Loris Magnani

Molecular gas at high galactic latitudes

The present quantitative results of a CO survey of high latitude molecular gas shows 57 clouds in 35 complexes at absolute values of b not smaller than 25 deg. Maps of 33 clouds are presented, together with C-13O and CO(J = 2-1) observations. The clouds, 70 percent of which are associated with optical emission, are noted to be distributed asymmetrically with respect to b = 0 deg; this is consistent with a displacement of the sun of 30 pc above the midplane. Since the mean distance to the clouds is about 100 pc, they are the nearest clouds to the sun and may contain the nearest regions of star formation. 51 references.

A Survey of High-Latitude Molecular Gas in the Northern Galactic Hemisphere

We surveyed the northern Galactic hemisphere (NGH) at b ≥ 30° in the CO (1-0) emission line to determine the surface filling factor of molecular gas at high Galactic latitudes and to search for heretofore unknown molecular clouds. The NGH was sampled on a locally Cartesian grid with 1° (true-angle) spacing in Galactic longitude and latitude. Of the 11,478 points in our grid, we observed all 10,562 positions that rise to an elevation above 30° in Cambridge, MA, the site of the 1.2 m millimeter-wave telescope that was used for the survey. Only 26 lines of sight showed CO emission. Monte Carlo simulations based on our sampling grid and with cloud sizes, in a uniform distribution, ranging from 0 to 2 deg2 suggest that the survey is ~70% complete. Power-law distributions yield fractional completenesses that are typically a factor of 2 lower. The surface filling factor, corrected for the incompleteness of our sampling grid, is 0.004-0.008, depending on which cloud size distribution is used. These values are substantially lower than what is found in the southern Galactic hemisphere at b ≤ -30°. Adopting as the CO to H2 conversion ratio NH2/WCO = 2.5 × 1020 cm-2 (K km s-1)-1, the mass surface density of molecular gas in the north ranges from 0.015 ± 0.009 to 0.035 ± 0.020 M☉ pc-2. With the exception of four fairly significant aggregations of clouds (the complexes associated with the Polaris flare, Ursa Major, Draco, and L134), and a handful of isolated cloudlets, the northern Galactic hemisphere at b ≥ 30° is found to be largely free of molecular gas.

Infrared cirrus and high-latitude molecular clouds

It is established that a close correlation exists between far-infrared cirrus emission observed with IRAS and the CO emission from high-latitude molecular clouds (HCLs). In all cases, the HLCs correspond to the central portions of 100-micron infrared cirrus features. This association firmly establishes at least some of the cirrus as features of the local interstellar medium with typical distances of 100 pc. The infrared energy distribution of the cirrus displays an excess of 12-micron and 25-micron emission over that expected from dust at equilibrium temperature, consistent with emission from very small (less than 10 A) transiently heated grains.

The Variation of the CO to H2 Conversion Factor in Two Translucent Clouds

The conversion factor, XCO, between the velocity-integrated CO(1-0) antenna temperature, W(CO), and the H2 column density, N(H2), is determined for 32 positions in two translucent high-latitude molecular clouds, MBM 40 and MBM 16. XCO is calculated using CH observations of the 2Π1/2 F = 1-1 hyperfine transition to infer N(H2). The latter quantity, divided by W(CO) yields XCO for various positions across the clouds. We observed 24 positions in MBM 16, and XCO values were derived in the range (1.6-17.3) × 1020 cm-2 (K km s-1)-1, with a mean value of 7.6 × 1020. Eleven lines of sight were sampled in MBM 40 yielding values of XCO in the range (0.7-9.7) × 1020, with a mean value of 2.6 × 1020. An inverse relationship between XCO and W(CO) may exist, suggesting that the variation in XCO for these two translucent clouds arises from varying CO abundances. This paper also reports the existence of a broad component in the CH spectra observed throughout much of MBM 16. This component possibly originates in the disturbed outer regions of the cloud where the gas is not gravitationally bound to the core of the cloud. It is unclear how sensitive the CO rotational transitions are to this component, but it is likely that the CO/H2 ratio in this broad-line gas is less than 10-5. However, if the CH/H2 ratio is the same for the gas in the extended wings as it is for the typical cloud gas, then up to 40% of the cloud mass could be contained in this difficult to trace molecular component.

CH, CO, and E(B–V) as Molecular Gas Tracers in a Translucent Cloud

The effectiveness of three molecular hydrogen gas tracers—the CH 2Π1/2, J = 1/2, F = 1-1 transition, the CO(1-0) transition, and the color excess, E(B-V)—is evaluated for the translucent molecular cloud MBM 16. The CH emission line and E(B-V) are well correlated with each other but not with the CO(1-0) emission line. It is likely that in translucent molecular clouds CO is not a linear tracer of the molecular hydrogen column density because, in these objects, the CO/H2 ratio is undergoing rapid fluctuations as a function of the molecular hydrogen column density. For translucent and diffuse molecular clouds, and possibly for the translucent envelopes of giant molecular clouds, CH observations should be used to calibrate the CO to H2 conversion factor.

A search for T Tauri stars in high-latitude molecular clouds. 2: The IRAS Faint Source Survey catalog

We present a catalog of infrared point sources from the IRAS Faint Source Survey at Galactic latitudes the absolute magnitude of b is greater than or equal to 30 deg. The aim of this paper is to provide a list of possible star-forming sites at high Galactic latitudes in order to address the question of whether or not the translucent molecular clouds (which are most easily identified at high latitudes) are capable of star formation. The primary list of sources has 12, 25, 60, and 100 micron fluxes within the range typical of pre-main-sequence or T Tauri stars. A secondary list has the same range of 12, 25, and 60 micron fluxes, but only upper limits at 100 microns. A total of 127 candidates from the first category and 65 candidates from the second category are identified and their positions and infrared spectral characteristics tabulated. Although the colors and fluxes of these sources are typical of T Tauri or pre-main-sequence stars and YSOs, extragalactic sources and planetary nebulae sometimes have similar colors. These lists provide a starting point for optical spectroscopy or other techniques to positively identify these objects. We can determine an upper limit to the star forming efficiency of high-latitude molecular clouds assuming all the candidates in our sample are pre-main sequence stars of one solar mass. The upper limit of a few tenths of 1% is less than the star-forming efficiency of local dark cloud complexes such as the Taurus-Auriga or rho Ophiuchus clouds.

The observation of correlated velocity structures in a translucent molecular cloud and implications for turbulence

We present a formaldehyde map of the translucent high-latitude molecular cloud MBM 16. The molecular gas traced by the H 2 CO is located in spatially distinct large structures that exhibit velocity coherence on a scale of 0.5 pc. These structures are not pressure-confined and are probably not self-gravitating. They may be transient structures. If so, we suggest that they are produced by shear flows whose scale length is of order the size of the cloud

A Dynamical Study of the Non-Star-forming Translucent Molecular Cloud MBM 16: Evidence for Shear-driven Turbulence in the Interstellar Medium

We present the results of a velocity correlation study of the high-latitude cloud MBM 16 using a fully sampled 12CO map, supplemented by new 13CO data. We find a correlation length of 0.4 pc. This is similar in size to the formaldehyde clumps described in our previous study. We associate this correlated motion with coherent structures within the turbulent flow. Such structures are generated by free shear flows. Their presence in this non-star-forming cloud indicates that kinetic energy is being supplied to the internal turbulence by an external shear flow. Such large-scale driving over long times is a possible solution to the dissipation problem for molecular cloud turbulence.

A search for pre-main-sequence stars in high-latitude molecular clouds. 3: A survey of the Einstein database

In order to discern whether the high-latitude molecular clouds are regions of ongoing star formation, we have used X-ray emission as a tracer of youthful stars. The entire Einstein database yields 18 images which overlap 10 of the clouds mapped partially or completely in the CO (1-0) transition, providing a total of approximately 6 deg squared of overlap. Five previously unidentified X-ray sources were detected: one has an optical counterpart which is a pre-main-sequence (PMS) star, and two have normal main-sequence stellar counterparts, while the other two are probably extragalactic sources. The PMS star is located in a high Galactic latitude Lynds dark cloud, so this result is not too suprising. The translucent clouds, though, have yet to reveal any evidence of star formation.

Molecular abundances in the high-latitude molecular clouds

This paper presents CO, H/sub 2/CO, OH, and extinction data for several high-latitude molecular clouds (HLMCs) in an attempt to determine the molecular abundances and gas properties of a typical HLMC. The CO abundance with respect to H/sub 2/ is comparable to what is found for dark clouds. The H/sub 2/CO abundance is within a few factors of the values found in dark cloud surveys. The OH abundance in the HLMCs is an order of magnitude greater than the value determined for dark clouds. The data indicate that the HLMCs show abundance and column densities more similar to those of the dark clouds than to those of the diffuse clouds. 61 references.