Draco Nebula Research Articles

CO in the Draco nebula: the atomic–molecular transition

ABSTRACT This paper presents maps of the J=2-1 transition of CO towards the Draco nebula intermediate-velocity cloud (IVC). The maps cover 8500 square arcmin with a velocity resolution of 0.33 km s$^{-1}$ and angular resolution of 38 ${\rm arcsec}$, or 0.11 pc at the cloud distance of 600 pc. The mapped area includes all the emission detected by the Herschel satellite with 250 $\mu$m intensity >5 MJy sr−1. Previously published observations of the far-infrared (far-IR) emission and the 21 cm line of HI are used to derive the column density distribution of H$_2$ and the abundance ratio CO/H$_2$, as well as the distribution of the molecular fraction of hydrogen, which approaches 90 per cent over much of the brighter parts of the nebula. The CO emission is highly clumpy and closely resembles the structures seen in far-IR images. The kinematics of the CO show supersonic motions between clumps but near-thermal to trans-sonic motions within clumps, consistent with model predictions that the scale length for dissipation of supersonic turbulence should be $\sim 0.1$ pc, mediated by kinematic viscosity and/or ambipolar diffusion. Different parts of the nebula show evidence for a spread of molecular formation time-scales of a few 10$^5$ yr, comparable with the dynamical time-scale of the infalling gas. The IVC will likely merge with the Galactic interstellar medium in $\sim 10^7$ yr, and the densest clumps may form an unbound cluster of low-mass stars.

Diagnostics from Polarization of Scattered Optical Light from Galactic Infrared Cirrus

We propose polarization of scattered optical light from intermediate Galactic latitude infrared cirrus as a new diagnostic to constrain models of interstellar dust and the anisotropic interstellar radiation field (aISRF). For single scattering by a sphere, with Mie scattering phase functions for intensity and polarized intensity for a dust model at a given wavelength (Sloan r and g bands), and with models of anisotropic illumination from the entire sky (represented in HEALPix), we develop the formalism for calculating useful summary parameters for an integrated flux nebula (IFN): the average of the phase function weighted by the illumination, polarization angle (ψ), and polarization fraction (p). To demonstrate the diagnostic discrimination of polarization from scattered light, we report on the effects of different anisotropic illumination models and different dust models on the summary parameters for the Spider IFN. The summary parameters are also sensitive to the IFN location, as we illustrate using FRaNKIE illumination models. For assessing the viability of dust and aISRF models, we find that observations of ψ and p of scattered light are indeed powerful new diagnostics to complement joint modeling of the intensity of scattered light (related to the average phase function) and the intensity of thermal dust emission. However, optically thin IFNs that can be modeled using single scattering are faint and p is not large, as it could be with Rayleigh scattering, and so these observations need to be carried out with care and precision. Results for the Draco nebula compared to the Spider illustrate the challenge.

Structure formation in a colliding flow: The Herschel view of the Draco nebula

Context. The Draco nebula is a high Galactic latitude interstellar cloud observed at velocities corresponding to the intermediate velocity cloud regime. This nebula shows unusually strong CO emission and remarkably high-contrast small-scale structures for such a diffuse high Galactic latitude cloud. The 21 cm emission of the Draco nebula reveals that it is likely to have been formed by the collision of a cloud entering the disk of the Milky Way. Such physical conditions are ideal to study the formation of cold and dense gas in colliding flows of diffuse and warm gas. Aims. The objective of this study is to better understand the process of structure formation in a colliding flow and to describe the effects of matter entering the disk on the interstellar medium. Methods. We conducted Herschel-SPIRE observations of the Draco nebula. The clumpfind algorithm was used to identify and characterize the small-scale structures of the cloud. Results. The high-resolution SPIRE map reveals the fragmented structure of the interface between the infalling cloud and the Galactic layer. This front is characterized by a Rayleigh-Taylor (RT) instability structure. From the determination of the typical length of the periodic structure (2.2 pc) we estimated the gas kinematic viscosity. This allowed us to estimate the dissipation scale of the warm neutral medium (0.1 pc), which was found to be compatible with that expected if ambipolar diffusion were the main mechanism of turbulent energy dissipation. The statistical properties of the small-scale structures identified with clumpfind are found to be typical of that seen in molecular clouds and hydrodynamical turbulence in general. The density of the gas has a log-normal distribution with an average value of 103 cm-3. The typical size of the structures is 0.1−0.2 pc, but this estimate is limited by the resolution of the observations. The mass of these structures ranges from 0.2 to 20 M⊙ and the distribution of the more massive structures follows a power-law dN/ dlog (M) ~ M-1.4. We identify a mass-size relation with the same exponent as that found in molecular clouds (M ~ L2.3). On the other hand, we found that only 15% of the mass of the cloud is in gravitationally bound structures. Conclusions. We conclude that the collision of diffuse gas from the Galactic halo with the diffuse interstellar medium of the outer layer of the disk is an efficient mechanism for producing dense structures. The increase of pressure induced by the collision is strong enough to trigger the formation of cold neutral medium out of the warm gas. It is likely that ambipolar diffusion is the mechanism dominating the turbulent energy dissipation. In that case the cold structures are a few times larger than the energy dissipation scale. The dense structures of Draco are the result of the interplay between magnetohydrodynamical turbulence and thermal instability as self-gravity is not dominating the dynamics. Interestingly they have properties typical of those found in more classical molecular clouds.

TheSpitzer Space TelescopeFirst Look Survey: Neutral Hydrogen Emission

The Spitzer Space Telescope (formerly SIRTF) extragalactic First Look Survey covered about 5 deg2 centered on (J2000.0) ? = 17h18m, ? = 59?30' in order to characterize the infrared sky with high sensitivity. We used the 100 m Green Bank Telescope to image the 21 cm Galactic H I emission over a 3? ? 3? square, covering this position with an effective angular resolution of 98 and a velocity resolution of 0.62 km s-1. In the central square degree of the image the average column density is N = 2.5 ? 1020 cm-2 with an rms fluctuation of 0.3 ? 1020 cm-2. The Galactic H I in this region has a very interesting structure. There is a high-velocity cloud, several intermediate-velocity clouds (one of which is probably part of the Draco Nebula), and narrow-line low-velocity filaments. The H I emission shows a strong and detailed correlation with dust. Except for the high-velocity cloud, all features in the NH I map have counterparts in an E(B - V) map derived from infrared data. Relatively high E(B - V)/NH I ratios in some directions suggest the presence of molecular gas. The best diagnostic of such regions is the peak H I line brightness temperature, not the total NH I: directions where Tb > 12 K have E(B - V)/NH I significantly above the average value. The data corrected for stray radiation have been released via the World Wide Web.

Neutral Atomic Carbon in Intermediate‐Velocity Clouds

Using the Heinrich-Hertz-Telescope we have detected for the —rst time the [C I] line tran3P 1 ] 3P 0 sition at 492 GHz of neutral atomic carbon from two intermediate-velocity clouds (IVCs): IVC 135]54[46 and the Draco Nebula. Both clouds are located at high Galactic altitudes, more than 200 pc above the Galactic plane. The width of the [C I] line is quantitatively comparable to that of the 12CO rather than to that of the 13CO line, as commonly found for Galactic plane clouds. This indicates that neutral atomic carbon within the diUuse medium at high Galactic altitudes —lls a signi—cantly larger volume in comparison to the situation in dark or denser cirrus clouds close to the Galactic plane. Moreover, our observations suggest that for IVCs cooling by the [C I] line is as important as or even more important than that by the low level CO lines. PDR models used to approximate the observed line intensity ratios indicate that the molecular gas in high-altitude IVCs is exposed to a weak interstellar radiation —eld while its density must be high. . . . . ..

Single-dish and aperture synthesis H I observations towards the Draco nebula. I. The data

We describe 21-cm line observations of the Draco nebula region and present the results as channel maps. These observations have been performed with the Effelsberg 100-m telescope, the Westerbork Synthesis Radiotelescope, and the Very Large Array. The measurements reveal the full extent of the cloud as well as the detailed morphological and kinematical structure of several subareas. A list of continuum point sources found in the aperture synthesis maps is included.

Soft X-ray and H I surveys of the low N[SUB]H[/SUB] region in Ursa Major

A region of almost 300 deg squared in Ursa Major around the direction of the lowest neutral hydrogen column density in the sky has been surveyed in the 1/4 keV X-ray band (approximately 0.12 to 0.284 keV at 10% of the peak response) using the ROSAT position sensitive proportional counter (PSPC) and has been fully mapped in the 21 cm line of neutral hydrogen with an angular resolution of 21' or better. We present maps of these two data sets and an analysis of the spatial correlation between them. Over the entire field, the 1/4 keV band X-ray count rate shows a strong negative correlation with the column density of Galactic neutral hydrogen. We discuss the local, extragalactic, and halo contributions to the observed diffuse X-ray intensity and derive an upper limit to the Galactic X-ray emission originating beyond this H I distribution that is more than a factor of 7 smaller than the inferred flux beyond the Draco nebula, an apparent halo object located approximately 41 degs away. This implies that while there may be significant amounts of hot gas in the Galactic halo, its properties must vary strongly across the sky.

Neutral clouds in a pervasive X-ray gas between the sun and northern HVCs

We present evidence that the soft X-ray distribution observed with the ROSAT PSPC instrument is not adequately explained by the standard Local Hot Bubble model (/1/). We discuss the X-ray absorbing cloud LVC 88 + 36 − 2 embedded in the hot plasma of the Local Hot Bubble, the X-ray shadow of the Draco nebula and other clouds inside and outside the galactic disk, and the X-ray emission associated with halo type objects like the HVC's M I and M II. They populate the distance range from about 60 pc to more than a few kpc and imply the presence of X-ray emitting plasma between the sun and and the outer galactic halo. These observations are consistent with a pervasive X-ray emitting plasma in which neutral clouds are embedded. However, the volume filling factor of this plasma is not known. A model which adequately describes the observed features has been developed and published by Hirth et al. (/28/).For the first time in the literature we present results of a correlation analysis of X-ray shadows and H I or IR images of a molecular cloud. This is a new technique for the determination of the total column density of hydrogen nuclei for molecular clouds.

The nature of unidentified far-infrared point sources

view Abstract Citations (16) References (50) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Nature of Unidentified Far-Infrared Point Sources Reach, William T. ; Heiles, Carl ; Koo, Bon-Chul Abstract Sources discovered by the Infrared Astronomical Satellite with spectra that rise steeply from 60 to 100 microns are a mixed sample of external galaxies, condensations in the interstellar medium, and potentially unknown astronomical objects. Sources in regions of low total H I column density (as determined with the all-sky 21 cm line surveys), and with no nearby cataloged external galaxy, were observed for presence of 20 cm continuum emission (with the NRAO Very Large Array), enhanced 21 cm H I line emission (with the NAIC Arecibo 305 m), and molecular line emission (with the NRAO 12 m, HCRO 6 m, and FCRAO 14 m). Based on the CO detection rate, 70% of our sample are small interstellar molecular clouds. The remainder are mostly faint external galaxies. The dust column densities of the interstellar condensations were determined from the infrared flux, using a spherical-cloud model for the radiative transfer of the interstellar radiation field (ISRF). The CO column densities were determined using the large-velocity-gradient model for the excitation and radiative transfer of the rotational transitions. About half of the interstellar sources have normal dust-to-gas ratios and ISRF heating. Sources with excess infrared emission, compared to the molecular column density, are protostellar candidates. Sources in the Draco nebula are rich in CO but deficient in dust. The remaining interstellar sources, which have pressure some two orders of magnitude higher than the average interstellar pressure, may be accretion nebulae around stellar remnants. Publication: The Astrophysical Journal Pub Date: July 1993 DOI: 10.1086/172906 Bibcode: 1993ApJ...412..127R Keywords: Far Infrared Radiation; Point Sources; H Ii Regions; Infrared Astronomy Satellite; Molecular Clouds; Nebulae; Protostars; Astrophysics; INFRARED: GALAXIES; INFRARED: INTERSTELLAR: CONTINUUM; ISM: MOLECULES; ISM: DUST; EXTINCTION full text sources ADS | data products SIMBAD (120) NED (34)

ROSAT Detection of an X-ray Shadow in the 1/4-keV Diffuse Background in the Draco Nebula

The detection by the Roentgen satellite (ROSAT) x-ray telescope of a shadow in the 1/4-kiloelectron volt (C band, 0.1 to 0.284 kiloelectron volt) cosmic diffuse background is reported. The location and morphology of the local minimum in x-rays are in clear agreement with a discrete H I cloud. The shadow is very deep with a minimum level at 50 percent of the surrounding emission; therefore, a minimum of 50 percent of the observed off-cloud flux must originate on the far side of the cloud. The analysis of H I velocity components links the cloud with the Draco nebula (distance approximately 600 parsecs); it then follows that there is significant 1/4-kiloelectron volt x-ray emission at a large distance (>400 parsecs) from the galactic plane along this line of sight. The extent of the distant emission region is uncertain, and, if it indicates the existence of a hot galactic corona, it must be patchy in nature.

Molecules at the Interface of an HVC and a High-z HI Filament

The Draco nebula, the faint reflection nebulosity LBN 406/412/415 (Lynds 1965) at 1≈91°, b≈38°, z≥500 pc (Goerigk and Mebold, 1986), contains delicate filaments at its low-1, low-b (SW) boundary (Goerigk et al. 1983, Mebold et al. 1985). The density in the boundary is characterized by the presence of molecules like CO and H2CO (Mebold et al. 1985) and NH3 (Mebold et al. 1987). Johnson (1986) found an excess of unidentified IRAS point sources in the area of the Draco nebula, a large fraction of which is located close to the SW boundary.

The Draco Nebula, a Molecular Cloud Associated with a High Velocity Cloud?

Extended and very faint bright nebulae are found in high (b≳ 30°) galactic latitudes at the Palomar Observatory Sky Survey (POSS) (Lynds, 1965) and even more pronounced at a very sensitive photographic survey of the galactic polar caps by Sandage (1976). Such a nebula, located in the constellation Draco and called “Draco Nebula” or “Dracula”, was found to be in detailed positional coincidence with a 21-cm emission line feature at a LSR velocity of VLSR ≈ -22 km s-1 by Goerigk et al. (1983).

A Neutral Gas Jet in a Low Velocity Shock Front at the Boundary of the Draco Nebula

Abstract21-cm line observations with the Westerbork Synthesis Radio Telescope of a dust and molecular filament at the boundary of the Draco Nebula (l≈ 91°, b≈ 38°) reveal a jet-like neutral hydrogen feature funneling through an outlet in the low velocity shock front at the interface between the Draco Nebula and the surrounding gas. The jet-like feature is apparently connected with a high velocity filament at VLSR = -180 km s-1. We suggest that the soft x-ray emission observed in the area is thermal bremsstrahlung produced by the deceleration of high velocity gas in galactic gas.