2. Previous distance estimates
The distances to MBM12 and MBM20 have previously been derived using both photometric and spectroscopic techniques. The spectroscopic technique used in this paper was first used by Hobbs et al. (1986) and Hobbs et al. (1988) to estimate the distance to MBM12 and by Penprase (1993) to estimate the distance to MBM20. These studies looked for interstellar NaI D lines at Å and Å in bright stars in the direction of each cloud. Since the distance to each star was known via its spectroscopic parallax, the distance to the cloud could be determined. The stars with interstellar NaI absorption features were presumed to be behind the cloud and those without interstellar features were presumed to be in front of the cloud or not located behind a sufficient column density to detect the interstellar line. Since the Hipparcos satellite measured the trigonometric parallax for most of the stars used to determine the distance to MBM12 and MBM20, it is no longer necessary to assume a spectral type or intrinsic luminosity (as is necessary for a spectroscopic parallax) to measure their distance.
The stars used by Hobbs et al. (1986), Hobbs et al. (1988), and Penprase (1993) to establish the distance to MBM12 and MBM20 are listed in Table 1 with their apparent magnitude, spectral type, distance based on spectroscopic parallax, distance based on the Hipparcos parallax 1 and whether the spectrum presented in Hobbs et al. (1986) and Hobbs et al. (1988) showed interstellar NaI absorption lines.
Table 1. Stars from Hobbs et al. (1986), Hobbs et al. (1988), and Penprase (1993).
Previous distance estimates to the cloud MBM12 have already been discussed by Hearty et al. (2000). We just note the Hipparcos results here. In their search for interstellar NaI D lines in the spectra of stars along the line of sight to MBM12, Hobbs et al. (1986) found that the star HD18404 (distance 60 pc) showed no interstellar absorption features and is therefore presumably in front of the cloud and the star HD18519/20 (distance 70 pc) did show interstellar absorption features and is therefore behind the cloud. According to Hipparcos , the distance to HD18404 is 321 pc and the distance to HD18519/20 is 9012 pc. Although the Hipparcos results indicate the distance to MBM12 is not as well constrained, it is consistent with previous estimates (i.e., 321 pc 9012 pc).
Hobbs et al. (1986) note that it remains possible, although unlikely, that a NaI cloud is located in the foreground of a more distant CO cloud at virtually the same radial velocity. Therefore it is important to note that there are at least two stars located behind the cloud which show a large extinction (see Fig. 1). However, both of these stars are too faint to have been observed with Hipparcos . One of the stars, H0253+193, is an eclipsing cataclysmic variable (Koyama et al. 1991; Zuckerman et al. 1992). Zuckerman et al. (1992) estimate a distance of 200 pc and a visual extinction 11.5 mag based on its near infrared colors. In addition, Hearty et al. (2000) showed that the optical spectrum of another star, DC48 (Duerr & Craine 1982), corresponds to a G9 star. Comparing the magnitude and color measured by Duerr & Craine (1982) for this star (V = 18.7 and V-I = 5.6 mag) to the intrinsic values of a G9 star, it could be a main sequence star with 8.9 mag at a distance of 63 pc or a giant star with 8.4 mag at a distance of 950 pc. Future observations to better constrain the distance, the luminosity class, and the spectral type of this star may prove that the CO and the NaI clouds along this line of sight are one and the same and provide a new upper limit on the distance to MBM12.
The cloud MBM20 is located southwest of several cometary globules probably associated with the Orion star forming region. However, previous photometric and spectroscopic observations of MBM20 and the morphology of the cometary globules suggest that MBM20 is much closer to the Sun than the cometary globules in the region. Studies of star formation in Bok globules in the Gum Nebula have identified Bok globules with comet-like tails pointed away from the central OB association (e.g., Reipurth 1983). This type of large scale morphology which Bally et al. (1991) argue is caused by the energy injected from massive stars can also be seen in the cometary clouds surrounding the Orion OB association where many clouds have dense heads with tails pointing away from the center of the Orion OB association. Whatever the mechanisms are that generate the cometary clouds, there are no signs that MBM20 is interacting with the bright stars in Orion (i.e., there is no cometary tail pointed away from the OB association).
Using the same technique we use in this paper, Penprase (1993) arrived at an upper limit for the distance to MBM20 ( 110 pc) by observing NaI D interstellar absorption features is stars for which a spectroscopic parallax could be determined (cf. Hobbs et al. 1986). The nearest star which showed interstellar NaI absorption, HD29851, is at a distance of 110 pc based on its spectroscopic parallax. Since none of the stars observed by Penprase (1993) were superimposed on the cloud contours, a lower limit for the distance to the cloud could not be determined. However, even without a lower limit on the distance, these observations showed that MBM20 is among the nearest star forming molecular clouds to the Sun. The Hipparcos parallax for HD29851 places it at pc. Therefore, the Hipparcos results indicate the distance to the cloud could be larger than previous estimates.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000