The cool half of the H-R diagram in soft X-rays

Ayres, Thomas R.; Linsky, J. L.; Vaiana, G. S.; Golub, L.; Rosner, R.

doi:10.1086/159374

Cited by 124 publications

(68 citation statements)

References 4 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Ayres et al (1981) attempted to observe soft X-ray emission from late-type stars, being a signature of stellar coronae (T > 10 6 K). They found an "X-ray DL" roughly coinciding with the "transition region DL" from Linsky & Haisch (1979); only stars to the blue side were detected in X-rays.…”

Section: Coronal Transition Region and Wind Dividing Linesmentioning

confidence: 99%

Structure of the outer layers of cool standard stars

Dehaes

Bauwens

Decin

et al. 2011

A&A

View full text Add to dashboard Cite

Context. Among late-type red giants, an interesting change occurs in the structure of the outer atmospheric layers as one moves to later spectral types in the Hertzsprung-Russell diagram: a chromosphere is always present, but the coronal emission diminishes and a cool massive wind steps in. Aims. Where most studies have focussed on short-wavelength observations, this article explores the influence of the chromosphere and the wind on long-wavelength photometric measurements. The goal of this study is to assess wether a set of standard near-infrared calibration sources are fiducial calibrators in the far-infrared, beyond 50 μm. Methods. The observational spectral energy distributions were compared with the theoretical model predictions for a sample of nine K-and M-giants. The discrepancies found are explained using basic models for flux emission originating in a chromosphere or an ionised wind. Results. For seven out of nine sample stars, a clear flux excess is detected at (sub)millimetre and/or centimetre wavelengths, while only observational upper limits are obtained for the other two. The precise start of the excess depends upon the star under consideration. For six sources the flux excess starts beyond 210 μm and they can be considered as fiducial calibrators for Herschel/PACS (60-210 μm). Out of this sample, four sources show no flux excess in the Herschel/SPIRE wavelength range (200-670 μm) and are good calibration sources for this instrument as well. The flux at wavelengths shorter than ∼1 mm is most likely dominated by an optically thick chromosphere, where an optically thick ionised wind is the main flux contributor at longer wavelengths. Conclusions. Although the optical to mid-infrared spectrum of the studied K-and M-type infrared standard stars is represented well by a radiative equilibrium atmospheric model, a chromosphere and/or ionised stellar wind at higher altitudes dominates the spectrum in the (sub)millimetre and centimetre wavelength ranges. The presence of a flux excess has implications on the role of the stars as fiducial spectrophotometric calibrators in these wavelength ranges.

show abstract

Section: Coronal Transition Region and Wind Dividing Linesmentioning

confidence: 99%

Structure of the outer layers of cool standard stars

Dehaes

Bauwens

Decin

et al. 2011

A&A

View full text Add to dashboard Cite

show abstract

“…The concept of dividing lines (DL) was introduced on the basis of early FUV observations by Linsky & Haisch (1979) and of soft X-ray observations by Ayres et al (1981) with the IUE and the Einstein observatory, respectively. These authors found a sharp division around mid K spectral type: hotter giants exhibit solar-like activity signatures with hot (10 5 −10 6 K) gas indicating the existence of chromospheres, transition regions and coronae, while cooler giants only show chromospheric temperatures around 10 4 K. Earlier, Reimers (1977) had found strong, cool outflows in giants cooler than mid K spectral type, which suggests a transition from hot coronae to massive, cool winds as a star crosses the DL region in the course of stellar evolution.…”

Section: Introductionmentioning

confidence: 99%

Buried magnetic flux tubes in giant stars near the "Coronal Dividing Line"

Holzwarth

Schüßler

2001

A&A

View full text Add to dashboard Cite

Abstract. We apply the "solar paradigm" for stellar magnetic activity to the post-main-sequence evolution of stars in the mass range 1 M ≤ M ≤ 3 M . The model starts from a strong toroidal magnetic field generated by a dynamo working in the overshoot layer below the convection envelope. Once a critical field strength is exceeded, an undulatory (Parker-type) instability leads to rising flux loops. Upon emergence at the stellar surface, they form bipolar magnetic regions and large-scale coronal loops. By considering the stability, dynamics, and rise of magnetic flux tubes along evolutionary sequences of stellar models, we find that the flux loops become trapped in the stellar interior when the depth of convective envelope exceeds about 80% of the stellar radius. Trapping is caused by an increase of field line curvature at the loop summit, so that eventually the magnetic tension force dominates over the buoyancy force. The magnetic loops find a stable equilibrium configuration within the convection zone and do not emerge at the stellar surface. The transition from emerging to trapped flux tubes falls in the range of spectral types G7 to K0 for luminosity class III giants, which is close to the observed "coronal dividing line" in the Hertzsprung-Russell diagram. This result is remarkably stable within large ranges of stellar parameters (mass, rotation) and flux tube parameters (field strength, magnetic flux) and depends practically exclusively on the fractional radius of the stellar radiative core. We suggest that flux tube trapping is the cause for the strong decline of stellar X-ray emission across the "coronal dividing line".

show abstract

“…1) showed evidence of neither ultraviolet ( UV ) radiation of C iv (T $ 10 5 K; Linsky & Haisch 1979) nor soft X-ray (T $ 10 6 K; Ayres et al 1981). Stencel & Mullan (1980) further pointed out that the stellar winds appear cool and massive after stars cross the DL.…”

mentioning

confidence: 99%

Structured Red Giant Winds with Magnetized Hot Bubbles and the Corona/Cool Wind Dividing Line

Suzuki

2007

ApJ

121

View full text Add to dashboard Cite

By performing MHD simulations, we investigate the mass loss of intermediate-and low-mass stars from MS to RGB phases. Alfvén waves, which are excited by the surface convections, travel outwardly and dissipate by nonlinear processes to accelerate and heat the stellar winds. We dynamically treat these processes in open magnetic field regions from the photospheres to '25 stellar radii. When the stars evolve to slightly blueward positions of the dividing line (Linsky & Haisch), the steady hot corona with temperature T % 10 6 K suddenly disappears. Instead, many hot ($10 6 K) and warm (k10 5 K) bubbles are formed in cool (T P 2 ; 10 4 K) chromospheric winds because of thermal instability; the red giant wind is not a steady stream but structured outflow. As a result, the mass-loss rates,Ṁ , largely vary in time by 3Y 4 orders of magnitude in the RGB stars. Supported by magnetic pressure, the density of hot bubbles can be kept low to reduce the radiative cooling and to maintain the high temperature a long time. Even in the stars redward of the dividing line, hot bubbles intermittently exist, and they can be sources of UV/soft X-ray emissions from hybrid stars. Nearly static regions are formed above the photospheres of the RGB stars, and the stellar winds are effectively accelerated from several stellar radii. Then, the wind velocity is much smaller than the surface escape speed because it is regulated by the slower escape speed at that location. We finally derive an equation that determinesṀ from the energetics of the simulated wave-driven winds in a forward manner. The relation explainsṀ from MS to RGB and can play a complementary role to the Reimers formula, which is mainly for more luminous stars.

show abstract

The cool half of the H-R diagram in soft X-rays

Cited by 124 publications

References 4 publications

Structure of the outer layers of cool standard stars

Structure of the outer layers of cool standard stars

Buried magnetic flux tubes in giant stars near the "Coronal Dividing Line"

Structured Red Giant Winds with Magnetized Hot Bubbles and the Corona/Cool Wind Dividing Line

Contact Info

Product

Resources

About