The spectral type of HD 358623 B was determined in Sect. 3.2 roughly to be
early- to mid-M from its JHK magnitudes.
To confirm and refine this classification,
we obtained an H-band spectrum on 8 Dec. 2001 with SofI.
We took 20 spectra with 60 s exposure each through a
slit with a red grism including both the H- and K-band
(1.53 to
m) with a resolution of
.
Data reduction was done in the normal way with IRAF:
Dark subtraction, normalization, flat fielding, sky subtraction,
wavelength calibration, co-adding the spectra, then
correction for instrumental sensitivity and atmospheric response.
The spectra were not flux-calibrated.
The final spectra of both the primary HD 358623 A
and the secondary HD 358623 B are shown in Fig. 4.
Because we have comparison spectra of K- to M-type dwarfs
available only for the H-band, we use and show only the H-band
part of the SofI H+K-band spectrum.
The H-band spectrum of the comparison star GSC 7210 1352, a known M1-dwarf
in the TW Hya region, has been taken by us with ISAAC at the VLT
and was published before in Neuhäuser et al. (2002).
As seen in Fig. 4, HD 358623 A shows the typical features of late-K to early-M type stars,
namely Mg, Si, Al, and Na absorption (see e.g. Green & Lada 1996; Meyer et al. 1998).
Those lines are clearly weaker in the comparison star GSC 7210 1352,
a known M1 dwarf.
It is very similar for the secondary, where these lines are even a bit weaker.
Hence, regarding the presence of lines, the relative strength of them,
and the general shape of the continuum, we can classify the secondary as
M2 dwarf (1 sub-class).
This is consistent with its JHK colors (JB01 and our Table 2).
It cannot be later than M3, because a stronger than observed
absorption line appears at
m in dwarfs later than M3
(Green & Lada 1996; Meyer et al. 1998).
This classification is also consistent with
the primary being K7-M0 and the magnitude difference between
primary and secondary being 1.7 mag in J and H (and 1.6 mag in K),
assuming that they are at the same age and distance.
Copyright ESO 2002