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Astron. Astrophys. 336, 637-647 (1998)

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5. Distance to BX Mon

The interstellar Na i [FORMULA] absorption lines which we observed at high spectral resolution allow us to derive a lower limit for the distance to BX Mon. Nulling the absorption feature at 2200 Å in the IUE low resolution data yields [FORMULA], which puts an upper limit on the distance.

5.1. Interstellar absorption lines

The galactic coordinates of BX Mon are [FORMULA], [FORMULA]. The velocity structure of the interstellar medium has been studied in detail by Brand & Blitz (1993). In the direction of BX Mon, the radial velocity of the interstellar Na i D lines is increasing with increasing distance. The Na i doublet in the spectra of BX Mon, shows a complex signature (see Fig. 6). As both components show the same structure, we can assume that the observed profiles are real, without significant noise. The radial velocities of the cool and of the hot component at the time of observation are [FORMULA] and [FORMULA] in the local standard of rest (LSR). Na i absorptions with radial velocities larger than these values are thus most probably caused by the interstellar medium. According to Brand & Blitz (1993), the strong absorption feature corresponding to [FORMULA] indicates a distance of [FORMULA] pc. The weak absorption feature at [FORMULA] would be associated with interstellar clouds at 3500 pc. We therefore use the 3000 pc as a lower limit for the distance to BX Mon.

[FIGURE] Fig. 6. Interstellar absorption lines Na i [FORMULA] (top, shifted by +0.25) and Na i [FORMULA] (bottom) in the spectrum of Nov 1, 1993, transformed into the local standard of rest (LSR).

5.2. Interstellar reddening

The better exposed IUE spectra of BX Mon show the broad [FORMULA] feature which can be used to determine the interstellar extinction. As the UV-continuum emission of BX Mon is similar to that of a late A or early F type star, which in itself has some spectral structure at [FORMULA], it is not advisable to simply straighten the spectrum to a steadily increasing continuum because that would tend to overestimate the extinction value. Instead we derive [FORMULA], using the mean interstellar extinction law of Seaton (1979), by comparing the BX Mon spectra with that of a spectral standard of known extinction. The spectral standard HD 59 612 fits the absorption line spectrum of BX Mon well in the UV. Fanelli et al. (1992) find for this A5I star [FORMULA]. For BX Mon we obtain [FORMULA], which agrees with earlier estimates by Viotti et al. (1986). Assuming [FORMULA] and using [FORMULA]/[FORMULA] as tabulated in Savage & Mathis (1979) leads to [FORMULA], [FORMULA] and [FORMULA].

Neckel & Klare (1980) have examined the spatial distribution of the interstellar extinction. Field 69 and 70 which are close to BX Mon can be used to estimate an upper limit for the distance of BX Mon. At a distance of 3000 pc, there is a steep increase in [FORMULA] from [FORMULA] to much higher values, making this value an upper limit for the distance. Together with the interstellar Na i absorption features we thus estimate the distance of BX Mon to be


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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998