Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://hea-www.harvard.edu/PINTofALE/papers/kgd_MM_head32_4217.ps.gz Äàòà èçìåíåíèÿ: Sat Nov 4 00:35:10 2000 Äàòà èíäåêñèðîâàíèÿ: Sat Dec 22 02:18:30 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: rainbow

GJ 205
Other Names HD 36395 / Wolf 1453
(RA, Dec) 2000 (05:31:24.85, ­03:38:53.4)
(l II b II ) (206.93, ­19.45)
Spectral Type dM1.5
Distance 5.7 pc
m V 7 m :98
B \Gamma V 1.5
[m=H] 0.5
ROSAT/PSPC 0.19 cts s \Gamma1
EUVE/DS 0.02 cts s \Gamma1
EUVE Observation Jan/Feb 1997 [394.1 ks]
GJ 411
Other Names HD 95735 / SAO 62377
(RA, Dec) 2000 (11:03:22.58, +36:02:09.6)
(l II b II ) (185.12, +65.43)
Spectral Type dM2
Distance 2.55 pc
m V 7 m :49
B \Gamma V 1.51
[m=H ­0.35
ROSAT/PSPC 0.18 cts s \Gamma1
EUVE/DS 0.02 cts s \Gamma1
EUVE Observation Mar 1995 [338.7 ks]
Emission due to lines of different species. The contributions
to the line spectrum from various species are shown separated out (in
different colors) and normalized. The line strengths were computed
using a solar­type Active Region DEM and a Quiet Sun DEM (Brosius
et al. 1996, Kashyap & Drake 1998), but abundances are rescaled such
that the strongest lines of each species in the EUVE/SW passband
all have the same intensity. Comparing this figure with the observed
spectrum of GJ 205, we can rule out the identification of the feature
at 118 š A with any lines of Fe, O, Ne, Ni, etc: any contribution to this
feature by these species results in a larger contribution elsewhere in the
spectrum, which are not observed. Only a blend of transition­region
lines of O VI and Si V at peak temperatures of 2 \Gamma 3 \Theta 10 5 K, for
a Quiet Sun type DEM, are left as possible contributors. However,
this requires apparently implausibly large abundances of Si, with
[Si/Fe] ?? 10 compared with Solar abundances.
ABSTRACT
Dwarf M stars are important probes for coronal heating and dynamo
theories: they span a region of the H­R diagram where stars are not
yet fully convective, and classical dynamo theories are thought to be
applicable, and contributions from non­magnetic heating processes
are presumably minimized. Further, unlike dMe flare stars, coronal
emission on dM stars is more likely to be dominated by steady
structures. Unfortunately, despite the large numbers of dM stars in
the Galaxy, their low fluxes make them hard to observe and analyze.
Here we present deep EUVE/SW spectrometer observations of two
inactive dM stars, GJ 411 and GJ 205. The former is metal­deficient,
while the latter appears to be metal­rich (Mould 1978; ApJ 226, 923).
Their spectra are remarkably unlike ``normal'' stellar coronal spectra.
We find lines of Ar, O, and possibly Si in the spectrum of GJ 205; no
lines from Ni or Fe could be identified. No feature in GJ 411 could be
conclusively identified.
The spectra from these dM stars are consistent with being produced
by a DEM of the same type as the Quiet Sun.
ANALYSIS
We analyzed 300+ ksec of EUVE/SW data on both stars, at full
resolution as well as rebinned to bin sizes of 0.3 š A.
We derived the background correction by including the effect of the
gradient in the cross­dispersion direction, and estimated the most
probable values of the source photons.
We then compared the derived spectra with line lists from CHIANTI
(v3) using PINTofALE (Kashyap & Drake 2000; BASI 28, 475;
Kashyap & Drake 2000, HEAD 27.05), for various metallicities
based on the standard abundances of Anders & Grevesse (1989,
Geo.Cos.Acta 53, 197), and adopting the ion balance calculations of
Arnaud & Raymond (1992, ApJ 398, 39).
We model these spectra with line fluxes and continuum emission
calculated using Differential Emission Measures (DEM) mimicing
Solar Active Regions and the Quiet Sun (Brosius et al. 1996 [ApJS
106, 143], Kashyap & Drake 1998 [ApJ 503, 450]).
The spectrum of GJ 205 shows a strong line blend feature at –118.
Note also the presence of statistically significant features coincident
with lines of O VII –136, Ar XII –154, and Ar XIII –165. No
lines from Fe and Ni could be positively identified, generally due to
the absence of accompanying lines. The spectrum is consistent with a
DEM of the same type as that of the Quiet Sun.
The spectrum of GJ 411 does not have identifiable features, and is
consistent with a metal­deficient corona with a Quiet Sun type DEM,
with enhanced emission measure at transition­region temperatures.
CONCLUSIONS
ffl The spectrum of GJ 411 is consistent with a metal­deficient corona
where the line fluxes are too low to be detected with EUVE.
While the data are insufficient to distinguish between an Active
Region type DEM and a Quiet Sun type DEM, the flatness of
the spectrum indicates possibly high Emission Measure at low­T
(which could be due to acoustic wave heating -- see Mullan &
Cheng [1994, ApJ 420, 392]), while ROSAT/PSPC data show
that the Emission Measure at T ? 10 6:5 K is very low: hence, an
Active Region type DEM may be ruled out.
ffl In the case of GJ 205 also, an Active Region type DEM may
be ruled out based on the observed lack of lines from highly
ionized species. The spectrum is inconsistent with emission
from a low­metallicity plasma. The observed feature at 118
š A may be explained as a blend of O VI and Si V lines, but
requires an improbably high [Si/Fe]?? 10. These abundance
measurements are subject to uncertainties in the DEM, which has
a minimum at log 10 T¸ 5:5. No Fe lines could be identified in the
spectrum, which suggests that the corona is depleted in Fe; this
is contrary to IR studies which suggest that [Fe/H]¸ 0:5 in the
photosphere. Ar also appears to be surprisingly overabundant,
with [Ar/Fe]? 10.
Acknowledgements: VK acknowledges support from AURA­C10255A, NASA
grants NAG5­3173, NAG5­6775, NAG5­3831, NAG5­3196, NAG5­7226 and
NAG5­3195. JJD was supported by NASA contract NAS8­39073 to the
Chandra X­ray Center
Comparison with continuum emission: Spectra are shown
rebinned by 5\Theta (black) in order to enhance features. Overlaid on it
are continuum spectra normalized to have the same counts as the
data, derived from 2 types of Differential Emission Measure (DEM)
curves (from Solar Active Region and Quiet Sun data; cf. Brosius et al.
1996 [ApJS 106, 143], Kashyap & Drake 1998 [ApJ 503, 450]), as well
as ffi­function Emission Measures at log 10 (T)=(5.3,5.5,5.8,6.1). This
explores the hypothesis that the reason for the lack of line emission
is that the coronal metallicity is very low. In the case of GJ 205, it
is clear that no combination of temperatures is sufficient to explain
the observed spectrum as due to low­metallicity plasma emission,
whereas a DEM with a sufficiently high transition­region component
may account for the featureless SW spectrum of GJ 411.
Observed EUVE/SW spectra: The background­corrected
spectra are shown at high­resolution (black curve). Overlaid (in red)
is the adaptively smoothed form (such that S=N ? 7; this smoothing
does not conserve flux).
Vinay Kashyap (SAO), Mark Giampapa (NOAO), Jeremy Drake (SAO)
42.17: The Coronae of Inactive Low-Mass Dwarfs