Issue |
A&A
Volume 516, June-July 2010
|
|
---|---|---|
Article Number | L8 | |
Number of page(s) | 4 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014914 | |
Published online | 23 June 2010 |
LETTER TO THE EDITOR
Correlated optical and X-ray variability in CTTS
Indications of absorption-modulated emission
E. Flaccomio1 - G. Micela1 - F. Favata2 - S. P. H. Alencar3
1 - INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
2 -
European Space Agency, 8-10 rue Mario Nikis, 75015 Paris, France
3 -
Departamento de Fisica - ICEx - UFMG, Av. Antônio Carlos 6627, 30270-901 Belo Horizonte, MG, Brazil
Received 3 May 2010 / Accepted 25 May 2010
Abstract
Aims. Optical and X-ray emission from classical T Tauri
stars (CTTSs) has long been known to be highly variable. Our long,
uninterrupted optical observation of the NGC 2264 region with CoRoT
allows the optical variability in CTTS to be studied with unprecedented
accuracy and time coverage. Two short Chandra observations obtained
during the CoRoT pointing with a separation of 16 days allow us to
study whether there is a correlation between optical and X-ray
variability on this timescale, thus probing the physical mechanisms
driving the variability in both bands.
Methods. We have computed the optical and X-ray fractional
variability between the two 30 ks duration windows covered by both the
Chandra and CoRoT observations, for a sample of classical and weak line
T Tauri stars (WTTSs) in NGC 2264. A scatter plot clearly shows
that the variability of CTTSs in the optical and soft X-ray
(0.5-1.5 keV) bands is correlated, while no correlation is
apparent in the hard (1.5-8.0 keV) band. Also, no correlation in
either band is present for WTTSs.
Results. We show that the correlation between soft X-ray and
optical variability of CTTSs can be naturally explained in terms of
time-variable shading (absorption) from circumstellar material orbiting
the star, in a scenario rather similar to the one invoked to explain
the observed phenomenology in the CTTS AA Tau. The slope
of the observed correlation implies (in the hypothesis of homogeneous
shading) a significant dust depletion in the circumstellar material
(with a gas-to-dust ratio approximately 5 times lower than the standard
value for interstellar material).
Key words: stars: variable: T Tauri, Herbig Ae/Be - stars: activity - stars: coronae - stars: formation - accretion, accretion disks - X-rays: stars
1 Introduction
Strong irregular optical variability is one of the defining characteristics of classical T-Tauri stars (CTTS, Joy 1945), together with their association with dark or bright nebulae and their strong optical emission lines. Following their discovery by Joy (1945), CTTS were then recognized as newly formed stars that have just completed their main accretion phase and are contracting toward the main sequence (e.g. Walker 1956). Unlike weak line T-Tauri stars (WTTS), CTTS are still undergoing mass-accretion: material from the inner edge of their truncated circumstellar disk is thought to be channeled along magnetic field lines toward an impact region on the stellar surface where it is shocked, producing an excess of emission with respect to the stellar photosphere, ranging from the X-ray band to the optical. The large and irregular optical variability of CTTSs has generally been linked with the accretion process, but the actual mechanism involved has remained elusive, with two classes of mechanisms being generally considered, i.e., i) variability of the emission from the accretion shock(s) due to variation in mass accretion rate and/or to their rotational modulation; or ii) variable absorption due to unstable and optically thick accretion streams and/or warps in the circumstellar disk that occult part of the photosphere. Recent statistical studies of CTTS observed over many years (Grankin et al. 2008,2007) suggest that in about 25% of the cases the optical variability may be attributed to absorption, while in the remaining cases time-variable accretion is favored. However, due to the similar effects of spots and absorption on the broad-band lightcurves, the absorption scenario cannot be excluded in most cases. A detailed study of the CTTS AA Tau by Bouvier and collaborators (Bouvier et al. 2003,1999,2007; Grosso et al. 2007; Ménard et al. 2003) has shown that, for this near edge-on star-disk system, the large, irregular optical variability is explained well by occultation of a significant fraction of the stellar surface by a warp in the inner disk, located at the corotation radius and thus rotating in and out of view with the same period as the stellar photosphere and evolving on similar timescales. The warp could be due to the misalignment of the rotation and magnetic axes and could correspond to the foot of the accretion stream. Until very recently the case of AA Tau could be considered peculiar, as no other similar systems had been reported. However, recent high-quality optical lightcurves of a large sample of young stars, CTTS and WTTS, in the NGC 2264 star-forming region, obtained with the CoRoT satellite, have shown that AA Tau-like variability is rather common (Alencar et al. 2010). This leads to the suggestion that time-dependent obscuration of part of the photosphere by disk warps, or else by the related accretion streams, might be an important mechanism to explain the optical variability of CTTS.
CTTS are also peculiar in the X-ray band; like WTTS, they show significant coronal emission from plasma at 10-30 MK, but with average luminosities lower than those of WTTSs, at any given stellar mass or bolometric luminosity, by a factor of 3-5, and with a significantly larger scatter (Flaccomio et al. 2003; Preibisch et al. 2005). Moreover, the X-ray emission of CTTSs may be more time variable and have a harder spectrum than the one of WTTSs (e.g. Flaccomio et al. 2006; Imanishi et al. 2001). These facts have so far remained unexplained. A variety of physical mechanisms have been suggested to explain the lower observed X-ray luminosity, such as mass-loading of coronal magnetic loops due to accretion material (leading to cooler plasma, not visible in X-rays) and the shielding of significant fractions of the coronal plasma by dense accretion streams (Gregory et al. 2007).
In addition to the hard coronal emission, a separate 2-3 MK X-ray spectral component related to accretion and possibly originating in the accretion shock is now believed to be common in CTTSs. This soft component has only been observed to date in a dozen such stars observed at high spectral resolution with either Chandra or XMM-Newton (e.g. Güdel & Telleschi 2007). With the notable exception of TW Hya, the coronal component seems to dominate the emission for E>500 eV.
Simultaneous optical and X-ray observations can constrain the physical
mechanisms responsible for the optical and X-ray variability and the
location of the X-ray emitting material relative to the photosphere.
For the case of AA Tau, for example, Grosso et al. (2007)
searched for X-ray eclipses corresponding to two of the optical
eclipses caused by the disk warp. Their failure to detect X-ray
eclipses was interpreted as evidence that the X-ray emitting plasma is
at high latitudes. Stassun et al. (2007,2006) examined ground-based optical photometry of 800 young stars in the Orion Nebula Cluster, overlapping, for
1 week, with the Chandra
Orion Ultradeep Project (COUP) observation of the same region. They
found ``very little evidence to suggest a direct causal link between
the sources of optical and X-ray variability in PMS stars''.
We have obtained two 30 ks Chandra observations of the star-forming region NGC 2264 overlapping with a dedicated CoRoT ``short run''. The two Chandra
observations, separated by approximately two weeks, were allocated from
the Director's Discretionary Time. We present evidence that the soft
X-ray and optical emissions are correlated for CTTSs (but not for
WTTSs) and discuss the implications in terms of location of the X-ray
emitting material and origins of the variability.
2 Data and sample selection
We observed the star-forming region NGC 2264 for 23.5 days with
the CoRoT satellite in March 2008. The two CCDs normally used for
exoplanet observations cover a 2 sq.degree field with the cluster fitting in a single CCD. High-quality, broadband (370-950
m),
optical lightcurves were obtained with a cadence of 512 or 32 s
for 8150 pre-selected targets in the field, with magnitudes down to
.
First results for NGC 2264 members were published by Alencar et al. (2010),
while a complete description of the observation is in preparation
(Favata et al.). We here use CoRoT ``white light'' lightcurves, as
produced by the standard pipeline, cleaned of datapoints of dubious
quality (status flag
0) and all rebinned to 512 s. The time series for
1/3
of the stars in our final sample actually contain color information.
We, however, decided to use only the sum of the fluxes in the three
available bands, given the poor definition of the CoRoT photometric
system.
During the CoRoT pointing, we obtained two Chandra ACIS-I observations of a
field
in NGC 2264, the first on 12 March, lasting 28 ks (ObsId:
9768), and the second on 28 March, lasting 30 ks (ObsId: 9769).
The aim points were the same within 2
(RA 6:41:12.5, Dec +9:29:32) and the roll angles differed by only
4 degrees, so that the two fields overlap almost completely.
A full account of the analysis of the Chandra
observations will be provided by Flaccomio et al. (in
preparation). In brief, we performed source detection on each field
with PWdetect (Damiani et al. 1997) and then used ACIS Extract (Broos et al. 2010)
to extract individual source spectra and lightcurves. We here make use
mainly of the mean observed fluxes during the two observations, in
units of counts s-1cm-2, i.e., countrates divided by the effective area of the detector at each source position.
In the following we study the optical and X-ray flux variations between the times of the two Chandra pointings. The reference source sample was defined starting with the 81 sources in common between the CoRoT and the Chandra datasets and with unambiguous cross-identifications with optical and NIR catalogs (Sung et al. 2009,2008,
2MASS). We then excluded five stars whose CoRoT lighturves are affected
by sudden flux variations, most likely spurious and due to cosmic rays.
In the X-ray band we consider two different energy ranges:
0.5-1.5 keV (soft) and 1.5-8.0 keV (hard). To limit the
uncertainties on the flux differences between the two observations we
restricted our sample to stars with 5 X-ray photons detected in the band of interest in at least one
of the two observations. This condition leads to samples of 69 and
62 objects for the soft and hard X-ray bands, respectively.
Finally, we excluded 14 objects from both samples: three are not
low-mass stars, but Herbig Ae/Be stars, based on their spectral types
and/or V-I colors; the other 11 showed evidence of strong X-ray flares
during the Chandra observations. Although of interest, flares
were excluded for the present investigation since our aim here is to
investigate flux variability due either to accretion or to variable
absorption, i.e. mechanisms unrelated to flaring activity. Our main
conclusions, however, are hardly affected by the exclusion of these 11
stars. The final samples of stars with ``good'' simultaneous optical
and X-ray fluxes count 55 and 48 objects considering the soft
and hard X-ray bands, respectively. The smaller sample is, with the
exception of one star, a subset of the larger one.
We finally classify the stars in our sample as CTTSs or WTTSs following
Alencar et al. (2010): CTTSs are defined as stars with an H
equivalent
width >10
,
or H
width at 10% of the peak intensity
greater than 270 km s-1 (data
from Furész et al. 2006; Dahm & Simon 2005)
. The
other stars were classified as WTTSs. Like CTTS, all WTTS in our sample
are almost certainly members of NGC 2264, being detected in X-rays and
satisfying, in the vast majority of cases, other membership criteria
based on radial velocity, optical variability, and position in optical
color-magnitude diagrams (Lamm et al. 2004; Furész et al. 2006).
![]() |
Figure 1:
Optical vs. X-ray flux variations between the two |
Open with DEXTER |
3 Results
Figure 1 shows scatter plots between the optical and X-ray fractional variability of CTTSs and WTTSs, separately for the soft and hard X-ray bands. Sample sizes, the results of the Spearman's (



No correlation is evident in the hard band. While the lower statistics make it harder to detect a correlation, the scatter of points in all quadrants point to a different physical mechanism driving variability in the soft and in the hard X-ray bands, with only the soft X-ray variability linked with the optical one. Given the error bars and the limited sample sizes, it is unfortunately impossible to determine whether the correlation for the soft X-ray band is common to all the CTTS in our sample, or if distinct behaviors characterize the stars with larger X-ray/optical variability and those with smaller amplitudes. However, the correlation is probably not driven just by an handful of stars: excluding the four most optically variable stars, the correlation tests still indicate a likely correlation, with null probabilities of 2.8-3.2%. The extension of the correlation to the low-variability stars is even more striking considering that whatever drives variability in WTTSs (e.g., dark spots in the optical and active region evolution/rotational modulation in the X-rays) is likely at work also on CTTSs, thus diluting any CTTS-specific effect at low amplitudes.
4 Discussion
The observation of correlated optical and soft X-ray variability for CTTSs in NGC 2264 is at odds with the negative results reported by Grosso et al. (2007), for AA Tau, and Stassun et al. (2006) for a sample of Orion stars. The different quality of the optical lightcurves and different timescales probed by these studies might explain the apparent contradiction.Our result points either to i) a single physical mechanism modulating the optical and soft X-ray emission or ii) the emitting regions being obscured by the same material and thus the emission being absorbed in a correlated fashion. In principle, the observed X-ray spectra would vary differently in these two scenarios. The limited statistics of the low-resolution X-ray spectra obtained in our Chandra observation and the known degeneracy between temperature and absorption when fitting these spectra with models made the spectral analysis inconclusive.
If both the varying optical emissions and the varying soft
X-ray emission were to come from the accretion shock, one could invoke
a mechanism of type i) above: modulation in the mass accretion rate. To explain the observed optical variability range (approximately ), this would require the CoRoT broad-band optical emission being dominated by accretion luminosity, a condition that is not verified for most CTTSs (cf. Gullbring et al. 1998).
At the same time, the soft X-ray spectrum should be dominated by
emission from the shock region, with little or no coronal contribution,
to explain the approximately
variability range. However, all these stars show evidence of
significant coronal emission (as shown by the thermal emission in the
harder band), and only in the peculiar CTTS TW Hya is the X-ray
emission dominated by the shock region. In order to explain the
correlated variability we might also speculate that the coronal
emission is directly correlated to accretion. This hypothesis is,
however, not supported by observations that, if anything, indicate an
inverse relation between accretion and coronal activity (e.g. Preibisch et al. 2005).
It thus appears unlikely that the large variations in the soft X-ray
and in the optical fluxes can be attributed to variability in the
accretion shock region.
The second class of mechanisms assume shading of the optical emission region (photosphere) and of the X-ray emission region (corona) by the same (or correlated) circumstellar material. Rotationally-modulated shading of the optical emission (photospheric and from the accretion spot) is observed in AA Tau, and similar variations in the optical lightcurve are observed in several CTTSs in NGC 2264. Under the assumption that the circumstellar material covers a similar fraction of coronal and photospheric material, the correlated flux variability may be explained quite naturally by time-variable shading.
The differences in the mechanisms leading to the absorption of X-ray
and optical photons give additional diagnostic power to the observed
correlation. X-ray photons are absorbed mainly by material in the gas
phase while optical photons are absorbed mainly by dust grains. In the
hypothesis that the optical emitting region (the photosphere and the
accretion spot) and the X-ray emitting one (the corona) are, from the
point of view of the shading material, similar in location and size
(i.e. that they are at all times subject to comparable shading), the
shape of the diagram in Fig. 1
will be determined by the extinction laws in the optical and X-rays and
by the gas-to-dust ratio in the shading material. We computed
theoretical loci in Fig. 1, starting from the ``standard'' relation between optical extinction, ,
and
,
the equivalent hydrogen column density with which X-ray absorption is usually parametrized:
cm-2 (Vuong et al. 2003),
valid for an interstellar gas-to-dust ratio. The effect of non-standard
gas-to-dust ratio is easily mimicked by multiplying the above
relation by a gas-enrichment factor.
The theoretical loci in Fig. 1
depend weakly on the incident X-ray spectrum (that we modeled as a
single-temperature thermal plasma) and on the photospheric temperature,
and we have assumed a plausible range for both parameters, computing
``families of loci'' as a function of the gas-enrichment factor. As
shown in Fig. 1, a
``standard/interstellar'' gas-to-dust ratio would require the X-rays
variations to be, in relation to the optical variations, significantly
smaller than observed.
One natural way of explaining the observed correlation slope is to assume an absorbing medium depleted in dust grains (thus primarily absorbing X-rays). The circumstellar material in the immediate vicinity of the star, inside the circumstellar inner rim, including the accretion streams, is indeed expected to be heavily gas-depleted (e.g. Isella et al. 2009; Kama et al. 2009) and an enhanced absorption of X-rays from CTTSs has actually been reported (e.g. Günther & Schmitt 2008). While we have not attempted to formally fit the data to derive the average degree of dust depletion in the absorbing medium, as shown in Fig. 1, the locii corresponding to a five-fold depletion factor in the dust content with respect to the ``standard'' interstellar value do provide a good explanation for the observed correlation.
An alternative explanation could be built by assuming a standard gas-to-dust ratio together with a more substantial shading of the X-ray emitting region with respect to the photosphere and the accretion spot. It is easy to imagine such situations: an equatorial corona with a disk warp only eclipsing the low-latitude regions of the photosphere would be a possible example, as would emission from a low-latitude flux tube extending from the photosphere to the inner disk, when the flux tube is pointing toward the observer. However, to explain a general correlation would require that these specific geometrical conditions be realized for all CTTSsin our sample, or at least for those with large variability amplitudes that determine the statistical correlation. This would require ad hoc assumptions about the shape of the warped disk (assuming an AA Tau-like configuration), the location of the corona, and the relative inclination of the systems as observed. Again, while this is not impossible to envisage, a higher gas-to-dust ratio in the absorbing medium provides a much more natural and simpler explanation.
The observed correlation and the amplitude of the variability imply that a significant fraction of the X-ray emission from CTTSs is affected by shading and obscuration. If this obscuration is sufficiently optically thick, it may be impossible to recognize it and to determine its amount with the usual X-ray spectral analysis techniques. Since part of the corona would remain unaccounted for, the shading thus also provides a natural explanation for the lower X-ray luminosity of CTTSs with respect to WTTSs for stars of the same mass, as well as for the wider range in observed X-ray luminosities (cf. Gregory et al. 2007).
If our favored interpretation of the observed correlation is correct, the observed X-ray variability, on the 16-day timescale probed by our observations, is for a large fraction not intrinsic to the emitting source but rather caused by variations in the absorbing material. Because of this, future simultaneous observations of CTTSs in the optical and soft X-rays, spanning a range of time scales comparable to the rotational period of the stars, can provide unique diagnostics of their circumstellar environment.
AcknowledgementsWe thank Costanza Argiroffi and Salvo Sciortino for discussions that helped shape our conclusions, and the anonymous referee for useful suggestions on how to improve this work.
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Footnotes
- ... CoRoT
- The CoRoT space mission was developed and is operated by the French space agency CNES, with participation of ESA's RSSD and Science Programs, Austria, Belgium, Brazil, Germany, and Spain.
- ...
) - A third criterium used by Alencar et al. (2010), based on the U-V excess, was redundant for our smaller sample.
All Figures
![]() |
Figure 1:
Optical vs. X-ray flux variations between the two |
Open with DEXTER | |
In the text |
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