AAS 200th meeting, Albuquerque, NM, June 2002
Session 74. Stellar Atmospheres and Circumstellar Material
Display, Thursday, June 6, 2002, 9:20am-4:00pm, SW Exhibit Hall

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[74.19] Chandra~HETGS Observes Tortured Coronae in the Rapid Braking Zone

T.R. Ayres, R.A. Osten, A. Brown (CASA), M. Gagne (West Chester Univ.), J.L. Linsky (JILA)

\noindent We have obtained {\em Chandra}\/ High Energy Transmission Grating Spectrometer observations of five moderate mass (2--3~M\odot) giants straddling the portion of the Hertzsprung gap where early-G~III stars---evolving rapidly toward the red giant branch---suffer strong rotational braking and dramatic changes in their X-ray emitting coronae. G0~III giants prior to the braking epoch are fast rotators (\upsilon\rm rot~50-100~km s-1) and display very hot (T> 107~K) coronae, but nevertheless have curiously depressed X-ray luminosities. The post-braking giants are slow rotators (\upsilon\rm rot< 10~km s-1) with cooler coronae (T~106.8~K), but nevertheless manage a healthy level of X-ray emission. We believe the differences reflect the violent replacement of a ``fossil'' magnetosphere---inherited from the late-B or early-A MS progenitor---by a solar-like regenerative magnetic dynamo. The latter becomes dominant when the initially shallow surface convection in yellow giants at the blue edge of the Hertzsprung gap gives way to deep convective layers as the stars evolve to the red edge.

Three of the targets were observed in Cycle~2: 31~Com (G0~III) on 2001-03-12 [132.0~ks]; HR~9024 (G1~III) on 2001-08-11 [96.9 ks]; and \mu~Vel (G5~III) on 2001-09-24 [19.9~ks], 2001-10-29 [58.1~ks], and 2001-12-18 [57.7~ks]. (The first \mu~Vel observation was scheduled for 80~ks, but was cut short by a solar flare. The second pointing was intended to complete the exposure, but was affected by ``threshold crossing plane'' latchup in the ACIS CCDs, and was repeated two months later, accounting for the third pointing.) The remaining two stars are: Cycle~3 target 24~UMa (G4 III; ~50~ks pointings on 2002-03-26 and 2002-03-29); and GTO target \beta~Ceti (K0 III) observed on 2001-06-29 [87.5~ks].

We describe the HETGS spectra and our efforts to infer plasma conditions (temperature/density models), chemical fractionation, gas dynamics (through emission line Doppler shifts), and coronal variability.\\[-3mm]

\noindent This work was supported by {\em Chandra}\/ grant GO1-2018X to the University of Colorado.

The author(s) of this abstract have provided an email address for comments about the abstract: ayres@casa.colorado.edu

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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.