Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userHI, CO, and Planck/IRAS dust properties in the high-latitude-cloud complex, MBM 53, 54, 55 and HLCG 92-35; Possible evidence for an optically thick HI envelope around the CO clouds
255
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present an analysis of the HI and CO gas in conjunction with the Planck/IRAS submillimeter/far-infrared dust properties toward the most outstanding high latitude clouds MBM 53, 54, 55 and HLCG 92-35 at b = -30 deg to -45 deg. The CO emission, dust opacity at 353 GHz (tau353), and dust temperature (Td) show generally good spatial correspondence. On the other hand, the correspondence between the HI emission and the dust properties is less clear than in CO. The integrated HI intensity WHI and tau353 show a large scatter with a correlation coefficient of ~0.6 for a Td range from 16 K to 22 K. We find, however, that WHI and tau353 show better correlation for smaller ranges of Td every 0.5 K, generally with a correlation coefficient of 0.7-0.9. We set up a hypothesis that the HI gas associated with the highest Td >= 21.5 K is optically thin, whereas the HI emission is generally optically thick for Td lower than 21.5 K. We have determined a relationship for the optically thin HI gas between atomic hydrogen column density and tau353, NHI (cm-2) = (1.5 x 10^26) x tau353, under the assumption that the dust properties are uniform and we have applied this to estimate NHI from tau353 for the whole cloud. NHI was then used to solve for Ts and tauHI over the region. The result shows that the HI is dominated by optically thick gas having a low spin temperature of 20-40 K and a density of 40-160 cm-3. The HI envelope has a total mass of ~1.2 x 10^4 Msol, an order of magnitude larger than that of the CO clouds. The HI envelope properties derived by this method do not rule out a mixture of HI and H2 in the dark gas, but we present indirect evidence that most of the gas mass is in the atomic state.
rate research
Read More
Gas and dust properties in the Chamaeleon molecular cloud complex have been investigated with emission lines from atomic hydrogen (HI) and 12CO molecule, dust optical depth at 353 GHz ($tau_{353}$), and $J$-band infrared extinction ($A_{J}$). We have found a scatter correlation between the HI integrated intensity ($W_{rm HI}$) and $tau_{353}$ in the Chamaeleon region. The scattering has been examined in terms of possible large optical depth in HI emission ($tau_{rm HI}$) using a total column density ($N_{rm H}$) model based on $tau_{353}$. A nonlinear relation of $tau_{353}$ with the $sim$1.2 power of $A_{J}$ has been found in opaque regions ($A_{J}$ $gtrsim$ 0.3 mag), which may indicate dust evolution effect. If we apply this nonlinear relation to the $N_{rm H}$ model (i.e., $N_{rm H} propto tau_{353}^{1/1.2}$) allowing arbitrary $tau_{rm HI}$, the model curve reproduces well the $W_{rm HI}$-$tau_{353}$ scatter correlation, suggesting optically thick HI ($tau_{rm HI} sim$1.3) extended around the molecular clouds. Based on the correlations between the CO integrated intensity and the $N_{rm H}$ model, we have then derived the CO-to-H$_{2}$ conversion factor ($X_{rm CO}$) on $sim$1.5$^{circ}$ scales (corresponding to $sim$4 persec) and found spatial variations of $X_{rm CO}$ $sim$(0.5-3)$times$10$^{20}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s across the cloud complex, possibly depending on the radiation field inside or surrounding the molecular clouds. These gas properties found in the Chamaeleon region are discussed through a comparison with other local molecular cloud complexes.
Comparison analyses between the gas emission data (HI 21cm line and CO 2.6 mm line) and the Planck/IRAS dust emission data (optical depth at 353 GHz tau353 and dust temperature Td) allow us to estimate the amount and distribution of the hydrogen gas more accurately, and our previous studies revealed the existence of a large amount of optically-thick HI gas in the solar neighborhood. Referring to this, we discuss the neutral hydrogen gas around the Perseus cloud in the present paper. By using the J-band extinction data, we found that tau353 increases as a function of the 1.3-th power of column number density of the total hydrogen (NH), and this implies dust evolution in high density regions. This calibrated tau353-NH relationship shows that the amount of the HI gas can be underestimated to be ~60% if the optically-thin HI method is used. Based on this relationship, we calculated optical depth of the 21 cm line (tauHI), and found that <tauHI> ~ 0.92 around the molecular cloud. The effect of tauHI is still significant even if we take into account the dust evolution. We also estimated a spatial distribution of the CO-to-H2 conversion factor (XCO), and we found its average value is <XCO> ~ 1.0x10^20 cm-2 K-1 km-1 s. Although these results are inconsistent with some previous studies, these discrepancies can be well explained by the difference of the data and analyses methods.
An isolated HI cloud with peculiar properties has recently been discovered by Dedes, Dedes, & Kalberla (2008, A&A, 491, L45) with the 300-m Arecibo telescope, and subsequently imaged with the VLA. It has an angular size of ~6, and the HI emission has a narrow line profile of width ~ 3 km/s. We explore the possibility that this cloud could be associated with a circumstellar envelope ejected by an evolved star. Observations were made in the rotational lines of CO with the IRAM-30m telescope, on three positions in the cloud, and a total-power mapping in the HI line was obtained with the Nancay Radio Telescope. CO was not detected and seems too underabundant in this cloud to be a classical late-type star circumstellar envelope. On the other hand, the HI emission is compatible with the detached-shell model that we developed for representing the external environments of AGB stars. We propose that this cloud could be a fossil circumstellar shell left over from a system that is now in a post-planetary-nebula phase. Nevertheless, we cannot rule out that it is a Galactic cloud or a member of the Local Group, although the narrow line profile would be atypical in both cases.
Based on the accurate color excess $E_{rm G_{BP},G_{RP}}$ of more than 4 million stars and $E_{rm NUV,G_{BP}}$ of more than 1 million stars from citet{2021ApJS..254...38S}, the distance and the extinction of the molecular clouds in the MBM catalog at $|b|>20^{circ}$ are studied in combination with the distance measurement of emph{Gaia}/EDR3. The distance as well as the color excess is determined for 66 molecular clouds. The color excess ratio $E_{rm G_{BP},G_{RP}}/E_{rm NUV,G_{BP}}$ is derived for 39 of them, which is obviously larger and implies more small particles at smaller extinction. In addition, the scale height of the dust disk is found to be about 100 pc and becomes large at the anticenter direction due to the disk flaring.
A study of the interstellar medium (ISM) and cosmic rays (CRs) using Fermi Large Area Telescope (LAT) data, in a region encompassing the nearby molecular clouds MBM 53, 54, and 55 and a far-infrared loop-like structure in Pegasus, is reported. By comparing Planck dust thermal emission model with Fermi-LAT gamma-ray data, it was found that neither the dust radiance (R) nor the dust opacity at 353 GHz (tau353) were proportional to the total gas column density N(Htot) primarily because N(Htot)/R and N(Htot)/tau353 depend on the dust temperature (Td). The N(Htot) distribution was evaluated using gamma-ray data by assuming the regions of high Td} to be dominated by optically thin atomic hydrogen (HI) and by employing an empirical linear relation of N(Htot)/R to Td. It was determined that the mass of the gas not traced by the 21-cm or 2.6-mm surveys is ~25% of the mass of HI in the optically thin case and is larger than the mass of the molecular gas traced by carbon monoxide by a factor of up to 5. The measured gamma-ray emissivity spectrum is consistent with a model based on CR spectra measured at the Earth and the nuclear enhancement factor of <=1.5. It is, however, lower than local HI emissivities reported by previous Fermi-LAT studies employing different analysis methods and assumptions on ISM properties by 15%-20% in energies below a few GeV, even if we take account of the statistical and systematic uncertainties. The origin of the discrepancy is also discussed.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
