A Survey of Interstellar Structures

The Importance of Large-Scale Interstellar Structures

Work by numerous researchers over the past few decades has led to the identification of numerous large-scale structures in the interstellar medium (ISM) of our galaxy. Detection of loops, shells, filaments, and interstellar clouds have all helped to shape our view of the nature of the ISM. This has led to a picture in which the energy input of supernovae(SNe) in the disk and halo of the galaxy is thought to be the principle driving force in structuring the interstellar medium. This structuring in turn has a profound influence on the life history of the galaxy, determining where and when stars will form, how the galaxy will look, and how the metal content of the galaxy changes over time.

Why do we need distances?

In order to understand processes relating to galactic chemical and dynamical evolution, it is necessary to observationally identify and theoretically understand the origin of these large scale stuctures in the ISM. One of the principal limitations in determining the basic physical parameters of these structure lies in the very large uncertainty in their distances. We intend to significantly reduce this uncertainty for several of the most interesting structures by means a coordinated program of high resolution optical spectroscopy of stars lying in the direction of the cloud. As is discussed below, this sort of study will be extremely valuable in elucidating the nature of the ISM and by extension, the hydrodynamical history of the solar neighborhood. The results of a preliminary study (Benjamin et al 1995), for example, has led to incontrovertible evidence for the existence of hot interstellar gas in the halo of our galaxy, as well as the placement of a molecular cloud well above the plane of our galaxy.

For a given interstellar structure, the distance is generally derived by measuring its velocity, and assuming a distance- velocity relationship as given by models of galactic differential rotation. Unfortunately, this method fails precisely for those structures that are the most interesting, e.g. superbubble shells produced by SNe, interstellar "worms", the mysterious intermediate- and high-velocity clouds (IVC and HVCs, respectively) in the galactic halo, etc. These objects are distinguished precisely because their motions are determined factors other than galactic rotation, namely mass flows driven by supernovae or non-azimuthal gas flows.

Research plans

With the recent improvements in the sensitivity of high resolution optical systems, such as the Sandiford echelle spectrograph on the 2.1m telescope at McDonald Observatory, it has become feasible to search for interstellar absorption in the spectra of stars as faint as V~=12, with a resolving power of R~50,000. The number of target stars per square degree is given as below (from Allen 1975) 
 
-------------------------------------------------------------------
           | # of A stars or earlier   | # of F stars or earlier  |
V (min)    |   b=0     b=60    b=90    |  b=0    b=60     b=90    |
-------------------------------------------------------------------
   10      |    5        1       0     |   8      2        1      |
   11      |   16        3       2     |  26      5        4      |
   12      |   46        8       3     |  75     13       10      |
-------------------------------------------------------------------
The likelihood of successfully putting upper and lower bounds on cloud distances with this number of target stars is quite good.

We intend to obtain spectra of several stars in the direction of interstellar structures of interest. The scientific payoff for accurate distance determination will vary from cloud to cloud, and comments on and parameters of individual clouds are given below. In general, the types of questions that this sort of study will address are the following:

Interstellar Structures Targeted for Distance Determination 

 G139-65   RA= 1h 16m Dec= -3 34' Priority=3

This cloud shows an X-ray shadow in ROSAT PSPC images (Juda 1995).

---------Target stars-full list --------- Target stars-short list---------

Progress to date 

 G192-67   RA= 2h 17m Dec=-17 46' Priority=3

This cloud shows an X-ray shadow in ROSAT PSPC images (Juda 1995).

---------Target stars-full list --------- Target stars-short list---------

Progress to date 

 G225-66   RA= 2h 39m Dec=-29 37' Priority=3

This cloud shows an X-ray shadow in ROSAT PSPC images (Juda 1995).

---------Target stars-full list --------- Target stars-short list---------

Progress to date 

 Field1    RA= 3h 18m Dec=-66 30' Priority=5

This cloud shows an X-ray shadow in ROSAT PSPC images and is designated by Wang & Yu (1995) as Field 1.

---------Target stars-full list --------- Target stars-short list---------

Progress to date 

 Field2    RA= 9h 59m Dec=+65 34' Priority=5

This cloud shows an X-ray shadow in ROSAT PSPC images and is designated by Wang & Yu (1995) as Field 2.

---------Target stars-full list --------- Target stars-short list---------

Progress to date 

 Field3    RA=10h 28m Dec=+68 25' Priority=5

This cloud shows an X-ray shadow in ROSAT PSPC images and is designated by Wang & Yu (1995) as Field 3.

---------Target stars-full list --------- Target stars-short list---------

Progress to date 

 G211+63   RA=10h 53m Dec=+24 56' Priority=3

This cloud shows an X-ray shadow in ROSAT PSPC images (Juda 1995).

---------Target stars-full list --------- Target stars-short list---------

Progress to date 

 UMa_v=0   RA=10h 55m Dec=+65 36' Priority=1

This cloud shows an X-ray shadow in ROSAT PSPC images taken by Snowden et al 1994.

---------Target stars-full list --------- Target stars-short list---------

Progress to date 

 UMa_v=-45 RA=11h 55m Dec=+62 31' Priority=0

This cloud shows an X-ray shadow in ROSAT PSPC images taken by Snowden et al 1994. Benjamin et al 1995, have detected this cloud in the spectrum of BD+63 985, and estimate the distance to the cloud to be 350 +/- 60 pc. This places it in the low halo, and demonstrates that X-ray emission arises in the halo. Moreover, pieces of it have been found to have molecular gas. This is one of three known molecular clouds known in the halo.

---------Target stars-full list --------- Target stars-short list---------

Progress to date 

 G107+71   RA=13h 18m Dec=+44 54' Priority=3

This cloud shows an X-ray shadow in ROSAT PSPC images (Juda 1995).

---------Target stars-full list --------- Target stars-short list---------

Progress to date 

 Field4    RA=13h 23m Dec=+54 55' Priority=5

This cloud shows an X-ray shadow in ROSAT PSPC images and is designated by Wang & Yu (1995) as Field 4.

---------Target stars-full list --------- Target stars-short list---------

Progress to date 

 HVC90.5+42.5-130  RA=16h 16m Dec=59 42' Priority=1

This cloud shows an X-ray enhancement according to Kerp, Lesch, and Mack (1994), and is interpreted as energy dissipation in front of the cloud due to magnetic reconnection occuring in swept up field lines in front of the cloud. Given the cloud's velocity (-200 < v < -80 km/s) it is unlikely that it is very close (d > 500 pc), leaving only one potential background star. Probably work on this cloud will yield only upper limits. This one is relatively high risk/ high payoff.

---------Target stars-full list --------- Target stars-short list---------

Progress to date 

 Field5    RA=18h 11m Dec=+40 46' Priority=5

This cloud shows an X-ray shadow in ROSAT PSPC images and is designated by Wang & Yu (1995) as Field 5.

---------Target stars-full list --------- Target stars-short list---------

Progress to date 

 G026-67   RA=23h  9m Dec=-27 26' Priority=3

This cloud shows an X-ray shadow in ROSAT PSPC images (Juda 1995). ---------Target stars-full list --------- Target stars-short list---------

Progress to date

References