Thomas R. Ayres

A solar photospheric thermal profiling analysis is presented, exploiting the infrared (2.3-4.6 μm) rovibrational bands of carbon monoxide (CO) as observed with the McMath-Pierce Fourier transform spectrometer (FTS) at Kitt Peak, and from above the Earth's atmosphere by the Shuttle-borne ATMOS experiment. Visible continuum intensities and center-limb behavior constrained the temperature profile of the deep photosphere, while CO center-limb behavior defined the thermal structure at higher altitudes. The oxygen abundance was self-consistently determined from weak CO absorptions (for C/O ≡ 0.5). Our analysis was meant to complement recent studies based on three-dimensional (3D) convection models, which, among other things, have revised the historical solar oxygen (and carbon) abundance downward by a factor of nearly 2, although in fact our conclusions do not support such a revision. Based on various considerations, an O = 700 ± 100 ppm (parts per million relative to hydrogen) is recommended; the large uncertainty reflects the model sensitivity of CO. New solar isotopic ratios also are reported: 12C/13C = 80 ± 1, 16O/17O = 1700 ± 220, and 16O/18O = 440 ± 6—all significantly lower than terrestrial. CO synthesis experiments utilizing a stripped down version of the 3D model—which has large temperature fluctuations in the middle photosphere, possibly inconsistent with CO movies from the Infrared Imaging Spectrometer (IRIS), and a steeper mean temperature gradient than matches visible continuum center-limb measurements—point to a lower oxygen abundance (~500 ppm) and isotopic ratios closer to terrestrial. A low oxygen abundance from CO—and other molecules like OH—thus hinges on the reality of the theoretically predicted midphotospheric convective properties.

A solar photospheric "thermal profiling" analysis is presented, exploiting the infrared rovibrational bands of carbon monoxide (CO) as observed with the McMath-Pierce Fourier transform spectrometer (FTS) at Kitt Peak, and from above the Earth's atmosphere by the Shuttle-borne ATMOS experiment. Visible continuum intensities and center-limb behavior constrained the temperature profile of the deep photosphere, while CO center-limb behavior defined the thermal structure at higher altitudes. The oxygen abundance was self consistently determined from weak CO absorptions. Our analysis was meant to complement recent studies based on 3-D convection models which, among other things, have revised the historical solar oxygen (and carbon) abundance downward by a factor of nearly two; although in fact our conclusions do not support such a revision. Based on various considerations, an oxygen abundance of 700+/-100 ppm (parts per million relative to hydrogen) is recommended; the large uncertainty reflects the model sensitivity of CO. New solar isotopic ratios also are reported for 13C, 17O, and 18O.

The late-type giants and supergiants of the "hybrid chromosphere" class display signatures of cool (T 2 × 10⁴ K) winds together with hot emission lines from species like C IV (T ~ 10⁵ K). A survey of such stars by Reimers et al. using ROSAT reported numerous X-ray detections (T ~ 10⁶ K), strengthening the (then heretical) idea that hot coronae and cool winds can coexist in luminous giants. However, several of the candidate sources were offset from the predicted stellar coordinates, calling into question the identifications. In an effort to secure better knowledge of the X-ray luminosities of the hybrids, the ROSAT fields from the Reimers et al. survey were reexamined, exploiting the USNO-A2.0 astrometric catalog to register the pointings to a few arcseconds accuracy. On the basis of positional mismatches, at least two of the previously reported detections of key hybrid stars—γ Dra (K5 III) and β Aqr (G0 Ib)—must be rejected. The new X-ray upper limits for these stars, combined with the remaining candidate detections (and nondetections) from the original survey, place the hybrids into the same "X-ray deficient" category as the "noncoronal" red giants like Arcturus (α Boo: K1.5 III) and Aldebaran (α Tau: K5 III). A few of the hybrid X-ray sources are exceptional, however. The archetype α TrA (K2 II-III), in particular, is securely detected in terms of positional coincidence, but its anomalous, contradictory coronal properties suggest that an unseen companion—a young hyperactive G dwarf—might dominate the X-ray emission.

We investigate the coronal structure of rapidly-rotating, solar-like stars using Chandra HETGS spectra of the short-period binary ER Vul, and by comparison with X-ray observations of the Sun and other dwarf stars. ER Vul consists of two solar-like (G0 + G5) dwarfs with rotation rates ~ 40 times that of the Sun. This binary is not interacting and these stars are the fastest rotating G dwarfs suitable for high resolution X-ray spectroscopy. X-ray (1.8-40 Å) spectra were obtained on 2001 March 29-30 along with 10.5 hours of simultaneous VLA monitoring at 3.6 and 20 cm. These spectra show hot, multi-temperature coronal emission with emission lines ranging in temperature from O VII (2 MK) to Fe XXIV (30 MK). ER Vul showed only low-level variability during the X-ray observation. Unlike the behaviour of longer period active binaries, no large, long-duration flares were detected, consistent with previous X-ray observations of this binary. No evidence for eclipses is seen in either the X-ray or radio emission. The coronal emission measure distribution and elemental abundances were derived for ER Vul.

A 34 day Extreme Ultraviolet Explorer (EUVE) pointing on the "clump" giant b Ceti (HD 4128; K0 III) recorded a series of striking coronal flare events, reminiscent of a singular outburst seen previously from m Velorum (HD 93497; G6 III ϩ dF). The recent flaring episode contrasts with a more placid behavior in a 6 day EUVE observation of b Cet 6 years earlier. The average 70-180 A ˚Deep Survey count rate in the new observation is twice as high, and the 75-150 A ˚spectrum displays a distinct hardening. The discovery of sustained flare activity on b Cet raises the possibility that such episodes are more common than suspected among the core helium-burning giants and sharpens the puzzle of the survival of magnetic activity beyond helium flash.

We have obtained long-slit far-ultraviolet (1150-1730 Å) spectra of the R Coronae Borealis (RCB) stars V854 Cen and RY Sgr near maximum light and pulsational phase zero with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The far-UV spectrum of each star shows a photospheric continuum rising steeply toward longer wavelengths and a prominent emission feature at C II λ1335. RY Sgr displays a second, but fainter, emission attributed to Cl I λ1351 (which is radiatively fluoresced by C II λ1335), but Cl I is weak or absent in V854 Cen. Most surprisingly, the C II emission of V854 Cen is significantly extended along the slit by ±25, about 6 × 10³ AU at the distance of the star. The C II feature of RY Sgr exhibits no such gross extension. Nevertheless, subtle broadenings of the C II emissions beyond the point response profile suggest inner clouds of radius ~01 (250 AU) around both stars. V854 Cen is only the third RCB star after R CrB and UW Cen known to have a resolved shell.

We have obtained long-slit far-ultraviolet (1150--1730 A) spectra of the R Coronae Borealis (RCB) stars V854 Cen and RY Sgr, near maximum light and pulsational phase zero, with the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). The far-UV spectrum of each star shows a photospheric continuum rising steeply toward longer wavelengths, and a prominent emission feature at C II 1335. RY Sgr displays a second, but fainter, emission attributed to Cl I 1351 (which is radiatively fluoresced by C II 1335), but Cl I is weak or absent in V854 Cen. Most surprisingly, the C II emission of V854 Cen is significantly extended along the slit by +/- 2.5 arcsec, about 6 x 10^3 AU at the distance of the star. The C II feature of RY Sgr exhibits no such gross extension. Nevertheless, subtle broadenings of the C II emissions beyond the point response profile suggests inner clouds of radius \~0.1 arcsec (250 AU) around both stars. V854 Cen is only the third RCB star after R CrB and UW Cen known to have a resolved shell.