from its gaseous envelope, it enters the T Tauri phase. In classical T Tauri stars (cTTS; age ~106"'7yr) the remaining circumstellar material has settled into a disk, from which it is accreted onto the star. In the infrared classification scheme, cTTS correspond roughly to Class II sources, dominated by emission from the disk (see below for differences between cTTS and Class II objects). Later on, when all circumstel-lar material has been dispersed and/or accreted, the stars' spectral energy distribution resembles that of main-sequence stars, although these weak-line T Tauri stars (wTTS) or Class III sources are still in their contraction phase to the main sequence.

Higher-mass premain sequence stars (M~2-10M0) are called Herbig Ae/Be stars (HAeBe stars). The evolutionary timescale of HAeBe stars is comparable to the dissipation timescale for their circumstellar matter. As a consequence - contrary to the T Tauri stars - all HAeBe stars show signatures for circumstellar material that in some cases has been shown to form a disk-like geometry.

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