An X-Ray Survey of the Taurus Star Formation Region
Manuel 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
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:
Surveyed
Detected
Protostars CTTS
21
70
10 (48%)
60 (86%)
WTTS
52
50 (96%)
BDs
17
9 (53%)
Accretion: Soft Excess
continuum
OVIII OVII
absorbed
continuum
total
169
136 (80%)
Active Brown Dwarfs
Jets: Double Absorbers
DG Tau
OVII triplet
of T Tau:
forbidden line
(f) strong 
low density
( 1010 cm-3)
r
inactive:
high OVII/OVIII
cts/s/kev
active:
low OVII/OVIII
others
9
7 (78%)
soft
hard
f
Energy
i
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").
time
New spectral phenomenology was found in a few jetdriving 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.
constant
jet
Photoelectric absorption
soft X
shocks
no excess visual extinction
flaring
hard X,
absorbed
dust
destruction
constant
at 10 R*
soft X,
counter
Photoelectric
absorbed
jet
absorption by disk
T Tau
detected
accreting
non-accreting
undetected
There is no significant
difference in the detection rates of accreting vs non-accreting
BDs.
Accretion does not significantly alter the Xray properties of BDs.
x
x
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!
CTTS
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 Xray detected. They fit
the stellar LX vs mass
and LX vs Lbol relations
quite well, i.e., young
BDs
behave
like
lowest-mass stars.
Gradual optical event in a BD
near V773 Tau:
Counter jet (blue)
harder than forward
jet (red/yellow):
absorption by disk
DG Tau (Chandra ACIS-S, 0.5-1.7keV)
counter jet
forward jet
hardness
WTTS
log NH
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.
(red to blue for
0.5-1.7 keV):
4"
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.
[OI] optical forward jet (Dougados et al. 2002)
New TMC Member Candidates
Gas-to-Dust Ratios
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.
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.
field around V410 Tau (L1495E)
known
members
special issue of A&A
containing 15 papers on various
topics. First authors and topics:
XEST sample
range of ISM
IR SED
and spectral
modeling
Spitzer IRS
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.
known members
mass-LX relation
IRAS 04158+2805
candidates
Güdel et al.: Introduction, Overview & Tables
Arzner et al.: Low-count spectra
Arzner et al.: Stochastic flaring (light curves)
Audard et al.: U-band/UV survey (XMM-OM)
Briggs et al.: Activity-rotation relations
Franciosini et al.: Flare geometric modeling
Grosso et al.: BD X-ray survey
Grosso et al.: BD U-band accretion event
Güdel et al.: X-rays from jet-driving CTTS
Güdel et al.: Case study of T Tau
Scelsi et al.: New TMC member candidates
Stelzer et al.: Flare & variability statistics
Telleschi et al.: High-res X-ray spectroscopy
Telleschi 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 04158
Güdel et al.: Soft excess in CTTS
Scelsi et al.: Coronal abundances
candidates
known members
IRAS 04158+2805 H-band
observation (left) and model
NOTE: This poster shows selected
results from a larger body of studies
related to XEST. A first series of
refereed papers will be published
shortly in a
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.