Coronal density diagnostics with Helium-like triplets: CHANDRA–LETGS observations of Algol, Capella, Procyon,<i>ϵ</i> Eri,<i>α</i> Cen A&amp;B, UX Ari, AD Leo, YY Gem, and HR 1099

Ness, J. U.; Schmitt, J. H. M. M.; Burwitz, V.; Mewe, R.; Raassen, A. J. J.; Meer, R. L. J. van der; Predehl, P.; Brinkman, A. C.

doi:10.1051/0004-6361:20021146

Cited by 82 publications

(56 citation statements)

References 37 publications

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“…This ratio is also sensitive to density and temperature. Interference with density effects is not severe as Ness et al (2002a) found low density limits for almost all stellar coronae. In Fig.…”

Section: He-like Line Ratio F/rmentioning

confidence: 67%

“…From the response matrices effective areas were calculated and stored in ASCII files which are used as look-up tables for converting measured line counts into line fluxes. Most of the LETGS data included have been introduced by Ness et al (2002a) and for details on the data reduction we refer to that paper (effective areas from Deron Pease, Aug. 2002).…”

Section: Reduction Of the Raw Datamentioning

confidence: 99%

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Are stellar coronae optically thin in X-rays?

Ness

Schmitt

Audard

et al. 2003

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Abstract. The relevance of resonant scattering in the solar corona has always been discussed controversially. Ratios of emission lines from identical ions but different oscillator strengths have been used in order to estimate damping of resonance lines due to possible resonant scattering, i.e., absorption by photo-excitation and re-emission out of the line of sight. The analysis of stellar spectra in analogy to previous works for the Sun is possible now with XMM-Newton and Chandra grating spectra and requires this issue to be considered again. In this work we present a sample of 45 X-ray spectra obtained for 26 stellar coronae with the RGS on board XMM-Newton and the LETGS and HETGS on board Chandra. We use ratios of the Fe  lines at 15.27 Å and 16.78 Å lines to the resonance line at 15.03 Å as well as the He-like f /r ratio of O  and Ne  to measure optical depth effects and compare them with ratios obtained from optically thin plasma atomic databases such as MEKAL, Chianti, and APEC. From the Fe  line ratios we find no convincing proof for resonance line scattering. Optical depths are basically identical for all kinds of stellar coronae and we conclude that identical optical depths are more probable when effects from resonant scattering are generally negligible. The 15.27/15.03 Å ratio shows a regular trend suggesting blending of the 15.27 Å line by a cooler Fe line, possibly Fe . The He-like f /r ratios for O and Ne show no indication for significant damping of the resonance lines. We mainly attribute deviations from the atomic databases to still uncertain emissivities which do not agree well with laboratory measurements and which come out with differing results when accounting for one or the other side effect. We attribute the discrepancies in the solar data to geometrical effects from observing individual emitting regions in the solar corona but only overall emission for stellar coronae including photons eventually scattered into the line of sight.

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Section: He-like Line Ratio F/rmentioning

confidence: 67%

Section: Reduction Of the Raw Datamentioning

confidence: 99%

Are stellar coronae optically thin in X-rays?

Ness

Schmitt

Audard

et al. 2003

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“…All but one of the CTTS for which the O vii triplet lines were detected showed cool plasma with high density, n e > 10 11 cm −3 (Kastner et al 2002;Stelzer & Schmitt 2004;Schmitt et al 2005;Günther et al 2006;Argiroffi et al 2007;. In contrast, the cool quiescent plasma of active stellar coronae is always dominated by low densities (n e < ∼ 10 10 cm −3 , Ness et al 2002;Testa et al 2004a). This basic difference suggests that the high-density cool plasma in CTTS is not coronal plasma but plasma heated in accretion shocks.…”

Section: Article Published By Edp Sciencesmentioning

confidence: 97%

X-ray optical depth diagnostics of T Tauri accretion shocks

Argiroffi

Maggio

Peres

et al. 2009

A&A

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Context. In classical T Tauri stars, X-rays are produced by two plasma components: a hot low-density plasma, with frequent flaring activity, and a high-density lower temperature plasma. The former is coronal plasma related to the stellar magnetic activity. The latter component, never observed in non-accreting stars, could be plasma heated by the shock formed by the accretion process. However its nature is still being debated. Aims. Our aim is to probe the soft X-ray emission from the high-density plasma component in classical T Tauri stars to check whether this plasma is heated in the accretion shock or whether it is coronal plasma. Methods. High-resolution X-ray spectroscopy allows us to measure individual line fluxes. We analyze X-ray spectra of the classical T Tauri stars MP Muscae and TW Hydrae. Our aim is to evaluate line ratios to search for optical depth effects, which are expected in the accretion-driven scenario. We also derive the plasma emission measure distributions EMD, to investigate whether and how the EMD of accreting and non accreting young stars differ. The results are compared to those obtained for the non-accreting weakline T Tauri star TWA 5. Results. We find evidence of resonance scattering in the strongest lines of MP Mus, supporting the idea that soft X-rays are produced by plasma heated in the accretion shock. We also find that the EMD of MP Mus has two peaks: a cool peak at temperatures expected for plasma heated in the accretion shock, and a hot peak typical of coronal plasma. The shape of the EMD of MP Mus appears to be the superposition of the EMD of a pure coronal source, like TWA 5, and an EMD alike that of TW Hya, which is instead dominated by shock-heated plasma.

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“…If low-level emission is made up of flares as the one discussed in this paper, then densities derived spectroscopically from the timeintegrated spectrum across the entire flare should be similar to densities measured in active stars during quiescence. For the Proxima Centauri flare, we obtained an average characteristic density from the O  triplet of log n e = 10.50 +0.32 −0.25 , while, for example, the low-level YY Gem spectrum reveals a similar value, namely log n e = 10.35 +0.13 −0.45 , as do many other active main-sequence stars (Güdel et al 2001a;Ness et al 2002).…”

Section: Small Flares and Coronal Heatingmentioning

confidence: 99%

Flares from small to large: X-ray spectroscopy of Proxima Centauri with XMM-Newton

Güdel¹,

Audard²,

Reale³

et al. 2004

A&A

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136

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Abstract.We report results from a comprehensive study of the nearby M dwarf Proxima Centauri with the XMM-Newton satellite, using simultaneously its X-ray detectors and the Optical Monitor with its U band filter. We find strongly variable coronal X-ray emission, with flares ranging over a factor of 100 in peak flux. The low-level emission is found to be continuously variable on at least three time scales (a slow decay of several hours, modulation on a time scale of 1 hr, and weak flares with time scales of a few minutes). Several weak flares are characteristically preceded by an optical burst, compatible with predictions from standard solar flare models. We propose that the U band bursts are proxies for the elusive stellar non-thermal hard X-ray bursts suggested from solar observations. In the course of the observation, a very large X-ray flare started and was observed essentially in its entirety. Its peak luminosity reached 3.9 × 10 28 erg s −1 [0.15-10 keV], and the total X-ray energy released in the same band is derived to be 1.5 × 10 32 ergs. This flare has for the first time allowed to measure significant density variations across several phases of the flare from X-ray spectroscopy of the O  He-like triplet; we find peak densities reaching up to 4 × 10 11 cm −3 for plasma of about 1-5 MK. Abundance ratios show little variability in time, with a tendency of elements with a high first ionization potential to be overabundant relative to solar photospheric values. Using Fe  lines with different oscillator strengths, we do not find significant effects due to opacity during the flare, indicating that large opacity increases are not the rule even in extreme flares. We model the large flare in terms of an analytic 2-Ribbon flare model and find that the flaring loop system should have large characteristic sizes (≈1R * ) within the framework of this simplistic model. These results are supported by full hydrodynamic simulations. Comparing the large flare to flares of similar size occurring much more frequently on more active stars, we propose that the X-ray properties of active stars are a consequence of superimposed flares such as the example analyzed in this paper. Since larger flares produce hotter plasma, such a model also explains why, during episodes of low-level emission, more active stars show hotter plasma than less active stars.

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Coronal density diagnostics with Helium-like triplets: CHANDRA–LETGS observations of Algol, Capella, Procyon,ϵ Eri,α Cen A&B, UX Ari, AD Leo, YY Gem, and HR 1099

Cited by 82 publications

References 37 publications

Are stellar coronae optically thin in X-rays?

Are stellar coronae optically thin in X-rays?

X-ray optical depth diagnostics of T Tauri accretion shocks

Flares from small to large: X-ray spectroscopy of Proxima Centauri with XMM-Newton

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