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5 Spectrocopy of HD 358623 A and B

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 $1^{\prime \prime}$ slit with a red grism including both the H- and K-band (1.53 to $2.52~\mu$m) with a resolution of $R \simeq 1000$. 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).


  \begin{figure}
\par\includegraphics[angle=270,width=8.8cm,clip]{MS02667f4.ps}\end{figure} Figure 4: H-band spectra of HD 358623 A (top) and B (bottom) taken with NTT/SofI as well as and GSC 7210 1352 (in the middle) taken with VLT/ISAAC for comparison. We spectra are not flux-calibrated; we plot a somewhat shifted flux, so that all three objects can be compared to each other. Star A is known to be K7-M0 with strong Mg, Si, and Al lines, GSC 7210 1352 is a known M1-dwarf with weaker lines. The lines in Star B is slightly weaker than in GSC 7210 1352, so that we classify it as M$2 \pm 1$.

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 ($\pm $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 $1.76~\mu$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.


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