High-resolution X-ray spectroscopy of CTTS and WTTS reveals an anomaly among CTTS: The observed OVIIr/OVIII Ly is large in CTTS (of order unity) whereas OVII is hardly detected in WTTS, similar to active ZAMS stars. In T Tau, the OVIIr line is intrinsically the most luminous X-ray line! In that respect, CTTS resemble inactive, old and cool coronae like Procyon's. On the other hand, a very prominent continuum and "hot" lines are seen in CTTS, as in extremely active field stars. The soft excess in CTTS may be due to accretion shocks, but in T Tau, the predicted high densities are not seen in the O VII line triplet. Alternatively, the cool accreting gas may stream into some of the coronal active regions, mixing with the hot plasma and thus adding large amounts of cool plasma ("coronal cooling").

OVIIr/OVIII Ly flux ratio as a function of NH. CTTS are in the upper part of the figure, WTTS (and ZAMS stars) in the lower. Curves indicate flux ratios for isothermal plasma as labeled.

New spectral phenomenology was found in a few jet-driving CTTS (e.g. DG Tau). The X-ray spectrum requires two unrelated components: a cool, very little absorbed ("soft", NH = few 1021 cm-2) component and a hot, very strongly absorbed ("hard", NH > 1022 cm-2) component.The hard component shows flares and is therefore coronal. But its absorption by gas is much higher than expected from the stellar visual extinction: This points to absorption by dust-depleted accreting gas streams: evidence for dust sublimation.

The soft component must therefore be emitted further out: We hypothesize that it is formed in shocks at the jet base. Chandra indeed sees faint jets with a spectrum similar to the soft component. The counter jet is ``harder'': its X-rays are absorbed by the gas disk!

Active Brown Dwarfs

The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) is a large X-ray (and U band) survey of the Taurus star-forming region designed to study the generation of high-energy radiation in young stars, its interaction with the surrounding molecular gas, and its potential impact on disks and forming planets. Stars in TMC form in relative isolation, high-mass stars being entirely absent. Strong mutual influence due to outflows, jets, winds, or UV radiation is therefore minimized. XEST thus ideally complements X-ray studies of clustered star formation regions such as Orion. It systematically surveys, for the first time, TMC protostars and substellar objects in X-rays. TMC surveys about 5 sq. degrees of the TMC cloud containing the most crowded stellar fields typically located in the densest molecular regions. The entire survey so far comprises 27 XMM-Newton EPIC fields of view, each being roughly circular with a diameter of 30 arcmin. The sensitivity is such (LX 1028 erg/s for average absorption) that almost every classical and weak-lined T Tauri star (CTTS/WTTS) has been detected, and so were about50% of the surveyed Class-I protostars and brown dwarfs (BDs). The detection statistics are as follows:

Protostars CTTS WTTS BDs others totalSurveyed 21 70 52 17 9 169Detected 10 (48%) 60 (86%) 50 (96%) 9 (53%) 7 (78%) 136 (80%)

An X-Ray Survey of the Taurus Star Formation RegionManuel Güdel (PSI/Switzerland) & the XEST Team (PSI/Switz.: K. Arzner, K. Briggs, A. Glauser, A. Telleschi;

LAOG Grenoble/Fr: J. Bouvier, C. Dougados, N. Grosso, S. Guieu, F. Ménard, J.-L. Monin, T. Montmerle; Palermo/It: E. Franciosini, G. Micela, I. Pillitteri, L. Scelsi, B. Stelzer; Colorado Univ./Boulder: S. L. Skinner;

Columbia Univ/Geneva Univ/Switz.: M. Audard; Firenze/It: F. Palla; MPIfR Bonn/Germany: T. Preibisch; PennState/USA: E. D. Feigelson; Caltech/USA: D. Padgett, L. Rebull; Porto/Pt: B. Silva

Jets: Double Absorbers Accretion: Soft Excess

r

if

absorbed

OVIII OVII

OVII tripletof T Tau:forbidden line(f) strong low density( 1010 cm-3)

active:low OVII/OVIII

inactive:high OVII/OVIII

continuum

continuum

CTTS

WTTS

T Tau

log NH 4"

time

cts/

s/ke

v

Energy

dust destructionat 10 R*

Photoelectric absorptionno excess visual extinction

constantsoft X

flaringhard X,absorbedx

jetshocks

hard

soft

DG Tau

Counter jet (blue) harder than forward jet (red/yellow):absorption by disk

[OI] optical forward jet (Dougados et al. 2002)

DG Tau (Chandra ACIS-S, 0.5-1.7keV)

hardness (red to blue for 0.5-1.7 keV):

counter

jet

constantsoft X,absorbed

x

counter jet

forward jet

non-accreting

accreting

detected

XEST detects of 9 out of 17 surveyed BDs, typically at LX of a few x1028 erg s-1.It is mostly the earlier spectral types/higher Lbol BDs that are X-ray detected. They fit the stellar LX vs mass and LX vs Lbol relations quite well, i.e., young BDs behave like lowest-mass stars. There is no

significantdifference in the de-tection rates of accre-ting vs non-accreting BDs.Accretion does not si-gnificantly alter the X-ray properties of BDs.

XEST observed a long, gradual U-band event in one of the observed BDs (2M J04141188+2811535). No simultaneous X-rays were detected (the star remained entirely undetected in X-rays). Although rotational modulation remains a possibility, the time scales also suggest an explanation in terms of an accretion event that increased the hot-spot luminosity: a temporary accretion rate increase by a factor of 6 is required.

Gradual optical event in a BD near V773 Tau:

undetected

Photoelectric absorption by disk

NOTE: This poster shows selectedresults from a larger body of studies related to XEST. A first series of refereed papers will be published shortly in a

special issue of A&A

containing 15 papers on various topics. First authors and topics:Güdel et al.: Introduction, Overview & TablesArzner et al.: Low-count spectraArzner et al.: Stochastic flaring (light curves)Audard et al.: U-band/UV survey (XMM-OM) Briggs et al.: Activity-rotation relationsFranciosini et al.: Flare geometric modelingGrosso et al.: BD X-ray surveyGrosso et al.: BD U-band accretion eventGüdel et al.: X-rays from jet-driving CTTSGüdel et al.: Case study of T TauScelsi et al.: New TMC member candidatesStelzer et al.: Flare & variability statisticsTelleschi et al.: High-res X-ray spectroscopyTelleschi et al.: Case study of AB Aur (Herbig)Telleschi et al.: Accretion & X-rays, C/WTTS

and in preparation:Glauser et al.: Gas/dust (NH-AV), IRAS 04158Güdel et al.: Soft excess in CTTSScelsi et al.: Coronal abundances

Acknowledgments: We acknowledge financial support from the International Space Science Institute (ISSI) in Bern, the Swiss NSF (grants 20-66875.01 and 20-109255/1), NASA (grant NNG05GF92G), and ASI/INAF (grant I/023/05/0). XMM-Newton is an ESA mission funded by ESA member states and the USA.

New TMC Member Candidates

Typically, 90% of ~100 detections per FoV are not known TMC members. IR color-color and color-magnitude diagrams find ~60 new potential stellar members of TMC. About one dozen show high probability given their thermal spectra and flares. They may be low-extinction WTTS, mostly following the mass-LX relation valid for TMC members.

mass-LX relationco

mm

on

rang

e fo

r

TMC

candidates

known members candidates

known members

field around V410 Tau (L1495E) known members

For every detected object, the X-ray spectrum has been used to determine the hydrogen column density NH from the absorption measured predominantly in the soft part ofthe spectrum. Together with extinction measurements (e.g., AV orAJ), we test whether the stellar environment (disk, envelope, cloud gas) agrees with a standard gas-to-dust (G/D) mix (as in the ISM). This is, overall, the case, although more detailed investigations using 2MASS and Spitzer are in progress.

Gas-to-Dust Ratios

We also model 3-D dust disks around highly extincted TTS and combine the modeled dust column density toward the star with our gas column densities. IRAS 04158+2805 shows a mass ratio G/D 220 +/- 160, excluding strong depletion of dust.

XEST sample

range of ISM

IR SED and spectralmodeling

Spitzer IRS

IRAS 04158+2805 H-band observation (left) and model

IRAS 04158+2805