Orion Compact Ridge Research Articles

SMA OBSERVATIONS OF THE W3(OH) COMPLEX: PHYSICAL AND CHEMICAL DIFFERENTIATION BETWEEN W3(H2O) AND W3(OH)

We report on the Submillimeter Array (SMA) observations of molecular lines at 270 GHz toward W3(OH) and W3(H$_2$O) complex. Although previous observations already resolved the W3(H$_2$O) into two or three sub-components, the physical and chemical properties of the two sources are not well constrained. Our SMA observations clearly resolved W3(OH) and W3(H$_2$O) continuum cores. Taking the advantage of the line fitting tool XCLASS, we identified and modeled a rich molecular spectrum in this complex, including multiple CH$_3$CN and CH$_3$OH transitions in both cores. HDO, C$_2$H$_5$CN, O$^{13}$CS, and vibrationally excited lines of HCN, CH$_3$CN, and CH$_3$OCHO were only detected in W3(H$_2$O). We calculate gas temperatures and column densities for both cores. The results show that W3(H$_{2}$O) has higher gas temperatures, and larger column densities than W3(OH) as previously observed, suggesting physical and chemical differences between the two cores. We compare the molecular abundances in W3(H$_2$O) to those in the Sgr B2(N) hot core, the Orion KL hot core and the Orion Compact Ridge, and discuss the chemical origin of specific species. An east-west velocity gradient is seen in W3(H$_2$O), and the extension is consistent with the bipolar outflow orientation traced by water masers and radio jets. A north-south velocity gradient across W3(OH) is also observed. However, with current observations we can not assure if the velocity gradients are caused by rotation, outflow or radial velocity differences of the sub-components in W3(OH).

VERA and ALMA observations of the H2O supermaser burst in Orion KL

Abstract The 22 GHz H2O maser in Orion KL has shown extraordinary burst events in 1979–1985 and 1998–1999, sometimes called supermaser. We have conducted monitoring observations of the supermaser in Orion KL using VERA (VLBI Exploration of Radio Astrometry) in the current third burst since 2011 March. Three flux maxima are detected in 2011 and 2012 with rising and falling timescales of 2–7 months. Time variations of the supermaser seem symmetric for all of the active phases. The maximum total flux density of 135000 Jy is observed in 2012 June while it is still one order of magnitude lower than those in previous bursts. The supermaser consists of two spatially different components at different velocities. They are elongated along a northwest–southeast direction perpendicular to the low-velocity outflow driven by Source I. Proper motions of the supermaser features with respect to Source I are measured toward west and southwest directions, almost parallel to the low-velocity outflow. The flux density and linewidth show an anti-correlation as expected for an unsaturated maser emission. The supermaser is located close to the methylformate (HCOOCH3) line and continuum emission peaks in the Orion Compact Ridge detected by ALMA (Atacama Large Millimeter/Submillimeter Array). The broader velocity range of the weak HCOOCH3 emission at the supermaser position would be evidence of a shock front. On the other hand, the 321 GHz H2O line is not detected at the position of the supermaser. It can be explained qualitatively by one of the theoretical H2O excitation models without extraordinary conditions. Our results support a scenario that the supermaser is excited in the dense gas interacting with the low-velocity outflow in the Compact Ridge. The extremely high flux density and its symmetric time variation for rising and falling phases could be explained by a beaming effect during the amplification process rather than changes in physical conditions.

VERA Observations of the H2O Maser Burst in Orion KL

AbstractIn 2011 February, a burst of the 22 GHz H2O maser in Orion KL was reported. In order to identify the bursting maser features, we have been carrying out observations of the 22 GHz H2O maser in Orion KL with VERA, a Japanese VLBI network dedicated for astrometry. The bursting maser turns out to consist of two spatially different features at 7.58 and 6.95 km s−1. We determine their absolute positions and find that they are coincident with the shocked molecular gas called the Orion Compact Ridge. We tentatively detect the absolute proper motions of the bursting features toward the southwest direction, perpendicular to the elongation of the maser features. It is most likely that the outflow from the radio source I or another young stellar object interacting with Compact Ridge is a possible origin of the H2O maser burst. We will also carry out observations with ALMA in the cycle 0 period to monitor the submillimeter H2O maser lines in the Orion Compact Ridge region. These follow-up observations will provide novel information on the physical and chemical properties of the mastering region.

A HIGH SPATIAL RESOLUTION STUDY OF THE λ = 3 mm CONTINUUM OF ORION-KL

Recent interferometric observations have called into question the traditional view of the Orion-KL region, which displays one of the most well-defined cases of chemical differentiation in a star-forming region. Previous, lower-resolution images of Orion-KL show emission signatures for oxygen-bearing organic molecules toward the Orion Compact Ridge, and emission for nitrogen-bearing organic molecules toward the Orion Hot Core. However, more recent observations at higher spatial resolution indicate that the bulk of the molecular emission is arising from many smaller, compact clumps that are spatially distinct from the traditional Hot Core and Compact Ridge sources. It is this type of observational information that is critical for guiding astrochemical models, as the spatial distribution of molecules and their relation to energetic sources will govern the chemical mechanisms at play in star-forming regions. We have conducted millimeter imaging studies of Orion-KL with various beam sizes using CARMA in order to investigate the continuum structure. These \lambda;=3mm observations have synthesized beam sizes of ~0.5"-5.0". These observations reveal the complex continuum structure of this region, which stands in sharp contrast to the previous structural models assumed for Orion-KL based on lower spatial resolution images. The new results indicate that the spatial scaling previously used in determination of molecular abundances for this region are in need of complete revision. Here we present the results of the continuum observations, discuss the sizes and structures of the detected sources, and suggest an observational strategy for determining the proper spatial scaling to accurately determine molecular abundances in the Orion-KL region.

IDENTIFICATION OF BURSTING WATER MASER FEATURES IN ORION KL

In February 2011, a burst event of the H$_{2}$O maser in Orion KL (Kleinmann-Low object) has started after 13-year silence. This is the third time to detect such phenomena in Orion KL, followed by those in 1979-1985 and 1998. We have carried out astrometric observations of the bursting H$_{2}$O maser features in Orion KL with VERA (VLBI Exploration of Radio Astrometry), a Japanese VLBI network dedicated for astrometry. The total flux of the bursting feature at the LSR velocity of 7.58 km s$^{-1}$ reaches 4.4$\times10^{4}$ Jy in March 2011. The intensity of the bursting feature is three orders of magnitudes larger than that of the same velocity feature in the quiescent phase in 2006. Two months later, another new feature appears at the LSR velocity of 6.95 km s$^{-1}$ in May 2011, separated by 12 mas north of the 7.58 km s$^{-1}$ feature. Thus, the current burst occurs at two spatially different features. The bursting masers are elongated along the northwest-southeast direction as reported in the previous burst in 1998. We determine the absolute positions of the bursting features for the first time ever with a submilli-arcsecond (mas) accuracy. Their positions are coincident with the shocked molecular gas called the Orion Compact Ridge. We tentatively detect the absolute proper motions of the bursting features toward southwest direction. It is most likely that the outflow from the radio source I or another young stellar object interacting with the Compact Ridge is a possible origin of the H$_{2}$O maser burst.

Molecular D/H ratios in the dense gas surrounding low-mass protostars

Observations of [HDCO]/[H 2CO] and [DCN]/[HCN] ratios have been made towards a selection of low-mass protostellar cores in three different star forming regions. Most [HDCO]/ [H 2CO] ratios are between 0.05 and 0.07, similar to the values observed towards the dark clouds, TMC-1 and L134N. [DCN]/[HCN] ratios are ∼0.04, higher than those seen in TMC-1, around the low-mass protostar, IRAS 16293, and in the Orion Hot Core, but similar values to the Orion Compact Ridge and L134N. These results are compared with predictions from chemical models, and to other observations made in these sources. We find best agreement between models and observations by assuming that interaction between gas phase molecules and dust grains has impacted on the chemistry during the cold pre-collapse phase of the cloud's history. There are no marked difference between molecular D/H ratios towards different regions, or between Class 0 and I protostars. However, the difference between the [DCN]/[HCN] ratios we have measured and those previously observed towards Hot Molecular Cores leads us to suggest that there are significant evolutionary differences between high and low mass star forming regions.

A survey of [HDCO]/[ H$_\mathsf{2}$CO] and [DCN]/[HCN] ratios towards low-mass protostellar cores

We present observations of [HDCO]/[H 2 CO] and [DCN]/[HCN] ratios towards a selection of low-mass protostellar cores in three different star forming regions. The best fit to the observed [HDCO]/[H 2 CO] ratios is ∼0.05-0.07, similar to the values observed towards the dark clouds, TMC-1 and L134N. [DCN]/[HCN] ratios are ∼0.04, higher than those seen in TMC-1, around the low-mass protostar IRAS 16293 and the Orion Hot Core, but similar values to the Orion Compact Ridge and L134N. We compare our results with predictions from detailed, chemical models, and to other observations made in these sources. We find best agreement between models and observations by assuming that interaction between gas phase molecules and dust grains has impacted on the chemistry during the cold pre-collapse phase of the cloud's history. The abundance of deuterated species indicates that the dense gas out of which a low mass protostar forms, evolves and collapses on a timescale of ≤50 000 years. We find no marked difference between molecular D/H ratios towards different regions, or between Class 0 and Class I protostars. However, the striking difference between the [DCN]/[HCN] ratios we have measured and those previously observed towards Hot Molecular Cores leads us to suggest that there are significant evolutionary differences between high and low mass star forming regions.

Deuterated Methanol in the Orion Compact Ridge

We have investigated the chemistry of hot cores containing methanol, formaldehyde, and their deuterated forms. We have computed the D fractionation ratios attainable by the addition of H and D atoms to CO on cold grain surfaces. We can easily reproduce the observed HDCO/H2CO and D2CO/H2CO ratios but find that, as there is an additional path to CH2DOH, the CH2DOH/CH3OD ratio is typically 3. This result is in conflict with the conclusion of Jacq et al. that the observed gas-phase CH2DOH/CH3OD ratio of ≈ 1.1-1.5 in the Orion Compact Ridge source is consistent with their production in grain mantles. We show that, when these mantles are evaporated, the CH2DOH/CH3OD ratio can be altered by gas-phase reactions that preferentially form CH3OD via reaction of H2DO+ with CH3OH. We find that an unrealistically high HDO/H2O ratio of ≈ 0.1 is necessary to drive the CH2DOH/CH3OD ratio to within the observed range. Before concluding that the observed gas-phase CH2DOH/CH3OD ratio is inconsistent with these grain-surface reactions, we recommend that more accurate determinations of this ratio be made in the Compact Ridge and in other hot cores.

D/H and 18O/16O ratio in the hydronium ion and in neutral water from in situ ion measurements in comet Halley

The D/H ratio and the 18O/16O ratio in hydronium ions in the gas coma of comet Halley have been determined from ion data obtained by the high intensity sensor (HIS) of the ion mass spectrometer (IMS) on board the spacecraft Giotto in 1986. The HIS instrument measured reliable data for the water group ions over a large distance range from the nucleus. We could therefore minimize the statistical errors. This large range also allowed us to study the systematic influence of formaldehyde on the D/H ratio in the hydronium ion and to deduce from the hydronium ion a D/H ratio for the water molecule. The measured 18O/16O ratio of (1.93±0.12) × 10−3 is compatible with the telluric value of 2.06 × 10−3. The D/H ratio for the hydronium ion measured is D/H = 3.08−0.53+0.38 × 10−4. We show that since the water chemistry is not significantly different for deuterated and nondeuterated water molecules and ions and since the contribution from other deuterated molecules to the deuterium abundance in the hydronium ion is small, the D/H value of the neutral water molecule is not significantly different from the value determined for the ion, although the uncertainty is somewhat larger. An upper limit for the deuterium abundance in formaldehyde in comet Halley can be derived from the D/H ratio in the hydronium ion as a function of distance from the nucleus. From our results we conclude that the D/H ratio in formaldehyde is certainly smaller than 2% and probably smaller than 0.5% and that the enrichment of the deuterium in the hydronium ion due to formaldehyde is lower than 5%. The D/H ratio in water in comet Halley is therefore significantly higher than the telluric sea water value, whereas the deuterium enrichment in formaldehyde is not as high as, for example, that observed in the Orion compact ridge.

The formation of oxygen-containing organic molecules in the Orion compact ridge

Following a suggestion of Blake et al. (1987), an attempt was made to account for the unusually large abundances of selected oxygen-containing organic molecules in the so-called 'compact ridge' source directed toward Orion KL by a gas-phase chemical model in which large amounts of water are injected into the source from the IRc2 outflow. Although quantitative model results show that the calculated abundances of methanol, methyl formate, and dimethyl ether can be enhanced relative to their values in the absence of water injection, the enhancements fall far short of explaining the very large observed abundances of these species. Models in which methanol is injected rather than water are more successful, although the source of the methanol is unclear.