Spectroscopic observations have been performed with the REOSC échelle
spectrograph at the 91-cm telescope of Catania Astrophysical Observatory -
M. G. Fracastoro station (Serra La Nave, Mt. Etna). The spectrograph is fed by
the telescope through an optical fibre (UV - NIR, 200m core diameter) and is
placed in a stable position in the room below the dome level.
Spectra were recorded on a CCD camera equipped with a thinned back-illuminated
SITe CCD of 1024
1024 pixels (size 24
24
m). The échelle crossed
configuration yields a resolution of about 14 000, as deduced from the FWHM
of the lines of the Th-Ar calibration lamp. The observations have been made in the red
region. The detector allows us to record five orders in each frame, spanning from about 5860
to 6700 Å. In this spectral region there are several line pairs of low and high
excitation potential, whose depth ratios are suitable for effective temperature determination.
The data reduction was performed by using the ECHELLE task of IRAF
package following the standard steps: background subtraction, division by a flat
field spectrum given by a halogen lamp, wavelength calibration using the
emission lines of a Th-Ar lamp, and normalization to the continuum through a polynomial fit.
Particular care was paid to the continuum level definition. The IRAF task CONTINUUM
was used for such a purpose and we chose a low-order (3rd-4th) Legendre polynomial
to follow the
continuum behaviour in each spectral order that is the result of the true spectral shape and
of residual instrumental effects, like the blazing curvature, spectrograph sensitivity, etc.,
that are not fully removed by the reduction process.
The choice of a low-order polynomial guarantees a good definition of the continuum level at least on a spatial scale of a few tens of Å, much more than the typical wavelength separation of line pairs.
HD | Name | Sp. Type | V | ![]() |
![]() |
(mag) | (km s-1) | (days) | |||
209813 | HK Lac | K0III | 6.52 | 15 | 24.4284 |
216489 | IM Peg | K2III-II | 5.60 | 26 | 24.494 |
17433 | VY Ari | K3-4V-IV | 6.9 | 6 | 16.1996 |
Observations were carried out from August 2000 to January 2001,
on three selected targets, i.e. VY Ari, HK Lac, IM Peg, whose main parameters
are reported in Table 1. The main requirement for the selection was the known
strong spottedness and low .
The
of the targets is in the 6-24 km s-1 range, so if the spots or the spotted
area is large enough to feel the rotation broadening effect the width
of the bumps caused by the spot visibility would be of the same order
as the width of a spectral resolution element. The slow rotation combined with
the relatively low spectral resolution will ensure the non-detection of the Doppler
shift of the bump along the spectral lines due to the spot rotation.
In addition to the active stars, a number of giant and main sequence stars of spectral type in the range G2III-M0III and F8V-K7V, respectively, have been observed to establish the temperature scale of the line-depth ratios.
The average signal-to-noise ratio (S/N) at continuum in the spectral region of interest was 200-500 for the calibration stars and about 100-150 for the active stars.
HD | Name | Sp. T. | V | ![]() |
B-V |
![]() |
(mag) | (mas) | (K) | ||||
GIANTS | ||||||
161239 | 84 Her | G2IIIb | 5.714 | 26.13 | 0.654 | 5732 |
196755 | ![]() |
G2IV | 5.069 | 33.27 | 0.705 | 5583 |
161797 | ![]() |
G5IV | 3.417 | 119.05 | 0.752 | 5451 |
188512 | ![]() |
G8IV | 3.715 | 72.95 | 0.855 | 5183 |
23249 | ![]() |
K0IV | 3.527 | 110.58 | 0.922 | 5024 |
62345 | ![]() |
G8IIIa | 3.568 | 22.73 | 0.932 | 5001 |
216131 | ![]() |
G8III | 3.488 | 27.95 | 0.934 | 4996 |
22796 | 12 Tau | G6III | 5.565 | 8.14 | 0.934 | 4996 |
28100 | ![]() |
G7IIIa | 4.692 | 7.17 | 0.982 | 4891 |
197989 | ![]() |
K0III | 2.467 | 45.26 | 1.034 | 4783 |
74442 | ![]() |
K0IIIb | 3.937 | 23.97 | 1.082 | 4687 |
12929 | ![]() |
K2III | 2.009 | 49.48 | 1.153 | 4552 |
54719 | ![]() |
K2III | 4.407 | 10.81 | 1.261 | 4350 |
43232 | ![]() |
K1.5III | 3.972 | 5.06 | 1.320 | 4239 |
49161 | 17 Mon | K4III | 4.758 | 6.73 | 1.394 | 4096 |
69267 | ![]() |
K4III | 3.532 | 11.23 | 1.481 | 3923 |
29139 | ![]() |
K5III | 0.868 | 50.09 | 1.537 | 3813 |
60522 | ![]() |
M0IIIb | 4.058 | 13.57 | 1.539 | 3809 |
DWARFS | ||||||
187691 | 54 Aql | F8V | 5.116 | 51.57 | 0.552 | 6045 |
22484 | 10 Tau | F9IV-V | 4.290 | 72.89 | 0.574 | 5976 |
157214 | 72 Her | G0V | 5.394 | 69.48 | 0.619 | 5837 |
186408 | 16 CygA | G1.5Vb | 5.960 | 46.25 | 0.645 | 5758 |
217014 | 51 Peg | G2.5IV | 5.463 | 65.10 | 0.665 | 5699 |
20630 | ![]() |
G5V | 4.836 | 109.18 | 0.679 | 5658 |
10700 | ![]() |
G8V | 3.496 | 274.17 | 0.727 | 5520 |
3651 | 54 Psc | K0V | 5.879 | 90.03 | 0.849 | 5197 |
22049 | ![]() |
K2V | 3.726 | 310.75 | 0.882 | 5117 |
16160 | HR 753 | K3V | 5.821 | 138.72 | 0.972 | 4912 |
201091 | 61 CygA | K5V | 5.224 | 287.13 | 1.169 | 4522 |
201092 | 61 CygB | K7V | 6.046 | 285.42 | 1.360 | 4162 |
To convert the depth ratio variation of our active stars into temperature variation we need to define a temperature scale for the measured line-depth ratios. We have then observed a number of single stars of different spectral type in the range from F8 to M0 and luminosity class from V to III. Main criteria for the selection were: (i) a low rotation velocity, (ii) a reasonably good parallax value, (iii) accurate B-V color index. Since the line-depth ratio is dependent on gravity also for some temperature-sensitive lines, we have observed main sequence and giant stars to correct the gravity effect and eventually set separate temperature scales to be used for active main sequence and giant stars. The calibration stars are listed in Table 2 together with their spectral type, V magnitude, parallax, B-V and effective temperature. Spectral types are from the Bright Star Catalogue (Hoffleit & Warren 1991), visual magnitudes V and B-V color indices are from the Geneva Web database (Mermilliod et al. 1997) and the parallaxes are from the Hipparcos Catalogue (ESA 1997).
Since effective temperatures are available for very few of our calibration stars, we
used color indices B-V to set the effective temperature of each calibration star.
Although interstellar reddening is not expected to be large,
since all the stars in Table 2, with few exceptions, are closer than 100 pc, we
applied an isotropic extinction correction to obtain the (B-V)0.
We used = 0.8 mag kpc-1 and a ratio of total to selective extinction
of 3.3 as suggested by Henry et al. (2000).
Conversion of (B-V)0 to effective temperature has been made through the empirical
relation proposed by Gray (1992):
![]() |
|
- 3.614(B-V)03 + 3.2637(B-V)04 | |
- 1.4727(B-V)05 + 0.2600(B-V)06. | (1) |
The metallicity effects can alter the B-V indices.
Gray (1994) has investigated the influence of metallicity on color indices, finding
an empirical relation between B-V and
.
The B-V color index is only very slightly
dependent on
,
its maximum variation being of about
.
According to the calibration relation given in Eq. (1), the corresponding
temperature
change is about 20-30 K. Given the uncertainties in the B-V values and in the setting of the
temperature scale, such effects appears to be statistically not significant in our
LDR-temperature calibrations.
Within the spectral range covered by our échelle frames, 5870-6700 Å, there are several pairs of lines suitable for temperature determination, the more frequently used being in the spectral region around 6200 Å (Gray & Johanson 1991; Gray & Brown 2001; Hatzes et al. 1998) and 6400 Å(Strassmeier & Fekel 1990; Strassmeier & Schordan 2000). We preferred to use lines in the 6100-6200 Å range because we were able to select a larger number of unblended pairs with separation smaller than 5 Å thus avoiding problems of different setting of the continuum, and less contamination from telluric lines that at our resolution is difficult to remove properly.
Figure 1 displays a portion of the 6200 Å region for a series of spectra
of giant stars representative of spectral type from K0III to K4III.
From this figure the strengthening of Fe I and V I lines with decreasing
temperature is evident, while the 6247 Fe II shows the opposite behaviour.
Furthermore, the growth of low-excitation lines (like those of V I) is faster than
that of iron lines.
Altogether we identify 15 spectral lines forming 10 pairs suitable for
line-depth ratios. These lines were identified through the solar spectral atlas (Moore et al.
1966), choosing the unblended lines. The only exception is the
6243 V I
line that is indeed composed of two very close V I lines of comparable intensities and
with the same temperature dependence that appear as a single line at our resolution.
Line identification and excitation potential,
,
taken from Moore et al. (1966) and
Bashkin & Stoner (1975) are listed in Table 3.
![]() |
Element | ![]() |
(Å) | (eV) | |
6199.19 | VI | 0.29 |
6200.32 | FeI | 2.61 |
6210.67 | ScI | 0.00 |
6215.15 | FeI | 4.19 |
6215.22 | TiI | 2.69 |
6216.36 | VI | 0.28 |
6243.11 | VI | 0.30 |
6246.33 | FeI | 3.60 |
6247.56 | FeII | 3.89 |
6251.83 | VI | 0.29 |
6252.57 | FeI | 2.40 |
6265.14 | FeI | 2.18 |
6266.33 | VI | 0.28 |
6268.87 | VI | 0.30 |
6270.23 | FeI | 2.86 |
6274.66 | VI | 0.27 |
The lines for each ratio are chosen to be close together in order to minimize
errors in choosing the continuum. The lowest five points in the core
of each measured line were fitted with a cubic spline
and the minimum of this cubic polynomial was taken as the line depth.
Writing the line depth d as
Copyright ESO 2002