薄分子云湍流结构函数的倾角改正及其应用

通过结构函数可以测量湍流的能量级联速率.在实际观测中,无法测量分子云中气体的3维速度,这使得其湍流结构函数难以测量.对垂直于视线方向的薄分子云的情形,结构函数Stt2可以通过云核速度弥散(core velocity dispersion,CVD)进行测量,CVD2=1/2Stt2.对此进行推广,对于不垂直于视线方向的薄分子云,CVD2=1/2Stt2(1-1/8cos2θ)R2/3,其中,θ是视线方向与投影方向的夹角,平均投影距离与3维距离之比R可以用第2类椭圆积分E(k,φ)表示为R=2/πE(cosθ,π/2).     

On the CH B‐X (1, 0) band in translucent clouds

We present observations of CH B‐X (1, 0) band toward 13 translucent sightlines. The given precise positions (at 3627.403, 3633.289, 3636.222 Å) are derived by means of shifting our spectra to the rest velocity frame using B‐X (0, 0) features (at 3878, 3886 and 3890 Å). The proposed oscillator strengths (equal to 0.00035, 0.00104, 0.00069, respectively) are based on f ‐values of the CH B‐X (0, 0) system, i.e. 0.00110, 0.00320, 0.00210 (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Unlocking the Keyhole: H2 and PAH emission from molecular clumps in the Keyhole Nebula

To better understand the environment surrounding CO emission clumps in the Keyhole Nebula, we have made images of the region in H₂ 1–0 S(1) (2.122-μm) emission and polycyclic aromatic hydrocarbon (PAH) emission at 3.29 μm. Our results show that the H₂ and PAH emission regions are morphologically similar, existing as several clumps, all of which correspond to CO emission clumps and dark optical features. The emission confirms the existence of photodissociation regions (PDRs) on the surface of the clumps. By comparing the velocity range of the CO emission with the optical appearance of the H₂ and PAH emission, we present a model of the Keyhole Nebula whereby the most negative velocity clumps are in front of the ionization region, the clumps at intermediate velocities are in it and those which have the least negative velocities are at the far side. It may be that these clumps, which appear to have been swept up from molecular gas by the stellar winds from η  Car, are now being overrun by the ionization region and forming PDRs on their surfaces. These clumps comprise the last remnants of the ambient molecular cloud around η Car.     

Molecular signatures of MHD waves

The small ion-neutral drift speeds attained in MHD waves lead to a significant increase in the rates of several important ion-molecule reactions. The effect on the deuterium chemistry of dark clouds could allow MHD wave motion to be observed directly. The results of some recent observations to test this theory are outlined.     

An infrared proper motion study of the Orion bullets

We report the first infrared proper motion measurements of the HerbigHaro objects in OMC-1 using a 4-yr time baseline. The [Fe  ii ]-emitting bullets are moving of the order of 0.08 arcsec per year, or at about 170 km s¹. The direction of motion is similar to that inferred from their morphology. The proper motions of H₂-emitting wakes behind the [Fe  ii ] bullets, and of newly found H₂ bullets, are also measured. H₂ bullets have smaller proper motion than [Fe  ii ] bullets, while H₂ wakes with leading [Fe  ii ] bullets appear to move at similar speeds to their associated bullets. A few instances of variability in the emission can be attributed to dense, stationary clumps in the ambient cloud being overrun, setting up a reverse-oriented bullet. Differential motion between [Fe  ii ] bullets and their trailing H₂ wakes is not observed, suggesting that these are not separating, and also that they have reached a steady-state configuration over at least 100 yr. The most distant bullets have, on average, larger proper motions, but are not consistent with free expansion. Nevertheless, an impulsive, or short-lived (<<1000 yr), duration for their origin seems likely.     

Shocked gas around Cepheus A: evidence for multiple outflows from H2S and SO2 observations

The Cepheus A star-forming region has been investigated through a multiline H₂S and SO₂ survey at millimetre wavelengths. Large-scale maps and high-resolution line profiles reveal the occurrence of several outflows. Cep A East is associated with multiple mass-loss processes: in particular, we detect a 0.6-pc jet-like structure which shows for the first time that the Cep A East young stellar objects are driving a collimated outflow moving towards the south.
The observed outflows show different clumps associated with definitely different H₂S/SO₂ integrated emission ratios, indicating that the gas chemistry in Cepheus A has been altered by the passage of shocks. H₂S appears to be more abundant than SO₂ in high-velocity clumps, in agreement with chemical models. However, we also find quite small H₂S linewidths, suggestive of regions where the evaporated H₂S molecules had enough time to slow down but not to freeze out on to dust grains. Finally, comparison between the line profiles indicates that the excitation conditions increase with the velocity, as expected for a propagation of collimated bow shocks.     

Hyperfine structure and interstellar curves of growth

The effects of the hyperfine structure (HFS) that is present in some interstellar absorption lines are investigated in the case of a single absorbing cloud. If the respective total equivalent widths of two or more unresolved HFS multiplets measured in relatively low-resolution spectra are analysed specifically by means of a curve of growth, the column density N (X) and the linewidth parameter b (X) inferred for absorbing species X in the cloud will generally be in error if the HFS is ignored. The fundamental physical effect is the reduced line saturation that arises because the total column density is divided among the HFS levels of the ground atomic level, each of which generally gives rise to an HFS line at a different wavelength. For nuclear spins   I = 3/2  and   I = 5/2  , theoretical curves of growth are calculated for some of the resonance lines of some alkali atoms, for each of four illustrative choices of the parameter  α=Δ/ b   , the ratio of the HFS splitting in the ground atomic level to the linewidth. Applications of the results to interstellar absorption by Na  i , K  i and Al  iii are emphasized. HFS is, fortunately, unimportant for most interstellar lines, however. Among the 35 elements that have been detected in diffuse clouds via interstellar absorption in the ultraviolet/optical spectral region, the most abundant isotope of each of 25 of these shows no HFS, because   I = 0  or, in the relevant ground atomic level,   J = 0  .     

The excitation and kinematical properties of H2 and [Fe ii] in the HH 46/47 bipolar outflow

Long-slit spectra of the molecular outflow Herbig–Haro (HH) 46/47 have been taken in the J and K near-infrared bands. The observed H₂ line emission confirms the existence of a bright and extended redshifted counter-jet outflow south-west of HH 46. In contrast with the optical appearance of this object, we show that this outflow seems to be composed of two different emission regions characterized by distinct heliocentric velocities. This implies an acceleration of the counter-jet.
The observed [Fe  ii ] emission suggests an average extinction of 7–9 visual magnitudes for the region associated with the counter-jet.
Through position–velocity diagrams, we show the existence of different morphologies for the H₂ and [Fe  ii ] emission regions in the northern part of the HH 46/47 outflow. We have detected for the first time high-velocity (−250 km s⁻¹) [Fe  ii ] emission in the region bridging HH 46 to HH 47A. The two strong peaks detected can be identified with the optical positions B8 and HH 47B.
The H₂ excitation diagrams for the counter-jet shock suggest an excitation temperature for the gas of T _ex≈2600 K . The lack of emission from the higher energy H₂ lines, such as the 4–3 S(3) transition, suggests a thermal excitation scenario for the origin of the observed emission. Comparison of the H₂ line ratios with various shock models yielded useful constraints about the geometry and type of these shocks. Planar shocks can be ruled out whereas curved or bow shocks (both J- and C-type) can be parametrized to fit our data.     

A burst of outflows from the Serpens cloud core: wide-field submillimetre continuum, CO J=2–1 and optical observations

Wide-field mapping of Serpens in submillimetre continuum emission and CO J =2–1 line emission is here complemented by optical imaging in [S  ii ] λλ 6716, 6731 line emission. Analysis of the 450- and 850-μm continuum data shows at least 10 separate sources, along with fainter diffuse background emission and filaments extending to the south and east of the core. These filaments describe 'cavity-like' structures that may have been shaped by the numerous outflows in the region. The dust opacity index, β , derived for the identifiable compact sources is of the order of 1.0±0.2, with dust temperatures in excess of 20 K. This value of β is somewhat lower than for typical class I YSOs; we suggest that the Serpens sources may be 'warm', late class 0 or early class I objects.
With the combined CO and optical data we also examine, on large scales, the outflows driven by the embedded sources in Serpens. In addition to a number of new Herbig–Haro flows (here denoted HH 455–460), a number of high-velocity CO lobes are observed; these extend radially outwards from the cluster of submillimetre sources in the core. A close association between the optical and molecular flows is also identified. The data suggest that many of the submillimetre sources power outflows. Collectively, the outflows traced in CO support the widely recognized correlation between source bolometric luminosity and outflow power, and imply a dynamical age for the whole protostellar cluster of ∼3×10⁴ yr. Notably, this is roughly equal to the proposed duration of the 'class 0' stage in protostellar evolution.     

Detection of CO Outflow in Rotating Cores

We investigate the effect of bulk motion on the detection of molecular outflows in the sources S 146, GGD27, and IRAS 22566 5830. The traditional techniques do allow for bulk motions or systematic VLSR shifts of the core emissions, which may cause contamination of the high velocity gas emissions, and outflows may either fail to be detected or have their properties miscalculated. We used a program to follow the systematic shift of VLSR and better results have been obtained.