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Astron. Astrophys. 357, 681-685 (2000)

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1. Introduction

The two nearest known star forming clouds to the Sun are MBM12 and MBM20. The MBM12 complex consists of clouds 11, 12, and 13 from the catalog of Magnani et al. (1985) and is located at (l,b) [FORMULA] ([FORMULA],[FORMULA]) (we will refer to the entire complex as MBM12). It was first identified by Lynds (1962) and appears as objects L1453-L1454, L1457, L1458 in her catalog of dark nebulae. The mass of the entire complex is estimated to be [FORMULA] 30-200 [FORMULA] based on radio maps of the region in 12CO, 13CO and C18O (Pound et al. 1990; Zimmermann & Ungerechts 1990). The cloud MBM20 was first identified as L1642 in the Lynds (1962) catalog of dark nebulae. The mass of MBM20 is [FORMULA] 84 [FORMULA] and it is located at (l,b) [FORMULA] ([FORMULA],[FORMULA]) southwest of the Orion star forming complex (Magnani et al. 1985).

Both MBM12 and MBM20 are star forming clouds. Several T Tauri stars have been identified in MBM12 via H[FORMULA] emission line and X-ray surveys (Herbig & Bell 1988; Stephenson 1986; Hearty et al. 2000). Two binary classical T Tauri stars have been identified in the central region of MBM20 (Sandell et al. 1987). Subsequent high-resolution 12CO(J=1-0) observations near the center of the cloud have shown that the infrared point source IRAS04325-1419 corresponding to one of the binaries is associated with a red and blue-shifted bipolar outflow (Liljeström et al. 1989).

Using ROSAT observations, Kuntz et al. (1997) find evidence based on the possible detection of 0.25 keV X-ray shadows of these clouds that both are located either within or at the edge of the Local Bubble. They estimate that distances, d, of [FORMULA] pc to MBM12 and d [FORMULA] 100 pc to MBM20 are consistent with the foreground 0.25 keV emission seen in the direction of each cloud. The distances are also consistent with previous distance estimates (see Sect. 2).

Since MBM12 and MBM20 are both star forming clouds which are probably located within or at the edge of the Local Bubble, determining an accurate distance to these clouds is important for studies of stars and of the interstellar medium. In addition, observations of MBM12 are already scheduled for XMM (50 ks) and Chandra (100 ks) to investigate the diffuse X-ray emission of the Local Bubble. Therefore we present observations to improve the distance estimates to both clouds. In Sect. 2 we review previous distance estimates for these clouds and the Hipparcos results that revise them. In Sect. 3 we present our spectroscopic observations of Hipparcos stars along the line of sight to both of the clouds which we use to improve the previous distance estimates. In Sect. 4 we summarize our investigations and suggest future observations.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000