1 Introduction

Magnetized hot outer atmospheres (coronae) are ubiquitous in cool stars (spectral classes F-M). The particular example of the solar corona has revealed rich details on coronal structures, thermal stratification, abundance patterns, and the physics of heating and mass motion. Nevertheless, in many other stars coronal phenomena not common to the Sun are regularly observed, such as persistent high-density ( $n_{\rm e}>10^{10}$ cm^-3) coronal plasmas, persistent very hot gas (T>10 MK), or abundances at variance with solar values (Bowyer et al. 2000). The Sun's relatively modest magnetic activity is representative for a particular evolutionary state of a $1~M_{\odot}$star (Güdel et al. 1997), while most stellar objects regularly observed by X-ray satellites belong either to the more abundant low-mass classes with some exceptional magnetic activity (M dwarfs and young K dwarfs) or to tidally coupled binary systems with strongly enhanced magnetic dynamos (RS CVn or Algol-type binaries).

Figure 1: From top to bottom the spectra of Procyon observed by RGS1, RGS2 and LETGS in the wavelength region from 10 to 37 Å.

Figure 2: The Procyon spectrum observed by LETGS in the region 36-174 Å.

High-resolution X-ray spectroscopy of such stellar systems now available from Chandra and XMM-Newton (Brinkman et al. 2000, 2001) has revealed coronal features clearly at variance with solar phenomena. However, to translate solar knowledge to stellar environments, it is important to study stars that are relatively similar to the Sun. Given the low X-ray luminosity of such stars, there are only a few in the solar neighborhood accessible to high-resolution X-ray spectroscopy. We present here a detailed analysis of the X-ray spectrum of Procyon, a nearby bright X-ray source with a coronal plasma of about 1-3 MK, exhibiting a cooler X-ray spectrum than magnetically active stars that have predominantly been studied so far with XMM-Newton and Chandra (Audard et al. 2001a,b; Güdel et al. 2001a,b; Mewe et al. 2001). The late-type (F5 IV-V) optically bright ( $m_{\rm v}=0.34$) star Procyon (with a faint white dwarf companion) at a distance of 3.5 pc has a line-rich coronal spectrum in the X-ray region. The mass of Procyon is $1.75~M_\odot$ and its radius $2.1~R_\odot$ (Irwin et al. 1992). The high-resolution spectrum of Procyon has been studied earlier using EUVE (Drake et al. 1995; Schrijver et al. 1995; Schmitt et al. 1996) and by means of the LETGS on board Chandra (Ness et al. 2001). Ness et al. focussed their efforts on the density-sensitive and temperature-sensitive lines of C V, N VI, and O VII only. Here we present an extended investigation of the LETGS spectrum covering the total spectral range from 5-175 Å together with the analysis of the RGS spectra from 5-37 Å. In Sect. 2 we describe the observations. Section 3 (Analysis) is divided into a part on global fitting (3.2) based on the total spectrum and a part that contains consistency checks based on individual line measurements (3.3).

 

 

Table 1: Wavelengths and fluxes for RGS1, RGS2, and LETGS, together with the line identifications from MEKAL and KELLY.

RGS1		RGS2		LETGS		Line identificationsa
$\lambda$(Å)	fluxb	$\lambda$(Å)	fluxb	$\lambda$(Å)	fluxb	MEKAL	fluxc	KELLY	Ion
gap	-	13.454(9)	0.17(4)	13.450(13)	0.14(5)	13.448	0.14	13.447	Ne IX
gap	-	13.701(6)	0.17(4)	13.690(15)	0.12(5)	13.700	0.10	13.700	Ne IX
15.018(7)	0.20(5)	15.024(9)	0.26(4)	15.015(18)	0.21(6)	15.014	0.16	15.013	Fe XVIId
15.176(13)	0.12(3)	15.161(21)	0.08(3)	-	-	15.175	0.03	15.176	O VIII
-	-	-	-	15.207(28)	0.13(5)	-	0.09	-	Fe XVIIe
15.281(12)	0.11(3)	15.258(12)	0.09(5)	-	-	15.265	0.04	15.260	Fe XVII
16.008(3)	0.17(3)	16.013(9)	0.20(5)	16.008(9)	0.18(5)	16.003	0.15	16.007	O VIIIf
16.776(9)	0.16(3)	16.776(15)	0.12(4)	16.790(14)	0.13(5)	16.780	0.10	16.775	Fe XVII
17.047(9)	0.23(4)	17.044(14)	0.19(5)	17.054(12)	0.22(9)	17.055	0.12	17.051	Fe XVII
-	-	17.099(12)	0.25(5)	17.102(9)	0.29(9)	17.100	0.10	17.100	Fe XVII
17.402(12)	0.07(3)	17.402(28)	0.06(3)	17.396(17)	0.08(5)	17.380	0.04	17.396	O VII
17.765(9)	0.10(3)	17.772(9)	0.11(4)	17.769(13)	0.14(5)	17.770	0.12	17.768	O VII
18.624(4)	0.30(4)	18.637(6)	0.37(5)	18.629(5)	0.39(8)	18.628	0.39	18.627	O VII
18.970(2)	1.61(8)	18.975(2)	1.78(11)	18.972(2)	1.83(15)	18.973	1.93	18.969	O VIII
20.918(27)	0.04(3)	gap	-	20.905(22)	0.14(7)	20.910	0.06	20.910	N VII
21.596(2)	2.36(11)	gap	-	21.597(2)	3.01(25)	21.602	3.35	21.602	O VII
21.797(4)	0.53(6)	gap	-	21.792(5)	0.90(14)	21.800	0.80	21.804	O VII
22.098(2)	2.20(11)	gap	-	22.089(2)	2.57(23)	22.100	2.33	22.101	O VII
-	-	24.780(3)	0.88(7)	24.790(4)	0.80(14)	24.781	0.76	24.781	N VII
-	-	24.907(22)	0.08(4)	24.906(11)	0.18(9)	24.900	0.08	24.898	N VI
27.001(11)	0.16(4)	26.994(12)	0.14(4)	26.979(19)	0.18(9)	26.990	0.08	26.990	C VI
28.460(7)	0.30(5)	28.465(6)	0.39(6)	28.470(6)	0.49(12)	28.470	0.39	28.466	C VI
28.785(4)	0.69(9)	28.775(6)	0.71(8)	28.785(5)	0.88(15)	28.790	0.77	28.787	N VI
29.078(9)	0.24(5)	29.084(9)	0.29(7)	29.082(12)	0.29(10)	29.090	0.32	29.084	N VI
29.524(6)	0.43(6)	29.520(8)	0.38(6)	29.546(11)	0.43(13)	29.530	0.41	29.534	N VI
30.445(12)	0.20(5)	30.446(13)	0.22(5)	30.450(25)	0.18(11)	30.448	0.22	30.448	Ca XI
31.027(16)	0.18(5)	31.021(12)	0.15(5)	31.054(15)	0.22(10)	31.015	0.19	31.015	Si XII
-	-	33.490(26)	0.15(7)	33.510(18)	0.17(10)	-	-	-	-
33.724(2)	3.49(17)	33.726(2)	4.15(30)	33.731(2)	4.02(32)	33.736	4.64	33.736	C VI
34.967(12)	0.19(6)	34.962(15)	0.17(7)	34.959(15)	0.27(17)	34.970	0.22	34.973	C V
35.198(30)	0.13(6)	35.193(12)	0.27(8)	35.188(18)	0.29(15)	35.212	0.12	35.212	Ca XI
35.566(16)	0.15(7)	35.562(12)	0.26(7)	35.566(16)	0.23(14)	35.576	0.27	35.576	Ca XI
35.682(8)	0.37(8)	35.676(9)	0.38(8)	35.672(9)	0.53(16)	35.665	0.28	35.665	S XIII
36.374(12)	0.35(10)	36.372(15)	0.28(7)	36.399(15)	0.34(14)	36.398	0.25	36.398	S XII
36.544(19)	0.15(6)	36.561(19)	0.29(9)	36.547(15)	0.24(13)	36.563	0.24	36.563	S XII

^a Identifications for MEKAL (Mewe et al. 1995) and KELLY (Kelly 1987) identical.

^b Observed flux in 10^-4 photons/cm²/s.

^c Model flux in 10^-4 photons/cm²/s, obtained from 3-T fitting of LETGS (see Sect. 3.2).

^d Fe XVII lines are strongly mixed with Fe XVI satellite lines.

^e Line not split up; mixture of 15.175 Å and 15.265 Å from O VIII and Fe XVII, respectively.

^f Small contamination by Fe XVIII possible.
Values in parentheses are statistical 1$\sigma$ uncertainties in the last digits.