High-latitude Clouds Research Articles

Parameterization of effective ice particle size for high‐latitude clouds

AbstractA parameterization has been developed for mean effective size Dge in terms of ice water content (IWC) and temperature using in situ measurements of ice crystal spectra, cloud particle shapes and particle cross‐sectional area A from four research projects conducted in latitudes north of 45° N. The cloud microphysical measurements were made using PMS 2D optical probes, a PMS forward scattering spectrometer probe (FSSP), and Nevzorov total water and liquid water content probes. The IWCs derived from particle spectra using three different methods were compared with IWC measured with the Nevzorov probe (IWCNev). The contribution of small particles to the total mass was estimated by integrating a gamma distribution function that was fitted to match the measured FSSP concentrations. The Dge was calculated from the derived IWC and total cross‐sectional area per unit volume Ac.This analysis indicates that there are significant differences among the schemes used to derive the IWC. It was found that the IWC derived based on the Cunningham scheme and IWCNev have the highest correlation: r2 = 0.78. After considering small particles, the derived IWC almost matched the IWCNev. The average estimated contribution of small particles to the Ac was 43%. The average estimated contribution of small particles to the total IWC, however, was 20%. Since Dge is directly proportional to the ratio IWC/Ac, the addition of small particles reduced the derived Dge considerably. The largest changes in Dge associated with small particles, however, occur at the coldest temperature and at low IWC, reaching up to 45% for temperatures less than −25° C. Generally, Dge and IWC increase with increasing temperature. Good agreement between the parameterized Dge and derived Dge from measurements were found when small particles were included. Copyright © 2002 Royal Meteorological Society.

CO OBSERVATIONS OF A HIGH LATITUDE CLOUD MBM 40 WITH A HIGH RESOLUTION AUTOCORRELATOR

We have mapped 1 <TEX>$deg^2$</TEX> region toward a high latitude cloud MBM 40 in the J = 1 - 0 transition of <TEX>$^{12}CO$</TEX> and <TEX>$^{13}CO$</TEX>, using the 3 mm SIS receiver on the 14 m telescope at Taeduk Radio Astronomy Observatory. We used a high resolution autocorrelator to resolve extremely narrow CO linewidths of the molecular gas. Though the linewidth of the molecular gas is very narrow (FWHP < 1 km <TEX>$s^{-1}$</TEX> ), it is found that there is an evident velocity difference between the middle upper part and the lower part of the cloud. Their spectra for both of <TEX>$^{12}CO$</TEX> and <TEX>$^{13}CO$</TEX> show blue wings, and the position-velocity map shows clear velocity difference of 0.4 km <TEX>$s^{-1}$</TEX> between two parts. The mean velocity of the cloud is 3.1 km <TEX>$s^{-1}$</TEX>. It is also found that the linewidths at the blueshifted region are broader than those of the rest of the cloud. We confirmed that the visual extinction is less than 3 magnitude, and the molecular gas is translucent. We discussed three mass estimates, and took a mass of 17 solar masses from CO integrated intensity using a conversion factor <TEX>$2.3 {\times} 10^{20} cm^{-2} (K\;km s^{-1})^{-1}$</TEX>. Spatial coincidence and close morphological similarity is found between the CO emission and dust far-infrared (FIR) emission. The ratio between the 100 f.Lm intensity and CO integrated intensity of MBM 40 is 0.7 (MJy/sr)/(K km <TEX>$s^{-1}$</TEX>), which is larger than those of dark clouds, but much smaller than those of GMCs. The low ratio found for MBM 40 probably results from the absence of internal heating sources, or significant nearby external heating sources.

Detailed Structure of Low‐Density Molecular Gas in High‐Latitude Clouds

Extensive strip-scan observations of three high-latitude clouds— MBM 32, MBM 54, and MBM 55—were carried out in 12CO J = 1-0 emission with an angular resolution of 15'', or 0.0072 pc (1500 AU) at 100 pc. We resolved clumps with sizes from ~0.2 pc down to ~0.02 pc. Most of them have low gas density (≲102 cm-3) and sizes significantly smaller than their corresponding Jeans lengths. Some of them are well-isolated from the rest of the molecular components, suggesting their (quasi-) stable dynamical state. They provide a clear case for structure formation not caused by gravitational contraction and are most likely formed through thermal instability in interstellar shocks.

A catalogue of dust clouds in the Galaxy

In this study 21 catalogues of dust clouds in the Galaxy were cross-identified by taking into account available properties such as position, angular dimensions, opacity class and velocity. An initial list of $\approx$6500 entries was condensed into a cross-identified all-sky catalogue containing 5004 dust clouds. In particular, the transition zone between high and low Galactic latitude studies was also cross-identified. The unified catalogue contains 525 high-latitude clouds. The catalogue deals primarily with optical dark nebulae and globules, but it includes as well substantial information from their molecular counterparts. Some previously uncatalogued clouds were detected on optical images and FIR maps. Finally, we address recent results and prospective work based on NIR imaging, especially for clouds detected in the 2MASS $K_s$ Atlas.

A Survey for High-Latitude Molecular Clouds toward Infrared-Excess Clouds with NANTEN

Abstract A survey for high Galactic latitude molecular clouds was carried out toward the far-infrared-excess clouds of Reach et al. (1998, AAA 070.131.207) using the NANTEN telescope. All 68 infrared-excess clouds that are observable from NANTEN were mapped in the $J = 1 \hbox{--} 0$ line of 12CO. CO emission was detected from 32 infrared excess clouds, corresponding to a CO detection rate of 47%. Most of these CO clouds were identified and mapped as high-latitude clouds for the first time. The CO detection rate for the cold ($T_\mathrm{dust} \lt 17 \,\mathrm{K}$) infrared excess clouds is 72%, which is more than a factor of two higher than that of warmer ones, 33%. This indicates that the cold clouds are well shielded from external UV radiation, resulting in a high CO abundance and a low temperature of the clouds. The infrared-excess clouds with no CO emission are most likely to be molecular hydrogen clouds because the temperature is similar to, or lower than, that of the surrounding H i gas. The molecular gas without CO emission seems to occupy more than 90% of the area of the infrared-excess clouds.

Protoplanetary Disks in the Nearest Star-Forming Cloud: Mid-Infrared Imaging and Optical Spectroscopy of MBM 12 Members

The recent identification of several groups of young stars within 100 pc of the Sun has generated widespread interest. Given their proximity and possible age differences, these systems are ideally suited for detailed studies of star and planet formation. Here we report on the first investigation of protoplanetary disks in one such group, the high-latitude cloud MBM 12 at a distance of ~65 pc. We present mid-infrared observations of the eight candidate pre-main-sequence (PMS) members and the two main-sequence (MS) stars in the same line of sight, which may or may not be associated with the group. We have also derived Hα and Li line widths from medium-resolution optical spectra. We report the discovery of significant mid-infrared excess from six PMS stars—LkHα 262, LkHα 263, LkHα 264, E02553+2018, RX J0258.3+1947, and S18—presumably due to optically thick circumstellar disks. Our flux measurements for the other two PMS stars and the two MS stars are consistent with photospheric emission, allowing us to rule out dusty inner disks. The disks we have found in MBM 12 represent the nearest known sample of very young protoplanetary systems and thus are prime targets for high-resolution imaging at infrared and millimeter wavelengths.

STIS and GHRS Observations of Warm and Hot Gas Overlying the Scutum Supershell (GS 018−04+44)

We present Space Telescope Imaging Spectrograph (STIS) and Goddard High Resolution Spectrograph (GHRS) observations of interstellar UV absorption toward HD 177989, a B0 III star at 4.9 kpc in the direction l = 178, b = -119. The line of sight passes through the high-latitude ejecta of the Scutum supershell (GS 018-06+44), which is ~5° in diameter extending ~7° below the Galactic plane at a kinematic distance of ~3.5 kpc in the Scutum spiral arm. The observations with the STIS E140H and GHRS echelle B gratings provide far- and middle-UV spectra at resolutions (FWHM) of ~3 km s-1 and a signal-to-noise ratio (S/N) of ~20:1 to 50:1. The observations reveal strong and broad absorption in the lines of Si IV and C IV centered on LSR velocities of +18 and +42 km s-1 and weaker absorption from these ions near -50 and -13 km s-1. Weak absorption by N V extends over the full velocity range traced by Si IV and C IV. The +18 km s-1 high-ionization absorption likely occurs in gas ~400 pc below the Sagittarius spiral arm, while the extremely strong +42 km s-1 absorption occurs in highly ionized gas in the Scutum supershell at a distance of ~700 pc below the Galactic plane. The properties of the highly ionized gas associated with the Scutum supershell are similar to the gas found in radio loops I and IV; in both cases there is a strong enhancement in the column density of C IV without a corresponding increase in the column density of N V, which causes N(C IV)/N(N V) to be among the largest measured in the interstellar medium. The low-ionization absorption lines of N I, S II, Si II, and Fe II produce narrow absorption features at +37, +40 km s-1 and +55, +60 km s-1. The strength and kinematic properties of these absorption features bear no resemblance to those expected for the high-latitude neutral cloud seen in the H I 21 cm line. This may be due to the relatively low angular resolution (FWHM ~21') of the 21 cm observations. The kinematic relationships among the high-ionization and low-ionization absorption lines observed in the UV suggest a related origin in a hot-warm gas interface region. We are possibly seeing the warm gas in the swept-up shell surrounding a region where hot gas is being vented into the halo. In the warm gas, N(N I)/N(S II) ~0.01 solar, which implies a similar value for N(H0)/[N(H0) + N(H+)]. The warm neutral and ionized gas in the matter overlying the Scutum supershell has values of Si /S and Fe /S roughly similar to those found in the warm neutral medium of the Galactic disk in the vicinity of the Sun. While there has been grain processing in the ejecta of the Scutum supershell, the processing has not been complete. Based on observations of interstellar C IV and Si IV at high S/N and high resolution toward four very distant stars, we determine that highly ionized gas absorption components occur at a frequency of ~1 component kpc-1. The strongest components are associated with lines of sight that pass over or under spiral arms or that pass though Galactic supershells.

Physical State of Molecular Gas in High Galactic Latitude Translucent Clouds

The rotational transitions of carbon monoxide (CO) are the primary means of investigating the density and velocity structure of the molecular interstellar medium. Here we study the lowest four rotational transitions of CO toward high-latitude translucent molecular clouds (HLCs). We report new observations of the J = (4-3), (2-1), and (1-0) transitions of CO toward eight high-latitude clouds. The new observations are combined with data from the literature to show that the emission from all observed CO transitions is linearly correlated. This implies that the excitation conditions that lead to emission in these transitions are uniform throughout the clouds. Observed 13CO/12CO (1-0) integrated intensity ratios are generally much greater than the expected abundance ratio of the two species, indicating that the regions that emit 12CO (1-0) radiation are optically thick. We develop a statistical method to compare the observed line ratios with models of CO excitation and radiative transfer. This enables us to determine the most likely portion of the physical parameter space that is compatible with the observations. The model enables us to rule out CO gas temperatures greater than ~30 K, since the most likely high-temperature configurations are 1 pc-sized structures aligned along the line of sight. The most probable solution is a high-density and low-temperature (HDLT) solution, with volume density, n = 104.5±0.5 cm-3, kinetic temperature, Tk ≈ 8 K, and CO column density per velocity interval NCO/ΔV = 1016.6±0.3 cm-2/( km s-1). The CO cell size is L ~ 0.01 pc (~2000 AU). These cells are thus tiny fragments within the ~100 times larger CO-emitting extent of a typical high-latitude cloud. We discuss the physical implications of HDLT cells, and we suggest ways to test for their existence.

Multiline CO observations of MBM 32

We present a detailed study of the high latitude cloud MBM 32. Observations were made in the , (2-1), and (3-2) transitions of 12 CO and in 13 CO(1-0) and (2-1). These data were complemented by 21 cm Hi data and by IRAS 60 and 100 μ m data. Our data show that MBM 32 consists of a main cloud component at km s-1 (mass about 16.9 ), and a smaller component at km s-1 (4.1 ), in addition to some emission in between those two velocities ( CO(CO(1-0) is lower in the cloud center. This suggests equal excitation conditions through MBM32 and line temperatures determined through clump filling factors. All CO components show associated Hi emission, but small velocity differences of typically 1 km s-1 exist between the Hi and CO gas. The mass of associated Hi gas is similar to the molecular mass for all components. There is a good correlation between FIR, CO, and Hi emission. The dust mass is about 0.073 , and the ratio of gas and dust mass (280) is lower than found from similar (IRAS) data for denser clouds, suggesting that the amount of dust colder than 20 K is relatively small. We subdivide the CO data cubes in Gaussian shaped clumps; 40-50% of the CO emission can be assigned to the larger clumps. The remaining emission comes from either more extended clumps or from overlapping unresolved clumps.

Observational Evidence of Supershell Blowout in GS 018−04+44: The Scutum Supershell

Emission in the H I 21 cm line has been mapped for a region of the Galaxy that includes two known supershells, GS 018(04)44 and GS 034(06)65. We focus on the GS 018(04)44, hereafter referred to as the Scutum Supershell, which is an elongated shell about 5i in diameter extending to (7i below the Galactic plane. The Scutum shell lies at a kinematic distance of D3300 pc, implying a shell diameter of D290 pc with a vertical extension of D400 pc away from the Galactic plane. The Scutum shell con- tains 6.2 ) 105 swept into the walls. We observe that the top of the shell is missing, and a substan- M _ tial column of H I rises from the shell walls to b \( 11i, culminating in a large cloud of neutral hydrogen, 3.74 ) 104 located D630 pc from the plane. ROSAT data show X-ray emission that M _ , closely anticorrelates with the 21 cm emission. This emission probably originates from hot gas within the Scutum Supershell. After approximately correcting for the foreground absorption, we —nd that the 1.5 keV X-rays peak at the base of the shell, the 0.75 keV emission peaks in the interior and at the top of the shell, and the 0.25 keV emission extends to high latitudes above the shell. The X-ray luminosity is roughly D5 ) 1036 ergs s~1. The Wisconsin H-Alpha Mapper (WHAM) survey shows the presence of Ha emission that exhibits a morphology similar to that of the H I. Spectra indicate the presence of ionized hydrogen at velocities similar to the H I, placing ionized material at the same kinematic distance as the neutral material. IRAS images in the 60 and 100 km wavebands reveal the presence of dust corre- lated with the neutral hydrogen. Infrared surface brightness indicates an excess in the 100 km emission, which could indicate a molecular hydrogen component with a column density of 2.4 ) 1021 cm~2 in the densest regions of the high-latitude cloud of neutral hydrogen. IUE ultraviolet high dispersion spectra of HD 177989 and HD 175754 reveal the presence of very (l 17i.89, b \( 11i.88) (l 16i.40, b \( 9i.92) strong absorption by highly ionized gas at a velocity that associates the absorption with the ejecta of the Scutum Supershell. In the case of HD 177989, the high ion column density ratios suggest an origin in a turbulent mixing layer where hot and cool gases mix in the presence of shear —ows. The Ha and X-ray emission suggest that a multitude of energetic phenomena exist in this region, providing the necessary ionizing radiation. Indeed, there are multiple supernova remnants, H II regions, and hot stars, which could all contribute sizeable amounts of energy and ionizing radiation. The combination of these data sets indicates observational evidence of a ii blowout ˇˇ phenomena whereby hot material produced within the Scutum Supershell has blown through the top of the shell and been pushed to high latitude. Subject headings: Galaxy: structureISM: bubblesISM: individual (Scutum Supershell) ¨ ISM: structureradio lines: ISMsupernova remnants

A Dark-Cloud Complex in Aquila: Small Molecular Clouds Possibly Associated with the Aquila Rift

Abstract A 12CO(J = 1−0) survey for local molecular clouds was performed toward dark clouds in Aquila (26° &amp;lt; l ≤ 42° and −25° ≤ b &amp;lt; −2°) by using the 4-meter millimeter wave telescope, NANTEN, at Las Campanas Observatory, Chile. A cloud complex consisting of 64 small clouds has been discovered in −25° ≲ b ≲ −12°; at a distance of 220 pc, its height from the galactic plane is ∼ 50–100 pc and the total mass is ∼ 4 × 103M⊙. The spatial and velocity distributions of the complex suggest that it may be connected to the Great Rift in Aquila. This complex, as a whole, has a significantly large virial mass compared with the mass derived from the CO intensities by an order of magnitude, though H I gas of ∼ 104M⊙, possibly associated, may contribute to bind them gravitationally. The individual CO clouds have velocity dispersion and mass similar to those of the high-latitude clouds; also, the clouds are not in gravitational equilibrium. There is no indication of active star formation.

Distances to the High Galactic Latitude Molecular Clouds G192−67 and MBM 23–24

We report on distance determinations for two high Galactic latitude cloud complexes, G192-67 and MBM 23-24. No distance determination exists in the literature for either cloud. Thirty-four early type stars were observed towards the two clouds, more than half of which have parallaxes measured by the Hipparcos satellite. For the remaining stars we have made spectroscopic distance estimates. The data consist of high resolution echelle spectra centered on the Na I D lines, and were obtained over six nights at the Coude Feed telescope at Kitt Peak National Observatory. Interstellar absorption lines were detected towards some of the stars, enabling estimates of the distances to the clouds of 109 +/- 14 pc for G192-67, and of 139 +/- 33 pc for MBM 23-24. We discuss the relationship of these clouds to other ISM features such as the Local Hot Bubble and the local cavity in neutral hydrogen.

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.

Star Formation at High Galactic Latitude: A Case for Extensions to the Hα Survey

AbstractThe AAO/UKST Hα Survey should be extended to high Galactic latitude (∣b∣ &gt; 25°) to search for T Tauri stars. The Hα Survey can contribute to a complete inventory of young stellar objects in high-latitude clouds, which will help define the limits of conditions and processes that lead to star formation.

The Attenuation of Sunlight by High-Latitude Clouds: Spectral Dependence and Its Physical Mechanisms

Abstract Measurements of the ground-level solar irradiance from Palmer Station, Antarctica, and Ushuaia, Argentina, reveal a systematic wavelength dependence in the attenuation provided by cloudy skies. As wavelength increases from 350 to 600 nm, the measured cloudy-sky irradiance, expressed as a fraction of the clear-sky value, decreases. Results from Ushuaia for a solar zenith angle of 45° show that a cloudy sky that reduces the spectral irradiance at 500 nm to 50% of that for clear skies is accompanied by irradiances at 350 and 600 nm, which are approximately 59% and 49%, respectively, of the clear sky value. A weaker wavelength dependence appears in the data for Palmer Station. The observed behavior can arise from Rayleigh backscattering of sunlight beneath the cloud, followed by reflection of this upwelling radiation from the cloud base back to the ground. This sequence of events is most effective at short wavelengths and leads to cloudy skies providing less overall attenuation as wavelength decreases.

A Sensitive Search for Molecular Gas in High‐Velocity Clouds

We report on observations of the J = 1-0 line of (CO)-C-12 at 115 GH, toward several high-velocity H I clouds (HVCs). High angular resolution observations at 21 cm show the presence of dense cores with N(H I) = (2-4) x 10(20) cm(-2) and n(H I) = 30-800(kpc)(-1) cm(-3). We observed the CO line toward six of the densest HVC cores. No (CO)-C-12 emission is detected down to a best 5 sigma limit of 0.077 K km s(-1). Using the median I(CO) to N(H-2) conversion factor appropriate for high-latitude translucent clouds, this limit corresponds to a column density limit for N(H-2) of 9 x 10(18) cm(-2). We calculate the expected value of N(H-2) assuming a standard formation mechanism for the molecules and accounting for the run of the interstellar radiation held with the assumed distance to the HVC. The nondetections then indicate that the observed HVCs are more distant than similar to 2 kpc. This is consistent with the results of interstellar absorption line studies, which yield lower limits of 2-5 kpc for most of the observed HVCs. The nondetections also suggest that HVC cores are not likely to be current sites of star formation in the halo.

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

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

X‐Ray Shadows by High‐latitude Molecular Clouds. I. Cartography

We have observed X-ray shadowing by nine nearby high-latitude molecular clouds with the ROSAT PSPC. The $f {1}{4} $ --> keV emission from the Local Bubble is used to calculate an average electron density, ne, toward all of our sample clouds. Given the uniformity of emission within the Local Bubble, and previously determined cloud distances, one can determine whether the clouds are within or without the Local Bubble. We confirm that three of the sample clouds, MBM 12, MBM 16, and possibly MBM 20, which have been thought to be within the Local Bubble, are actually within. A combination of $f {1}{4} $ --> keV and ne measures suggest that MBM 6 and LDN 1563 may also lie within the Local Bubble or its boundary layer. The shadows cast by the clouds at $f {3}{4} $ --> keV and 1.5 keV imply little if any foreground emission in those bands, and produce lower limits for the distance to the hotter emission component responsible for the observed flux. The results are in agreement with previous estimates for the distances to the Loop I Bubble and the Eridion Bubble.

Atomic Carbon in Southern Hemisphere High‐Latitude Clouds

We report the detection of atomic carbon in a sample of eight southern hemisphere high Galactic latitude molecular clouds, using the Antarctic Submillimeter Telescope and Remote Observatory. The 492 GHz (3P1 →3P0) transition of [C I] was detected in all of the clouds observed. The C/CO column density ratio ranges from 0.4 to 2.5 and is similar to the values previously measured in high-latitude clouds MBM 12 and HD 210121. For all 10 high-latitude clouds observed in [C I], C/CO averages ~1.2 and decreases with increasing total gas column density NH, as predicted by translucent cloud models. Quantitative comparison with chemical models of homogeneous clouds is unsatisfactory, however, and we conclude that the clumpy structure of clouds must be taken into account in order to interpret the data properly.

The Structure of the Galactic Magnetic Field toward the High‐Latitude Clouds

We present the results of an optical polarization survey toward the galactic anticenter, in the area 5(h) greater than or equal to alpha greater than or equal to 2(h) and 6 degrees less than or equal to delta less than or equal to 12 degrees. This region is characterized by the presence of a stream of high-velocity H I as well as high galactic latitude molecular clouds. We used our polarization data together with 100 mu m IRAS maps of the region to study the relation between the dust distribution and the geometry of the magnetic field. We find that there is a correlation between the percent polarization and the 100 mu m flux such that P(%) less than or equal to (0.16 +/- 0.05)F-100. When the IRAS flux is converted into H I column densities this becomes P(%) less than or equal to (0.13 +/- 0.03)N-20 - 0.22, which is consistent with previous interstellar medium studies on the relation between reddening and polarization. This correlation indicates that our survey is as deep as IRAS and that the magnetic field geometry does not change strongly with the optical depth in the lines of sight that we have studied. The implied lower limit to the distance of our survey is 500-700 pc at galactic latitudes b = -20 degrees and 100 pc at b = -50 degrees. Our main finding is that the magnetic field is perpendicular to the H I high-velocity stream as well as the molecular cloud MBM 16 in the high-latitude region. The field is also perpendicular to the velocity gradient in the stream. Closer to the plane, the magnetic field is parallel to the dust filaments that extend like a plume toward the halo. Our observations indicate that the galactic magnetic field toward the high-latitude clouds is toroidal. We propose a model in which flux tubes that rise out of the galactic plane become force-free and predominantly toroidal at high latitudes. The high-latitude clouds may be gas streams that are falling back toward the galactic plane within such buoyant braided ropes of magnetic flux.