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SCIENCE DIRECT.

TlJV AND EUV OBSERVATIONS OF STELLAR CORONAL ST.RUCTURE AND ACTIVITY

Alexander Brown]

’ Center for Astrophysics and Space Astronomy (CASA), 389 UCB, University of Colorado, Boulder, CO 80309, USA

ABSTRACT

Ultraviolet (HST, FUSE) and extreme-ultraviolet (EUVE) observations of stellar coronae and flare-related activity are reviewed, with particular attention being devoted to the coronal information that is’obtainable in the ultraviolet but not from the X-ray region. The coronal temperature, density, and emission measure distributions of active stars are significantly different from quiescent solar coronal conditions, thus allow- ing broader study of the underlying physical processes. Stellar coronae are magnetically-confined plasmas typically with substantially higher heating rates and temperatures than encountered in the Quiet Sun. Conditions are similar to solar flares but the question of the flare contribution during “quiescence” is still unresolved. The properties of stellar flares observed in long EUVE monitoring observations and the rela- tionship between. events seen in different atmospheric regions are discussed. UV coronal forbidden lines, e.g. Fe XXI 135,4 A, :Fe XII 1242, 1349 A , Fe XVIII 975 A, and Fe XIX 1118 A are commonly detected in active dwarfs and giants; these lines currently provide the only resolved spectroscopic diagnostics of conditions within stellar coronae. The UV coronal lines generally have widths consistent with thermal broadening and are unshifted. Currently only tantalising glimpses of flare-related motions have been detected. 0 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

INTRODUCTION

While the bulk of coronal emission is radiated in the X-ray spectral region (as a combined emission line and co.ntinuum spectrum), the extreme-ultraviolet (100 5 X 5 400 A) and far-ultraviolet (912 5 X 5 2000 A) regions provide unique information on the physical conditions in stellar coronae that are otherwise currently unobservable. X-ray observatories, such as the Chandra X-ray Observatory and XMM-Newton, use transmission gratings to obtain spectra with resolutions (x/Ax) of 500 - 1000; X-ray spectra are thus limited to a resolution of N 300 km s-l and, as we shall see, these spectra do not resolve stellar coronal emission lines. In contrast, UV spectra obtained with the Goddard High Resolution Spectrograph (GHRS - Brandt et al., 1994) and Space Telescope Imaging Spectrograph (STIS - Woodgate et al., 1998) on the Hubble Space Telescope (HST), and with the Far Ultraviolet Spectroscopic Explorer (FUSE - Moos et al., 2000) provide significantly higher resolution spectra that can resolve the coronal forbidden emission lines accessible to these instruments. Typical resolutions for such observations are 7.5 km s-l for STIS-E140M, 15 km s-l for GHRS-GlGOM, and 30 km s-l for FUSE.

Over the period 1992 - 2001 the Extreme Ultraviolet Explorer (EUVE - Bowyer and Malina, 1991) satellite allowed access tso the 80 - 400 A spectral region for spectroscopic and photometric study of stellar coronal emission. Such studies were of great importance in establishing the scientific context for current on-going X-ray studies with Chandra and XMM-Newton. While X-ray spectroscopy is providing more detailed in- vestigations of stellar coronal structure and elemental abundances, there are other areas where these X-ray instruments are not able to supplant EUVE; in particular, the opportunities offered by EUVE for long monitoring observations are unlikely to be duplicated in the foreseeable future.

Adv. Space Res. Vol. 32, No. 6, pp. 977-984, 2003 0 2003 COX4R. Published by Elsevier Ltd. All rights reserved Printed in Gre,at Britain 0273-1177/$30.00 + 0.00

978 A. Brown

STELLAR FLARES

Flaring is inherent to the structure of stellar coronae and is likely to be an important factor in the heating of coronae. Magnetic reconnection events constantly rearrange the coronal magnetic structure, often on vast scales. Numerous authors have attempted to use observations of coronal variability to estimate how important the energy input from flares is for the overall heating of coronae. Studies of the importance of microflaring generally attempt to determine the slope of the flare energy distribution, a, where the number N and energy E of the flares are related by

&y&-“. ClE (1)

Even the most comprehensive existing studies have only sampled the uppermost portion of this distribution and must extrapolate by several orders of magnitude to reach the small reconnection events that might be associated with the majority of coronal heating. As the parameter a approaches a value of 2, flare heating becomes sufficient to be the basic coronal heating mechanism.

EUVE observations have provided a unique sampling of coronal variability, because typical observations lasted from several days to more than a month during the later stages of the satellite’s lifetime. Such long term monitoring has rarely been obtained by earlier X-ray satellites and is not likely to be feasible with the current generation of highly over-subscribed “Great” space observatories. The EUVE Deep Survey (DS) detector was sensitive to the wavelength range 80-150 A, which for most late type stars is dominated by coronal emission lines formed close to lo7 K. The DS light curves provide the most detailed records of coronal variations, because of the lower sensitivity of the EWE spectroscopic channels.

EUV Variability of Active G, K, and M Dwarfs The most extensive examination of EUV variability of active dwarfs is that by Audard et al. (ZOOO),

who studied EUVE DS data for ten dwarfs with spectral types between F and M. They found values of Q reasonably close to 2, particularly for the F and G stars. The detected flares represented - 10% of the energy needed to heat the coronae and the observed flare distributions would need to continue down to energies of order 10” ergs to provide the total energy needed to account for the observed coronal radiative losses.

One of the most valuable EUVE datasets for the study of coronal variability is the 45 day observation of the M3.5 flare star AD Leo analysed by Giidel et al. (2003). Stars like AD Leo have radii of N 0.4 solar radii, and thus their surface area is roughly 20% that of the Sun, yet their coronal luminosities are a factor of lo2 - lo3 larger. The DS lightcurve from this observation is shown in Figure 1. This observation is the longest, near-continuous, monitoring of a stellar corona and illustrates the continuous flaring and variability that is fundamental characteristic of stellar coronae. Flares on dwarf stars have relatively short durations (typically a few hours), when compared to those seen on active binaries and giant stars. Flares were detected with a wide range of range of flare energy. Giidel et al. provide a detailed analysis of the flare energy distribution using a variety of statistical methods that all result in values for a of 2.1-2.3 with an uncertainty of no more than 0.1. Therefore, if this distribution does extend to sufficiently small energies, it seems entirely possible that the coronal heating of stars like AD Leo may be entirely due to flaring and the associated stochastic magnetic reconnection events.

EUV Variability of RS CVn Binaries RS CVn binaries, whose orbital and rotational periods are typically synchronised by tidal forces, are

among the brightest late-type stars in the extreme ultraviolet. EUVE spent a considerable time observing these short-period (0.7 - 20 days) binaries, often with multiple observations. Variability within the EUVE DS observations of 16 RS CVn binary systems was studied systematically by Osten and Brown (1999). These RS CVn binaries showed large (log E = 33.5 -35.5 ergs), long duration (6 - 85 hours), frequent (0.1 - 1.5 Aares day-‘) flares in the EUV. These flares are pervasive and flaring was present for 40% of the time. Thus, flaring is a normal state for short-period binaries. The flare energy distribution for this heterogeneous sample had a slope a! N 1.6.

Spectroscopic EUVE data are less useful for flare studies because of the poorer time resolution inherent in the measurement of individual emission lines. Some flares did show spectral changes associated with the

UV and EUV Observations of Stellar Coronae

112’70 11280 11290 11300 11310 time (HJD - 2440000.5)

Fig. 1. 1.5 months in the life of the flare star AD Leo - the EWE Deep Survey (DS) light curve of AD Leo obtained1 by Giidel et al. (2003) between 1999 April 2 and May 16. This observation is the longest, near continuous, monit:oring of a stellar corona. The final segment of the observation (labeled V) has a lower count rate because the star was located close to an area of lower detector sensitivity - the DS “dead spot”.

0 10 20 30

1 [? Cet [1994]

Fig. 2. EUVE Deep Survey (DS) detector light curve for a 34 day observation of ,0 Ceti (KO Ill) from Ayres, Osten, and Brown (2001). The shaded areas mark different temporal ranges used by Ayres et al. for their variability analysis. Two (lower) insets show the earlier EWE light curves of p Cet and p Vel using the same elapsed time scaliing for direct comparison.

980 A. Brown

heating of the flare plasma, such as the flares observed on the 1.14 day binary g2 CrB (F6 V + GOV) where Fe XXIV emission lines were detected only during flares (&ten et al., 2000). However, the more common effect was an overall increase in the strength of the complete spectrum during large flares without any strong changes in the spectral distribution, implying that the peak flare temperature is typically significantly higher (i.e. 2 60 - 70106 K) th an the formation temperature of Fe XXIV, which is N 16106 K.

EUV Variability of Active Giants While coronal flaring from dMe stars and active binaries was expected, EUVE observations of evolved

single giant stars were not expected to show highly variable coronal emission. Discovery of a flare from the G6 giant p Vel with EUVE by Ayres, Osten, and Brown (1999) was therefore somewhat surprising. A subsequent month-long observation of the KO giant ,0 Cet showed even more remarkable variability (Ayres, Osten, and Brown, 2001). The coronal variations of these two giants are shown in Figure 2. The longer flare events on p Cet last over six days. Occurrence of such flares on helium-burning giants, which have already ascended the red giant branch once, is very intriguing, because it suggests a more vigorous rekindling of coronal heating than previously thought and indicates that angular momentum loss associated with flare mass ejections might be more severe than would be expected from a low velocity red giant wind alone.

Flares Seen Only in the Ultraviolet A number of studies have used UV observations to search for flare-related phenomena that might give

information of flaring rates and dynamics. Ayres et al. (2001) studied simultaneous UV (HST/STIS) and X-ray (Chandra) observations of the RS

CVn binary HR1099 and found two flares in UV emission lines (Si IV, C IV) formed at N lo5 K with durations of tens of minutes that showed no coronal counterparts. No response was seen in either the Chandra X- ray spectrum or the Fe XXI flux recorded by STIS. Any coronal (2 lo6 K) plasma associated with these flares should have been readily detectable in either the X-ray emission line or continuum spectrum. Coronal plasma at temperatures too hot to be seen in Fe XXI are prodigious continuum emitters. The simplest interpretation is that the peak temperature of these flares was less than lo6 K. The Fe XXI emission did show a small (20 km s-l) blue-shifted displacement during one of the flares, but is not certain if these are directly related.

The existence of “UV-only” flares is perhaps not surprising given the occurrence of solar transition region explosive events (see Dere, Bartoe, and Brueckner, 1989), which may be very analogous. HST spectra have been used to study stellar transition region (TR) emission line profiles in detail and all active dwarf stars are found to show excess high velocity emission, so called “broad components” (Linsky and Wood, 1994), that implies TR plasma motions of N 100-200 km s-r. The high velocity emission is symmetric and typically contributes 25-50% of the line flux (Wood, Linsky, and Ayres, 1997).

Flares in UV emission lines and the UV continuum have been investigated by a number of authors, although it is unclear how direct a connection can be made between the variability seen in the lo4 - lo5 K plasma and any associated changes in the corona. Rapid UV variations on seconds to minutes time- scales have been detected on dMe flare stars using the HST High Speed Photometer (Robinson et al., 1995, 1999). Similar flaring in emission lines was seen in STIS spectra of the M dwarf AU Mic (Robinson, Linsky, Woodgate, and Timothy, 2001).

CORONAL FORBIDDEN LINES

Coronal forbidden lines have been detected from a wide range of stars, and observation of these lines with HST and FUSE is presently the only way to resolve the profiles of stellar coronal emission lines.

The first detection was of the Fe XXI 1354 A line from the dMe flare star AU Mic using the HST/GHRS (Maran et al, 1994), with subsequent confirmation in the active binary HR 1099 (Robinson et al., 1996). For less active stars there can be problems from blending with nearby C I lines (see Figure 3). However, the Fe XXI line is the strongest forbidden line and has been detected from over 20 stars in an HST/STIS survey (Ayres et al., 2003). Fe XXI emission is present in the spectra of F, G, K, and M dwarfs and active F-G giants. Jordan et al. (2001) also detected the Fe XII 1242 and 1349 A forbidden lines in the STIS spectrum of the K dwarf e Eri. The Fe XXI line fluxes correlates extremely well with the soft X-ray luminosities of the

UV and EUV Observations of Stellar Coronae 981

i 4 i

UJ

M

i 0 - V

42

’

1200 1200 1300 1300 1400 1400 1500 1500 1600 1600 1700 1700

WAVELENGTH (ii)

Fig. 3. The far-UV spectrum of the flare star AD Leo showing the typical spectral context of stellar Fe XXI 1354 A coronal forbidden line observations (from Ayres et al., 2003). Coronal forbidden lines are intrinsically weak and require deep, high signal-to-noise spectra for detailed study. Careful spectral analysis is required to accurately account for potential blending by other emission lines.

same stars; the slope of the correlation is close to unity, which is to be expected because both diagnostics are formed close to IO7 K and are presumably emitted by the same coronal plasma. This correlation validates the use of the Pe XXI as a direct proxy of the overall coronal behaviour. The correlation for the Fe XII ‘emission is :much. shallower, presumably because of its significantly lower (- lo6 K) formation temperature. Ayres et al. (20103) searched unsuccessfully for a wide range of other Ne, Fe, Si, S, Ar, and Ca forbidden lines. At shorter wavelengths the FUSE satellite has detected the Fe XVIII 974 A coronal line. This line was first detected in the spectrum of the binary Capella (o Aur) by Young et al. (2001) and subsequently found in the spectra of ten other stars in the survey of Redfield et al. (2002). Redfield et al. also detected weak Fe XIX 1118 A emission from these same stars.

Ayres et al. (2003) found that a common profile fitted their ensemble of Fe XXI profiles remarkably well (see Figure 4). This fitted profile is only slightly broader (full width half maximum N 110 km s-l) than the thermal broadening from a lo7 K plasma, which is near 90 km s-l. The profiles show only a negligible shift relative to the stellar rest velocities and also are symmetric about the rest velocity; clearly the average coronal plasma is not incredibly dynamic over and above the inherent thermal motions. This is further confirma,tion that stellar coronal plasma arises predominantly from confined, presumably magnetic, structures. The full set of STIS spectra are shown in Figure 5.

Interestingly, there is suggestive evidence for excess rotational broadening of the Fe XXI line for two rapidly-rotating early G giants (31 Comae and the Gl III component of Capella) and for the Kl IV primary of the RS CVn binary HR 1099; if this were true, the coronae of these stars would have to be significantly extended. The e:xcess broadening for the G giants, if solely due to solid body rotation, would imply coronal extents of roughly one stellar radius.

982 A.Brown

8 Cet

I Cap

p Vel

24 UMa

HR 9024

AD Leo

I-- AUMic I

VELOCITY (km 8)

Fig. 4. A superposition of the Fe XXI 1354 A profiles from seven stars in velocity space (from Ayres et al.,2003). Note the remarkable similarity of these profiles, even though the stars involved are a mixed groups of active giants, and dMe flare stars. A common profile, shown as a solid band, can be fitted well to this ensemble of profiles. This fitted profile is slightly broader than the thermal broadening from a lo7 K plasma.

SUMMARY

This review has highlighted two important aspects of stellar coronal studies in the EUV and FUV spectral regions that demonstrate the important opportunities that spectral diagnostics in these regions offer.

o EWE has left an amazing legacy of data that detail stellar coronal variability on time scales (days - weeks) that were previously impossible to study. Studies of the flare distributions within these data strongly support the idea that flares are an important, indeed perhaps dominant, source of coronal heating for active stars. Further progress on this topic requires new longer observations with higher sensitivity X-ray/EUV detectors; perhaps these are obtainable from sufficiently large time allocations on the XMM observatory, which has the advantage of providing days-long uninterrupted monitoring of X-ray emission. A complete picture of the flare process requires simultaneous multi-spectral-region observations, so the energetic and dynamical response of the whole stellar atmosphere can be evaluated.

* Currently FUV coronal forbidden lines are the only viable tool for the study of stellar coronal dynamical processes, using currently operational space-based observatories. The dynamical energy losses through coronal mass ejections and flare plasma acceleration are currently very poorly known for stars. These processes are potentially measurable in the X-ray and EUV spectral regions, but new space-based spectrographs would be needed with a spectral resolution of better than 3000 (ideally 9000) - no such instruments are currently approved. Existing coronal forbidden line profiles show that typically the coronal plasma shows little additional broadening beyond that expected from inherent thermal motions, unlike the behavior of cooler transition region emission lines that show significant ubiquitous excess broadening in “quiescent” spectra.

UV and EUV Observations of Stellar Coronae 983

G8I GOI GOI K2 II K5 III Kl III KO III G8 III G8 III Gl III G5 III G4 III Gl III GO III GO III F8 III Kl IV M5V M3V M3V MOV K2V KOV G8V G8V G5V G2V GOV F7V

L-1 I’I’III 1 I II 1 I IIIlI 111 134iB 1350 1352 1354 1356 1358 1360

WAVELENGTH (A)

Fig. 5. A mlontage of STIS far UV spectra in the wavelength range containing the Fe XXI 1354 A and Fe XII 1349 A coronal forbidden lines for 28 stars in the Ayres et al. (2003) survey. Fe XXI is clearly detected from dMe flare stars, active {giants, active short-period binaries, and the most active solar-like dwarfs.

ACKNOW’LELIGEMENTS

This work was Impported by NASA grants NAG5-3226 and NAG5-12233 and HST grant GO-08280.01-97A to the University of Colorado.

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E-mail address of A. Brown:- ab@casa.colorado.edu

Manuscript received 3 December 2002; accepted 7 February 2003