On the interstellar extinction law toward young stars

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.

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“…Some energy will be conducted to cooler layers, and the majority of the X-rays may be entirely absorbed (Lamzin 1999).…”

Section: Observational Signaturesmentioning

confidence: 99%

X-rays from T Tauri: a test case for accreting T Tauri stars

Güdel¹,

Skinner

Melnikov

et al. 2007

A&A

104

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“…In standard accretion shock models, the shock heats up to a few times 10 6 K, with the ensuing X-rays heating the underlying photosphere to produce an UV excess (Calvet & Gullbring 1998). Lamzin (1999) concluded that a typically small fraction of the X-rays escape from the shock that can be seen in soft X-rays. However, it will be important to estimate whether the shock is above the photosphere at all and therefore visible, and this is not normally the case for T Tau stars with average accretion characteristics (Calvet & Gullbring 1998).…”

Section: Accretion-induced X-raysmentioning

confidence: 99%

The first high-resolution X-ray spectrum of a Herbig star: AB Aurigae

Telleschi

Güdel

Briggs

et al. 2006

A&A

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Context. The X-ray emission from Herbig Ae/Be stars remains to be explained. In later-type T Tauri stars, X-rays are thought to be produced by magnetically trapped coronal plasma, although accretion-shock induced X-rays have also been suggested. In earlier-type (OB) stars, shocks in unstable winds are thought to produce X-rays. Aims. We present the first high-resolution X-ray spectrum of a prototypical Herbig star AB Aurigae), measure and interpret various spectral features, and compare our results with model predictions. Methods. We use X-ray spectroscopy data from the XMM-Newton Reflection Grating Spectrometers and the EPIC instruments. The spectra are interpreted using thermal, optically thin emission models with variable element abundances and a photoelectric absorption component. We interpret line flux ratios in the He-like triplet of O vii as a function of electron density and the UV radiation field. We use the nearby co-eval classical T Tauri star SU Aur as a comparison. Results. AB Aurigae reveals a soft X-ray spectrum, most plasma being concentrated at 1-6 MK. The He-like triplet reveals no signs of increased densities as reported for some accreting T Tau stars in the literature. There are also no clear indications of strong abundance anomalies in the emitting plasma. The light curve displays modulated variability, with a period of ≈42 h. Conclusions. It is unlikely that a nearby, undetected lower-mass companion is the source of the X-rays. Accretion shocks close to the star would be expected to be irradiated by the photosphere, leading to alteration in the He-like triplet fluxes of O vii, which we do not measure. Also, no indications for high densities are found, although the mass accretion rate is presently unknown. Emission from wind shocks is unlikely, given the weak radiation pressure. A possible explanation would be a solar-like magnetic corona. Magnetically confined winds are a very promising alternative. The X-ray period is indeed close to periods previously measured in optical lines from the wind.

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“…This technique uses the central star as the background source and measures the N(H I) from the heavily damped Lyα absorption line profile observed against the red side of the broad Lyα emission line (see also Lamzin 2006). Since the absorbing material is fixed in velocity 5 , this method is insensitive to changes in the accretion rate (i.e., the background Lyα spectrum) and veiling does not contribute at these wavelengths.…”

Section: Optical Reddening Values and The Extinctionmentioning

confidence: 99%

“…Lyα contributes, on average, ∼ 10 times the intrinsic flux as the second largest contributor to the FUV luminosity, the continuum emission (France et al 2014b). However, direct measurements of the intrinsic Lyα luminosity are impossible owing to resonant scattering by circumstellar and interstellar hydrogen atoms Lamzin 2006;McJunkin et al 2014). The best indirect method for measuring the Lyα emission line profile and the total Lyα power is a fluorescent H 2 -based profile reconstruction.…”

Section: Fuv Continuum and 1600å Bumpmentioning

confidence: 99%

The 1600 Å Emission Bump in Protoplanetary Disks: A Spectral Signature of H₂O Dissociation^∗

France

Roueff

Abgrall

2017

ApJ

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The FUV continuum spectrum of many accreting pre-main sequence stars, Classical T Tauri Stars (CTTSs), does not continue smoothly from the well-studied Balmer continuum emission in the NUV, suggesting that additional processes contribute to the short-wavelength emission in these objects. The most notable spectral feature in the FUV continuum of some CTTSs is a broad emission approximately centered at 1600Å, which has been referred to as the "1600Å Bump". The origin of this feature remains unclear. In an effort to better understand the molecular properties of planet-forming disks and the UV spectral properties of accreting protostars, we have assembled archival FUV spectra of 37 disk-hosting systems observed by the Hubble Space Telescope-Cosmic Origins Spectrograph. Clear 1600Å Bump emission is observed above the smooth, underlying 1100 -1800Å continuum spectrum in 19/37 Classical T Tauri disks in the HST -COS sample, with the detection rate in transition disks (8/8) being much higher than in primordial or non-transition sources (11/29). We describe a spectral deconvolution analysis to separate the Bump (spanning 1490 -1690Å) from the underlying FUV continuum, finding an average Bump luminosity, L(Bump) ≈ 7 × 10 29 erg s −1 . Parameterizing the Bump with a combination of Gaussian and polynomial components, we find that the 1600Å Bump is characterized by a peak wavelength λ o = 1598.6 ± 3.3Å, with FWHM = 35.8 ± 19.1Å.Contrary to previous studies, we find that this feature is inconsistent with models of H 2 excited by electron-impact. We show that this Bump makes up between 5 -50% of the total FUV continuum emission in the 1490 -1690Å band and emits roughly 10 -80% of the total fluorescent H 2 luminosity for stars with well-defined Bump features. Energetically, this suggests that the carrier of the 1600Å Bump emission is powered by Lyα photons. We argue that the most likely mechanism is Lyα-driven dissociation of H 2 O in the inner disk, r 2 AU. We demonstrate that non-thermally populated H 2 O fragments can qualitatively account for the observed emission (discrete and continuum), and find that the average Lyα-driven H 2 O dissociation rate is 1.7 × 10 42 water molecules s −1 .

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On the interstellar extinction law toward young stars

Cited by 27 publications

References 26 publications

X-rays from T Tauri: a test case for accreting T Tauri stars

X-rays from T Tauri: a test case for accreting T Tauri stars

The first high-resolution X-ray spectrum of a Herbig star: AB Aurigae

The 1600 Å Emission Bump in Protoplanetary Disks: A Spectral Signature of H₂O Dissociation^∗

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On the interstellar extinction law toward young stars

Cited by 27 publications

References 26 publications

X-rays from T Tauri: a test case for accreting T Tauri stars

X-rays from T Tauri: a test case for accreting T Tauri stars

The first high-resolution X-ray spectrum of a Herbig star: AB Aurigae

The 1600 Å Emission Bump in Protoplanetary Disks: A Spectral Signature of H2O Dissociation∗

Contact Info

Product

Resources

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The 1600 Å Emission Bump in Protoplanetary Disks: A Spectral Signature of H₂O Dissociation^∗