High-latitude Clouds Research Articles

High-latitude cold season frontal cloud systems and their precipitation efficiency

A weak high-latitude warm-frontal precipitation system was simulated by using a 2D cloudresolving model. It was found that many of the structural characteristics of the system were consequences of the subfreezing temperatures and enhanced planetary rotation in the region. The coupling of cloud microphysical processes to frontal scale dynamics caused the precipitation efficiency (defined as the ratio of integrated surface precipitation rate to the net water vapor influx into the storm) of the model storm to exhibit strong sensitivity to low-level humidity conditions. A relatively high (zero) precipitation efficiency is associated with a storm if it develops over regions having near surface relative humidity with respect to ice ≤ (>) 80%. In addition, the model results show that the precipitation efficiency of a system is decreased (increased) when the background static stability is increased (decreased) or when the Coriolis parameter is increased (decreased). These model results may have great implications for the Arctic water cycle as well as for the parameterization of Arctic clouds and precipitation in GCMs.

Interstellar Extinction, Polarization, and Grain Alignment in the High‐Latitude Molecular Cloud toward HD 210121

We present near-infrared photometry and optical and near-infrared polarimetry for the line of sight toward HD 210121, obscured by the high-latitude cloud DBB 80. From the data, we determine remarkably small values of the ratio of total to selective extinction, RV = 2.1 ± 0.2, and the wavelength of maximum polarization, λmax = 0.38 ± 0.03 μm, as compared to average values in the diffuse Galactic interstellar medium. These values nevertheless follow the linear relationship between RV and λmax seen closer to the Galactic plane. Our results imply an abundance of small grains, which we interpret as evidence for lack of grain growth in this high-latitude cloud. This is consistent with previous observations of high molecular abundances and extremely steep far-ultraviolet extinction toward the star and also with the presence of mid-infrared emission in the foreground cloud. We show that populations of small grains responsible for extinction and polarization in DBB 80 respond to prevailing physical conditions in a correlated way. Based on its extremely short value of λmax, we predict that the line of sight to HD 210121 will be highly polarized in the ultraviolet.

A Catalog of Molecular Gas at High Galactic Latitudes

view Abstract Citations (82) References (108) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Catalog of Molecular Gas at High Galactic Latitudes Magnani, Loris ; Hartmann, Dap ; Speck, Bryan G. Abstract The global distribution and physical properties of the high-latitude molecular clouds are tabulated and discussed. As of this writing, more than 100 objects have been identified at Galactic latitudes |b| > 25°. Positions are listed for all the known clouds and LSR velocity, angular extent, mass, and internal velocity dispersion are listed when such data are available from the literature. The high-latitude molecular clouds consist primarily of translucent clouds, so that this catalog is the largest, most complete compendium of these objects at high Galactic latitudes. The velocity dispersion of the ensemble of clouds is 5.8 km s-1 if seven intermediate velocity objects are excluded and 9.9 km s-1 otherwise. The former dispersion implies a scale height for the clouds of 124 pc and a mean distance of 150 pc; the latter dispersion yields 210 pc and 260 pc, respectively. Thus, the majority of high-latitude clouds are local objects that may be situated at the nearer edges of the Local Bubble. As a group, they are the nearest molecular clouds to the Sun. The average velocity of the ensemble of clouds is -0.3 km s-1 without the intermediate velocity objects and -2.4 km s-1 including these clouds. From the catalog of clouds, we determine values for the filling fraction and mass surface density of the high-latitude molecular clouds (and thus of translucent gas) of 0.005 and 0.1-0.2 M0 pc -2, respectively, at |b| > 25°. Including the lower latitude contribution, we estimate the total local mass surface density of translucent clouds to be 0.2-0.3 Msun pc-2 In the local interstellar medium, these objects represent from 10% to 20% of the molecular gas inventory. Individual clouds range in size from less than 10-1 to 101 pc and in mass from 10-1 to 103 Msun. An asymmetry between the northern and southern Galactic hemisphere distribution of the clouds can be interpreted as a displacement of the Sun from the Galactic midplane of 18 pc, a value consistent with determinations based on other tracers. Publication: The Astrophysical Journal Supplement Series Pub Date: October 1996 DOI: 10.1086/192344 Bibcode: 1996ApJS..106..447M Keywords: CATALOGS; GALAXY: STRUCTURE; ISM: CLOUDS; ISM: KINEMATICS AND DYNAMICS; ISM: MOLECULES full text sources ADS | data products SIMBAD (121)

Limits to the 14 keV Extragalactic X-Ray Background

We observed several nearby face-on spiral galaxies with the ROSAT PSPC. The apparent deficiency in soft X-ray surface brightness observed at the outer portion of their disks is consistent with the absorption of the extragalactic soft X-ray background by material associated with these galaxies, and allows us to place a lower limit on the intensity of this cosmologically important background. From the depth of the soft X-ray shadow observed in NGC 3184, a 95% confidence lower limit was derived to be $32 keV cm^{-2} s^{-1} keV^{-1}$ at 1/4 keV. This was obtained by assuming that there is no unresolved 1/4 keV X-ray emission from the outer region of the galaxy which may otherwise partially fill in the shadow: any such emission, or any unresolved structure in the absorbing gas, would imply a larger value. In the deepest exposure to date in this energy range, Hasinger et al. (1993) resolved about $30 keV cm^{-2} s^{-1} keV^{-1}$ at 1/4 keV into discrete sources; our current limit is therefore consistent with an extragalactic origin for all of these sources. Our results can also be directly compared with the corresponding upper limit derived from the ROSAT PSPC detection of soft X-ray shadows cast by high-latitude clouds in Ursa Major, $\simeq 65 keV cm^{-2} s^{-1} keV^{-1}$ at 1/4 keV. The lower and upper limits are only a factor of 2 apart, and begin to provide a reasonable measurement of the intensity of the 1/4 keV extragalactic X-ray background.

The high-latitude cloud MBM 7. I. H I and CO observations.

The high-latitude cloud (HLC) MBM 7 has been observed in the 21 cm H I line and the 12CO(1-0) and 13CO(1-0) lines with similar spatial resolutions. The data reveal a total mass approximately 30 M solar for MBM 7 and a complex morphology. The cloud consists of a cold dense core of 5 M solar surrounded by atomic and molecular gas with about 25 M solar, which is embedded in hotter and more diffuse H I gas. We derive a total column density N(H I + 2H2) of 1 x 10(21) cm-2 toward the center and 1 x 10(20) cm-3 toward the envelope of MBM 7. The CO line indicates the existence of dense cores [n(H2) > or = 2000 cm-3] of size (FWHM) approximately 0.5 pc. The morphology suggests shock compression from the southwest direction, which can form molecular cores along the direction perpendicular to the H I distribution. The H I cloud extends to the northeast, and the velocity gradient appears to be about 2.8 km s-1 pc-1 in this direction, which indicates a systematic outward motion which will disrupt the cloud in approximately 10(6) yr. The observed large line widths of approximately 2 km s-1 for CO suggest that turbulent motions exist in the cloud, and hydrodynamical turbulence may dominate the line broadening. Considering the energy and pressure of MBM 7, the dense cores appear not to be bound by gravity, and the whole cloud including the dense cores seem to be expanding. The distance to HLCs suggest that they belong to the galactic plane, since the scale height of the cloud is < or approximately equal to 100 pc. Compared to the more familiar dense dark clouds, HLCs may differ only in their small mass and low density, with their proximity reducing the filling factor and enhancing the contrast of the core and envelope structure.

A Search for Star Formation in the Translucent Cloud MBM 40

The star formation status of the translucent high-latitude molecular cloud, MBM 40, is explored through analysis of radio, infrared, optical, and X-ray data. With a peak visual extinction of 1 to 2 mag, MBM 40 is an example of a high-latitude cloud near the diffuse/translucent demarcation. However, unlike most translucent clouds, MBM 40 exhibits a compact morphology and a kinetic energy-to gravitational potential energy ratio near unity. Our radio data, encompassing the CO (J = 1-0), CS (J = 2-1), and H2CO 1(sub 11-1(sub 10), spectral line transitions, reveal that the cloud contains a ridge of molecular gas with n greater than or equal to 10(exp 3)/ cc. In addition, the molecular data, together with IRAS data, indicate that the mass of MBM 40 is approx. 40 solar mass. In light of the ever-increasing number of recently formed stars far from any dense molecular clouds or cores, we searched the environs of MBM 40 for any trace of recent star formation. We used the ROSAT All-Sky Survey X-ray data and a ROSAT PSPC pointed observation toward MBM 40 to identify 33 stellar candidates with properties consistent with pre-main-sequence (PMS) stars. Follow-up optical spectroscopy of the candidates with V less than 15.5 was conducted with the 1.5 m Fred Lawrence Whipple Observatory telescope in order to identify signatures of T Tauri or pre-main- sequence stars (such as the Li 6708 A resonance line). Since none of our optically observed candidates display standard PMS signatures, we conclude that MBM 40 displays no evidence of recent or ongoing star formation. The absence of high-density molecular cores in the cloud and the relatively low column density compared to star-forming interstellar clouds may be the principal reasons that MBM 40 is devoid of star formation. More detailed comparison between this cloud and other, higher extinction translucent and dark clouds may elucidate the necessary initial conditions for the onset of low-mass star formation.

Spectroscopy of possible Hα emission stars in regions of high galactic latitude molecular clouds

We present mid-resolution spectroscopic observations of 63 faint (V =12-16) stars identified by Kun (1992) as Hα emission candidates in an objective prism survey of several high galactic latitude molecular clouds. Only 4 stars in our sample (6%) are bona fide T Tauri stars on the basis of their strong Li I absorption and Hα emission features. They have late M spectral types (M4–M5.5) and two of them form a visual binary with separation . The new T Tauris are associated with the L 134 molecular complex, the northernmost extension of the Scorpio – Ophiuchus star forming region, which has been traditionally considered as a non-star forming region. In the other high latitude clouds surveyed by Kun (1992), we have not found any T Tauri star in our follow-up spectroscopy. Most of the observed stars (~80%) are late type dwarfs without detectable Hα emission in our spectra. Eight stars are M-dwarfs with Hα in emission, but no detectable Li I λ 6708 absorption. They could be post T Tauri stars or older dMe stars; more data is necessary in order to establish their evolutionary status. Our results show that Kun (1992) was able to detect weak Hα emission lines (down to equivalent width of ~ 1.5 A) in faint stars near the plate magnitude limit (V ~16), but in many cases plate defects, absorption bands and/or overlying stars were taken as possible Hα emission. In this paper we correct Kun's previous indication that there may be numerous young stars associated to high latitude molecular clouds, and we severely constrain the presence of a population of T Tauri stars in these clouds. We note that none of our four T Tauri stars is located in an isolated translucent molecular cloud.

Atmospheric carbon dioxide and early Eocene climate: A general circulation modeling sensitivity study

The early Eocene was the warmest time interval of the past 65 million years. We have investigated the hypothesis that the warm climate of the early Eocene may have been caused by higher-than-present levels of atmospheric carbon dioxide (CO 2 ). We have used an atmospheric general circulation model, the NCAR genesis model, coupled to a land surface model and a mixed layer ocean, to examine the sensitivity of early Eocene climate to varying levels of atmospheric CO 2 . Model cases contained a description of the early Eocene Earth and included either 300, 560, or 1680 ppm atmospheric CO 2 . Climate responses to increased atmospheric CO 2 include (1) increased mean annual global temperature by 2–6°C, and decreased latitudinal surface temperature gradients; (2) maximum warming at high latitudes, especially in the winter season; (3) increased mean global precipitation; (4) increased high latitude clouds. In comparison with geological paleoclimate data, the climate response to increased CO 2 does not appear to explain completely the early Eocene latitudinal surface temperature gradient; for example, with the 1680 ppm CO 2 case, high latitude oceans and continents are minimally warm enough to match paleoclimate data, while low latitude regions are overly warm when compared to sea surface temperature reconstructions. We conclude that CO 2 was very likely at least double the present level during the early Eocene and contributed to the overall character of early Eocene climate. However, consideration of increased poleward oceanic heat transport and/or other factors, may be required to produce model results that more closely agree with marine and terrestrial paleoclimate data, especially at low latitudes.

High altitude molecular clouds

A population of molecular clouds with a significantly greater scale height than that of Giant Molecular Clouds has been identified by examining maps of the latitude distribution of the $^{12}CO(1-0)$ emission in the first quadrant of the Galaxy. These clouds are found by identifying emission more than 2.6 times the scale-height away from the galactic midplane (centroid of CO emission) at the tangent points. Since the distance to the tangent points is known, we know the height and the sizes of these clouds. They are smaller and fainter than the GMCs and do not seem to be gravitationally bound. These clouds have properties similar to the high latitude clouds in the solar neighborhood. Although they lie outside the molecular cloud layer, the high altitude clouds are well within the HI layer in the Galaxy and coincide with distinct peaks in the HI distribution. These clouds represent a galaxy wide population of small molecular clouds having a larger scale height. They may be clouds in transition between molecular and atomic phases.

Atomic carbon in the high-latitude molecular cloud MBM 12

We report the first detection of neutral atomic carbon emission from a high-latitude cloud. (C I) 492 GHz emission was detected from seven (13)CO cores in MBM 12, the nearest known molecular cloud (d = 65 pc; b = -34 deg). There is strong evidence that the lines of (C I) and CO are formed in the same regions of the cloud. The average C/CO column density ratio is approximately 0.7, which is a factor of approximately 1.4-7 times higher than the same value measured toward dense Galactic photodissociation regions. Recent chemical models of translucent clouds with I<SUB>UV</SUB> = 1 Habing field can account for this ratio. Due to the proximity of MBM 12, we have discovered evidence for density clumps with sizes less than or approximately 2000 AU. The clumps contain H2 masses of order the mass of Jupiter.

Infrared cirrus, what have we learned?

IRAS discovered thermal emission from infrared cirrus in 1983 and COBE has extended observations of these interstellar dust clouds to additional IR wavelengths. IR studies of individual high galactic latitude clouds are combined with sensitive optical measurements of scattered light from the same clouds. At least 4 distinctly different types of grains are required to explain the complex relationships revealed by combining observations of various samples of clouds made on different spatial scales and in a number of spectral bands. In addition to the dominant 22 K component seen most prominently at 100 μm, many clouds are bright at 12 and 25 μm, indicating dust at temperatures well above 100 K. Optical studies suggest at least two more types of grains are needed to explain the measured blue and red colors and the highly variable ratio of blue light to l00 μm thermal emission that is observed. We show a new display of 100 μm cirrus on a global scale and we determine the power density in a typical cloud to spatial scales of a few arcsec.

Proto--Brown Dwarfs. I. Methods and Results for High-Latitude Clouds

view Abstract Citations (31) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Proto--Brown Dwarfs. I. Methods and Results for High-Latitude Clouds Pound, Marc W. ; Blitz, Leo Abstract We describe a method for finding substellar-mass self-gravitating clumps ("proto-brown dwarfs") in nearby molecular clouds, using millimeter-wave telescopes. This method represents a novel approach to looking for brown dwarfs and, for the nearest molecular clouds, has a greater mass sensitivity than traditional methods. We describe how the method can distinguish between various functional forms of the low-mass (<0.2 Msun) stellar initial mass function, which is currently poorly constrained. We present results of a search for proto-brown dwarfs in high galactic latitude molecular clouds. The clouds have relatively low densities [n(H2) ≲ 104 cm-3], are at an average distance of 100 pc, and are apparently not forming stars. We find little unambiguous evidence for self-gravitating objects of any mass in them, although there are clearly clumps with masses as low as 3 MJupiter. We find two clumps in MBM 12(=L1457), the nearest molecular cloud to the sun, which have luminous masses within a factor of 3 of that needed to be gravitationally bound. One of these has a dense core which appears to be in hydrostatic equilibrium. If so, then it is the first known self-gravitating core in a high-latitude cloud. The mean pressure in the clumps is P/k = 2.9 × 105 K cm-3, at least one order of magnitude higher than the general ISM pressure. We therefore conclude that either the clumps are transient structures in regions of typical ISM pressure, or the clouds themselves are short-lived objects in overpressured regions. In the latter case, the clouds will disperse in less than about 106 yr. The search for pro to-brown dwarfs is being expanded to include known star-forming regions in the Ophiuchus and Taurus complexes. The results in Ophiuchus and Taurus will tightly constrain the functional form of the stellar initial mass function at very low masses regardless of whether or not any proto-brown dwarfs are found. The present results already exclude a single power-law initial mass function with index ≳2. Publication: The Astrophysical Journal Pub Date: November 1993 DOI: 10.1086/173394 Bibcode: 1993ApJ...418..328P Keywords: ISM: CLOUDS; ISM: INDIVIDUAL ALPHANUMERIC: L1257; ISM: STRUCTURE; RADIO CONTINUUM: STARS; STARS: LOW-MASS; BROWN DWARFS full text sources ADS | data products SIMBAD (14) Related Materials (2) Erratum: 1995ApJ...444..487P Part 2: 1995ApJ...444..270P

IRAS 2306 + 1450 - A cold dust component in the nearby, high-latitude cloud MBM 55

view Abstract Citations (5) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS IRAS 2306+1450: A Cold Dust Component in the Nearby, High-Latitude Cloud MBM 55 Vallee, J. P. ; Koempe, C. Abstract The infrared source IRAS 230604+145055 is located in the nearby, high-latitude cloud MBM 55, and appeared to be associated with red- and blue-shifted CO line emission, extended in opposite directions from the source. In order to investigate the nature of this object, we have mapped the region around the IRAS source at 3.1 mm wavelength in the C-12(S-32) J = 2-1 molecular line with the 12-m telescope at Kitt Peak. The CS emission originates from several clumps which exhibit velocity shifts analogous to those of the CO emission. Attempts to detect continuum emission from the region at 2.0 cm wavelength using the C configuration of the Very Large Array telescope gave negative results. It is now established that the IRAS source is a 2 arcmin cirrus component, part of the larger MBM 55 complex. Publication: The Astronomical Journal Pub Date: October 1993 DOI: 10.1086/116747 Bibcode: 1993AJ....106.1561V Keywords: Astronomical Maps; Emission Spectra; Infrared Sources (Astronomy); Mass Distribution; Molecular Clouds; Astronomical Models; Carbon Monoxide; Line Spectra; Radio Astronomy; Astrophysics; ISM: CLOUDS; Surveys; Galaxies: optical; Positional data; Photometry: CCD full text sources ADS | data products SIMBAD (2)

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

Ultraviolet, optical, and infrared observations of the high-latitude molecular cloud toward HD 210121

Low-resolution UV spectra of the B3 V star HD 210121, located behind the high-latitude molecular cloud DBB 80, yield an extinction curve exhibiting a far-UV rise that is among the steepest known. The apparently simple line of sight affords an excellent opportunity for investigating the absorption and emission characteristics of a single, isolated interstellar cloud characterized by extreme UV extinction. The low ratios of the IRAS bands with respect to I(100 microns) suggest that the radiation field incident on the cloud is lower than the average interstellar field, with further attenuation of the field within the cloud. The apparent relative enhancement of I(12 microns) compared with models of dust emission, and the extremely steep far-UV extinction together are consistent with the presence of an enhanced population of very small grains; the normal calcium depletion suggests that there has been little wholesale grain destruction. The steep far-UV extinction may help to explain the relatively high abundances of CO and CN. The disagreement in density for this cloud inferred from C2 absorption versus that inferred from CO emission may be due in part to clumping in the gas sample by the radio beams.

Polarization measurements of stars in the region of the nearby molecular cloud MBM 12

Linear polarization measurements of stars in the region of the nearby high-latitude molecular cloud MBM 12 are presented. Stars behind the cloud show a polarization ∼ 2-3 per cent, while stars that are in front of the cloud or are projected outside the cloud boundary show very low values (≤0.5 per cent) of polarization. If the polarization is caused by dust grains aligned by a magnetic field, the observed polarization position angles suggest that the field in the cloud is more or less unidirectional and is roughly parallel to the long axis of the cloud

The effect of resolution on high latitude cloud track winds produced from AVHRR imagery

Sequences of images from geostationary satellites have been used for some time to track clouds and so produce an estimate of the wind at cloud height (see/1/ and /2/). However, in the polar regions the resolution of geostationary imagery is normally too poor to allow the tracking of cloud features. Fortunately at high latitudes the swaths from the imaging instruments aboard polar orbiting satellites overlap. This means that the NOAA series of polar orbiting satellites can be used to obtain cloud track winds in the polar regions using both automatic and manual tracking. The tracking is best if full resolution (1km) imagery is used rather than 4km Global Area Coverage imagery.

On the nature of the molecular cores in high-latitude cirrus clouds. II - Structure, stability, and physical conditions from CO-18) observations

Extensive observations of the J=2-1 and 1-0 lines of C 18 O, 13 CO, and 12 CO have been made of 33 high-latitude cirrus cloud cores and two high-latitude, cold, dense molecular clouds. It is argued that C 18 O is an essential observational tool for the understanding of molecular cloud structure. We have developed the formalism of molecular clouds both in polytropic hydrostatic equilibrium and also undergoing self-similar dynamical collapse, and we apply these to our C 18 O observations

Diffuse, Translucent &amp; High-Latitude Clouds: Theoretical Considerations

The relative importance of gas-phase, shock and grain-surface chemistry is discussed with reference to recent observational data. The relation between diffuse, translucent and high-latitude clouds is illustrated with models in which carbon is transformed from atomic (C+, C) to molecular (CO) form with increasing extinction.

CO J = 3-2 observations of translucent and high-latitude molecular clouds

view Abstract Citations (87) References (59) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CO J = 3→2 Observations of Translucent and High-Latitude Molecular Clouds van Dishoeck, Ewine F. ; Black, John H. ; Phillips, T. G. ; Gredel, R. Abstract Measurements of the CO J = 3→2 emission line at 345 GHz in translucent and high-latitude clouds are reported. Together with data on the J = 2→1 and J = 1→0 lines of both 12CO and 13CO they are used to constrain the densities in the clouds. The physical conditions in the high-latitude clouds are found to be very similar to those in the translucent clouds. The CO emission data complement optical absorption line observations the C2 molecule in the same region, which provide independent constraints on temperature and density. The densities derived from the 12CO 1→0/3→2 ratios are often higher than those inferred from the C2 excitation, whereas densities derived from the 3→2/13CO 1→0 ratios are found to be very similar to those based on C2. Several explanations for these differences are discussed, including the possibility that the CO and C2 molecules are distributed differently throughout a cloud. Various "clumpy" cloud models have been developed to explore this question. No CO J = 3→2 emission has been detected from the classical diffuse cloud toward ζOph. The very low limit, T*Λ(3→2) ≤ 0.08 K, excludes high-density (nH > 700 cm-3) models for this line of sight. Evidence is found for a slight decrease in the J = 1→0/3→2 and 2→1/3→2 line intensity ratios from the edge to the center of one cloud, which can be explained by a corresponding increase in the density into the center. The CO lines often show complex profiles consisting of several narrow (ΔV ≤1 km s-1) components; in some cases the profiles show centrally reversed line cores that can be interporeted in terms of gradients in density, temperature, and molecular abundance within a single kinematic component. Publication: The Astrophysical Journal Pub Date: January 1991 DOI: 10.1086/169547 Bibcode: 1991ApJ...366..141V Keywords: Carbon Monoxide; Interstellar Matter; Molecular Clouds; Abundance; Astronomical Models; Astronomical Spectroscopy; Emission Spectra; Line Spectra; Astrophysics; HIGH-LATITUDE OBJECTS; INTERSTELLAR: MOLECULES full text sources ADS | data products SIMBAD (37)