Rotationally modulated X-ray emission from T Tauri stars (original) (raw)
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The Origin of T Tauri X‐Ray Emission: New Insights from the Chandra Orion Ultradeep Project
The Astrophysical Journal Supplement Series, 2005
The Chandra Orion Ultradeep Project (COUP) provides the most comprehensive dataset ever acquired on the X-ray emission of pre-main sequence stars. In this paper, we study the nearly 600 X-ray sources that can be reliably identified with optically well characterized T Tauri stars (TTS) in the Orion Nebula Cluster. With a detection limit of L X,min ∼ 10 27.3 erg/sec for lightly absorbed sources, we detect X-ray emission from more than 97% of the optically visible late-type (spectral types F to M) cluster stars. This proofs that there is no "Xray quiet" population of late-type stars with suppressed magnetic activity. We use this exceptional optical, infrared, and X-ray data set to study the dependencies of the X-ray properties on other stellar parameters. All TTS with known rotation periods lie in the saturated or super-saturated regime of the relation between activity and Rossby numbers seen for main-sequence (MS) stars, but -2the TTS show a much larger scatter in X-ray activity than seen for the MS stars. Strong near-linear relations between X-ray luminosities, bolometric luminosities and mass are present. We also find that the fractional X-ray luminosity L X /L bol rises slowly with mass over the 0.1 − 2 M ⊙ range. The plasma temperatures determined from the X-ray spectra of the TTS are much hotter than in MS stars, but seem to follow a general solar-stellar correlation between plasma temperature and activity level. The scatter about the relations between X-ray activity and stellar parameters is larger than the expected effects of X-ray variability, uncertainties in the variables, and unresolved binaries. This large scatter seems to be related to the influence of accretion on the X-ray emission. While the X-ray activity of the non-accreting TTS is consistent with that of rapidly rotating MS stars, the accreting stars are less X-ray active (by a factor of ∼ 2 − 3 on average) and produce much less well defined correlations than the non-accretors. We discuss possible reasons for the suppression of X-ray emission by accretion and the implications of our findings on long-standing questions related to the origin of the X-ray emission from young stars, considering in particular the location of the X-ray emitting structures and inferences for pre-main-sequence magnetic dynamos.
The X-ray activity-rotation relation of T Tauri stars in Taurus-Auriga
Astronomy & Astrophysics, 2007
Context. The Taurus-Auriga star-forming complex hosts the only population of T Tauri stars in which an anticorrelation of X-ray activity and rotation period has been observed. Aims. We aim to explain the origin of the X-ray activity-rotation relation in Taurus-Auriga. We also aim to put the X-ray activity of these stars into the context of the activity of late-type main-sequence stars and T Tauri stars in the Orion Nebula Cluster. Methods. We have used XMM-Newton's European Photon Imaging Cameras to perform the most sensitive survey to date of X-ray emission (0.3-10 keV) from young stars in Taurus-Auriga. We investigated the dependences of X-ray activity measures-X-ray luminosity, L X , its ratio with the stellar luminosity, L X /L , and the surface-averaged X-ray flux, F XS-on rotation period and compared them with predictions based solely on the observed dependence of L X on a star's L and whether it is accreting or not. We tested for differences in the distributions of L X /L of fast and slow rotators, accretors and non-accretors, and compared the dependence of L X /L on the ratio of the rotation period and the convective turnover timescale, the Rossby number, with that of late-type main-sequence stars. Results. We found significant anticorrelations of L X and F XS with rotation period, but these could be explained by the typically higher stellar luminosity and effective temperature of fast-rotators in Taurus-Auriga and a near-linear dependence of L X on L. We found no evidence for a dependence of L X /L on rotation period, but for accretors to have lower L X /L than non-accretors at all rotation periods. The Rossby numbers of accretors and non-accretors were found to be the same as those of late-type main-sequence stars showing saturated X-ray emission. Conclusions. Non-accreting T Tauri stars show X-ray activity entirely consistent with the saturated activity of fast-rotating late-type main-sequence stars. Accreting T Tauri stars show lower X-ray activity, but this cannot be attributed to their slower rotation.
Astronomy & Astrophysics, 2007
Context. T Tau stars display different X-ray properties depending on whether they are accreting (classical T Tau stars; CTTS) or not (weak-line T Tau stars; WTTS). X-ray properties may provide insight into the accretion process between disk and stellar surface. Aims. We use data from the XMM-Newton Extended Survey of the Taurus molecular cloud (XEST) to study differences in X-ray properties between CTTS and WTTS. Methods. XEST data are used to perform correlation and regression analysis between X-ray parameters and stellar properties. Results. We confirm the existence of a X-ray luminosity (L X) vs. mass (M) relation, L X ∝ M 1.69 ± 0.11 , but this relation is a consequence of X-ray saturation and a mass vs. bolometric luminosity (L *) relation for the TTS with an average age of 2.4 Myr. X-ray saturation indicates L X = const.L * , although the constant is different for the two subsamples: const. = 10 −3.73 ± 0.05 for CTTS and const. = 10 −3.39 ± 0.06 for WTTS. Given a similar L * distribution of both samples, the X-ray luminosity function also reflects a real X-ray deficiency in CTTS, by a factor of ≈2 compared to WTTS. The average electron temperatures T av are correlated with L X in WTTS but not in CTTS; CTTS sources are on average hotter than WTTS sources. At best marginal dependencies are found between X-ray properties and mass accretion rates or age. Conclusions. The most fundamental properties are the two saturation laws, indicating suppressed L X for CTTS. We speculate that some of the accreting material in CTTS is cooling active regions to temperatures that may not significantly emit in the X-ray band, and if they do, high-resolution spectroscopy may be required to identify lines formed in such plasma, while CCD cameras do not detect these components. The similarity of the L X vs. T av dependencies in WTTS and main-sequence stars as well as their similar X-ray saturation laws suggests similar physical processes for the hot plasma, i.e., heating and radiation of a magnetic corona.
Coronal abundances of X-ray bright pre-main sequence stars in the Taurus molecular cloud
Astronomy and Astrophysics, 2007
Aims. We studied the thermal properties and chemical composition of the X-ray emitting plasma of a sample of bright members of the Taurus Molecular Cloud to investigate possible differences among classical and weak-lined T Tauri stars and possible dependences of the abundances on the stellar activity level and/or on the presence of accretion/circumstellar material. Methods. We used medium-resolution X-ray spectra obtained with the sensitive EPIC/PN camera in order to analyse the possible sample. The PN spectra of 20 bright (L X ∼ 10 30 − 10 31 erg s −1 ) Taurus members, with at least ∼ 4500 counts, were fitted using thermal models of optically thin plasma with two components and variable abundances of O, Ne, Mg, Si, S, Ar, Ca, and Fe. Extensive preliminary investigations were employed to study the performances of the PN detectors regarding abundance determinations, and finally to check the results of the fittings. Results. We found that the observed X-ray emission of the studied stars can be attributed to coronal plasma having similar thermal properties and chemical composition both in the classical and in the weak-lined T Tauri stars. The results of the fittings did not show evidence for correlations of the abundance patterns with activity or accretion/disk presence. The iron abundance of these active stars is significantly lower than (∼ 0.2 of) the solar photospheric value. An indication of slightly different coronal properties in stars with different spectral type is found from this study. G-type and early K-type stars have, on average, slightly higher Fe abundances (Fe ∼ 0.24 solar) with respect to stars with later spectral type (Fe ∼ 0.15 solar), confirming previous findings from high-resolution X-ray spectroscopy; stars of the former group are also found to have, on average, hotter coronae.
Accretion and outflow-related X-rays in T Tauri stars
Proceedings of the International Astronomical Union, 2007
We report on accretion- and outflow-related X-rays from T Tauri stars, based on results from the “XMM-Newton Extended Survey of the Taurus Molecular Cloud.” X-rays potentially form in shocks of accretion streams near the stellar surface, although we hypothesize that direct interactions between the streams and magnetic coronae may occur as well. We report on the discovery of a “soft excess” in accreting T Tauri stars supporting these scenarios. We further discuss a new type of X-ray source in jet-driving T Tauri stars. It shows a strongly absorbed coronal component and a very soft, weakly absorbed component probably related to shocks in microjets. The excessive coronal absorption points to dust-depletion in the accretion streams.
T Tauri Stars: The X-Ray Connection
The Non-Sleeping Universe, 1999
We have analysed newly calibrated IUE and ROSAT data for three T Tauri stars from different subclasses: TW Hya, V410 Tau and CS Cha, a Classical T Tauri star (CTTS), a Weak T Tauri star (WTTS) and a T Tauri star intermediate between CTTS and WTTS, respectively. In the ultraviolet the continuum seems to be well explained by the sum of the stellar black body emission plus a hydrogenic component with temperature in the range 1.4 to 5 × 10 4 K. TW Hya requires additionally a third component at 7900 K covering approximately 5% of the stellar surface. Using UV line fluxes, we have also analysed the energy emitted by these stars up to temperatures of 10 5 K. All three stars show emission strongly enhanced relatively to the Sun and peaking at temperatures characteristic of the transition region. In this respect the behaviour of the three stars seems to extend in a harmonic way from the ultraviolet into the X-ray band. The X-ray spectral analyses are consistent with emission from a twotemperature, optically thin plasma in collisional equilibrium. The lowest temperature plasma is at about 10 6 K while the highest reaches 10 7 K. We have also checked for and found no evidence for flaring activity in our sources at the time of the observations. Nevertheless, some level of variability is present for the stars in the sample. The ROSAT datasets for V410 Tau show a variation in terms of count rate on timescales of months to years.
Rotational Modulation of X‐Ray Emission in Orion Nebula Young Stars
The Astrophysical Journal Supplement Series, 2005
We investigate the spatial distribution of X-ray emitting plasma in a sample of young Orion Nebula Cluster stars by modulation of their X-ray light-curves due to stellar rotation. The study, part of the Chandra Orion Ultradeep Project (COUP), is made possible by the exceptional length of the observation: 10 days of ACIS integration during a time span of 13 days, yielding a total of 1616 detected sources in the 17 × 17 arcmin field of view. We here focus on a subsample of 233 X-ray-bright stars with known rotational periods. We search for X-ray modulation using the Lomb Normalized Periodogram method.
X-rays from T Tauri: a test case for accreting T Tauri stars
Astronomy & Astrophysics, 2007
Context. The generation of X-rays in accreting T Tauri stars (TTS) is thought to be predominantly due to energy dissipation in magnetic fields, but alternative X-ray generation mechanisms have been proposed, such as heating in accretion shocks near the stellar surface, or in shocks forming in jets. Aims. We test models and trends discussed in the literature using X-ray data from the classical TTS T Tau. Methods. High-resolution spectroscopy from the Reflection Grating Spectrometers on XMM-Newton is used to infer electron densities, element abundances and the thermal structure of the X-ray source. We also discuss the ultraviolet light curve obtained by the Optical Monitor, and complementary ground-based photometry. A high-resolution image from Chandra constrains contributions from the two companions of T Tau N. Results. The X-ray grating spectrum is rich in emission lines, but shows an unusual mixture of features from very hot (≈30 MK) and very cool (1-3 MK) plasma, both emitted by similar amounts of emission measure. The cool plasma confirms the picture of a soft excess in the form of an enhanced O vii/O viii Lyα flux ratio, similar to that previously reported for other accreting TTS. Diagnostics from lines formed by this plasma indicate low electron densities (< ∼ 10 10 cm −3). The Ne/Fe abundance ratio is consistent with a trend in pre-main sequence stars in which this ratio depends on spectral type, but not on accretion. Conclusions. On the basis of line density diagnostics, we conclude that the density of the cool "soft-excess" plasma is orders of magnitude below that predicted for an accretion shock, assuming previously determined accretion rates of (3−6) × 10 −8 M yr −1. We argue that loading of magnetic field lines with infalling material suppresses the heating process in a part of the corona. We thus suggest that the X-ray production of T Tau is influenced by the accretion process although the X-rays may not form in the bulk of the accretion footpoints.
Absorption of X-ray emission of t tauri stars by circumstellar material
Astrophysics and Space Science, 1995
The study of star forming regions (SFR) allows us to observe many young stellar objects with both the same metallicities and distances but with different masses. Because of its close distance (,-~ 140 pc) Taurus-Auriga is one of the best studied SFR with more than 100 well-studied, low-mass, pre-main sequence stars, T Tauri stars (TTS). A motivation for studying X-ray emission of T associations is to understand the origin of X-rays and coronal activity. The large sample observed with the ROSAT All-Sky Survey (RASS) also enables us to compare different types of young stars. Other primary goals include star formation efficiency and the interaction of young stars with their intermediate environment (probed by absorption of X-rays). RASS detection rates are comparable with Einstein Observatory results: 43 out of 65 (66%) weak-lined TTS (WTTS) and 9 out of 79 (11%) classical TTS (CTTS) exhibit X-ray emission above RASS detection limit. A strong correlation between X-ray surface flux and stellar rotation indicates that WTTS are intrinsically more X-ray active than CTTS, because WTTS rotate faster. However, rotation is not the only parameter that determines X-ray activity. Also, we compare Taurus-Auriga TTS with TTS of southern SFR like ScoCen, Lupus, Chamaeleon, and CrA. A new result is that CTTS and WTTS can be discriminated reliably by their X-ray spectral hardness ratios. X-ray emission of CTTS appears to be harder, partly because of circumstellar absorption. Spectra] fits give results consistent with Raymond-Smith spectra and emission temperatures of ~ 1.0 keV for both WTTS and CTTS. However, we find that CTTS and WTTS have significantly different X-ray luminosity functions. Medians of absorption corrected X-ray luminosities (log Lx in cgs units) are 29.701-4-0.045 for WTTS and 29.091 q-0.032 for CTTS. WTTS are intrinsically more luminous than CTTS, most likely because WTTS rotate on average faster than CTTS and are less absorbed. This paper concentrates on differences between CTTS and WTTS and indirect clues to be drawn from X-ray absorption and hardness ratios about circumstellar material around TTS.
High‐Resolution X‐Ray Spectroscopy of the Post–T Tauri Star PZ Telescopii
The Astrophysical Journal, 2004
We present an analysis of the Chandra High Energy Transmission Grating Spectrometer observation of the rapidly rotating (P rot = 0.94 d) post T Tauri (∼ 20 Myr old) star PZ Telescopii, in the Tucana association. Using two different methods we have derived the coronal emission measure distribution, em(T ), and chemical abundances. The em(T ) peaks at log T = 6.9 and exhibits a significant emission measure at temperatures log T > 7. The coronal abundances are generally ∼ 0.5 times the solar photospheric values that are presumed fairly representative of the composition of the underlying star. A minimum in abundance is seen at a first ionization potential (FIP) of 7-8 eV, with evidence for higher abundances at both lower and higher FIP, similar to patterns seen in other active stars. From an analysis of the He-like triplet of Mg XI we have estimated electron densities of ∼ 10 12 − 10 13 cm −3 . All the coronal properties found for PZ Tel are much more similar to those of AB Dor, which is slightly older than PZ Tel, than to those of the younger T Tauri star TW Hya. These results support earlier conclusions that the soft X-ray emission of TW Hya is likely dominated by accretion activity rather than by a magnetically-heated corona. Our results also suggest that the coronae of pre-main sequence stars rapidly become similar to those of older active main-sequence stars soon after the accretion stage has ended.