Antarctic Submillimeter Telescope And Remote Observatory Research Articles

Submillimeter Astronomy from the South Pole (AST/RO)

AbstractThe Antarctic Submillimeter Telescope and Remote Observatory (AST/RO), a 1.7 m diameter offset Gregorian telescope for astronomy and aeronomy studies at wavelengths between 200 and 2000 μm, saw first light in 1995 and operated until 2005. It was the first radio telescope to operate continuously throughout the winter on the Antarctic Plateau. It served as a site testing instrument and prototype for later instruments, as well as executing a wide variety of scientific programs that resulted in six doctoral theses and more than one hundred scientific publications. The South Pole environment is unique among observatory sites for unusually low wind speeds, low absolute humidity, and the consistent clarity of the submillimeter sky. Especially significant are the exceptionally low values of sky noise found at this site, a result of the small water vapor content of the atmosphere. Multiple submillimeter-wave and Terahertz detector systems were in operation on AST/RO, including heterodyne and bolometric arrays. AST/RO's legacy includes comprehensive submillimeter-wave site testing of the South Pole, spectroscopic studies of 492 GHz and 809 GHz neutral atomic carbon and 460 GHz and 806 GHz carbon monoxide in the Milky Way and Magellanic Clouds, and the first detection of the 1.46 THz [N II] line from a ground-based observatory.

A 205 μm [N II] MAP OF THE CARINA NEBULA

We present the results of a � 250 arcmin 2 mapping of the 205 �m [NII] fine-structure emission over the northern Carina Nebula, including the Car I and Car II HII regions. Spectra were obtained using the South Pole Imaging Fabry-Perot Interferometer (SPIFI) at the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) at South Pole. We supplement the 205 �m data with new reductions of far-IR fine-structure spectra from the Infrared Space Observatory (ISO) Long Wavelength Spectrometer (LWS) in 63 �m [OI], 122 �m [NII], 146 �m [OI], and 158 �m [CII]; the 146 �m [OI] data include 90 raster positions which have not been previously published. Morphological comparisons are made with optical, radio continuum and CO maps. The 122/205 line ratio is used to probe the density of the low-ionization gas, and the 158/205 line ratio is used to probe the fraction of C + arising from photodissociation regions (PDRs). The [OI] and [CII] lines are used to construct a PDR model of Carina. When the PDR properties are compared with other sources, Carina is found to be more akin to 30 Doradus than galactic star-forming regions such as Orion, M17, or W49; this is consistent with the view of Carina as a more evolved region, where much of the parent molecular cloud has been ionized or swept away. These data constitute the first ground-based detection of the 205 �m [NII] line, and the third detection overall since those of the COBE FIRAS and the KAO in the early 1990s. Subject headings: HII regions — infrared: ISM — ISM: individual (Carina nebula) — ISM: lines and bands — photon-dominated region (PDR) — submillimeter: ISM

IRAC Observations of COJ= 4 → 3 High‐Velocity Cloud in the 30 Doradus Complex in the Large Magellanic Cloud

We present the results of 12CO J = 2 → 1 observations of the X-ray-bright giant shell complex 30 Doradus in the Large Magellanic Cloud (LMC) using the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). This is the one of the largest H II complexes in the Local Group. We compare the 12CO J = 2 → 1 observations against previously made 12CO J = 4 → 3 observations and analyze the spatial distribution of young stellar objects (YSOs) within the cloud using the Spitzer IRAC observations of the 30 Doradus complex. Both peaks of 12CO J = 2 → 1 and J = 4 → 3 emitting clouds coincide with the densest region of the filaments in which multiple shells are colliding. The YSOs are clustered in the southern ridge of the warm and dense molecular gas clouds traced by 12CO J = 4 → 3, indicating a filamentary structure of star formation throughout 30 Doradus. We also find an excess of Class I YSO candidates close to the clouds, which likely represent the most recent phase of star formation in this region. This is a region where the triggered star formation has actually occurred, and newly formed stars may have produced such a high-velocity outflow through interacting with the surrounding molecular cloud material.

AST/RO 13 CO( J = 2→1) and 12 CO( J = 4→3) Mapping of Southern Spitzer c2d Small Clouds and Cores

Forty molecular cloud cores in the southern hemisphere from the initial Spitzer Space Telescope Cores-to-Disks (c2d) Legacy program source list have been surveyed in 13CO(2→1), 12CO(4→3), and 12CO(7→6) with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). The cores, 10 of which contain embedded sources, are located mostly in the Vela, Ophiuchus, Lupus, Chamaeleon, Musca, and Scorpius complexes. 12CO(7→6) emission was undetected in all 40 clouds. We present data of 40 sources in 13CO(2→1) and 12CO(4→3), significant upper limits of 12CO(7→6), as well as a statistical analysis of the observed properties of the clouds. We find the typical 13CO(2→1) line width to be 2.0 km s-1 for cores with embedded stars and 1.8 km s-1 for all others. The typical 12CO(4→3) line width is 2.6-3.7 km s-1 for cores with known embedded sources, and 1.6-2.3 km s-1 for all others. The average 13CO column density derived from the line intensities was found to be 1.9 × 1015 cm-2 for cores with embedded stars and 1.5 × 1015 cm-2 for all others. The average kinetic temperature in the molecular cores, determined through a large velocity gradient analysis of a set of nine cores, has an average lower limit of 16 K and an average upper limit of 26 K. The average molecular hydrogen density has an average lower limit of 10 2.9 cm-3 and an average upper limit of 103.3 cm-3 for all cores. For a different subset of nine cores, we have derived masses. They range from 4 to 255 M☉. Overall, our c2d sample of southern molecular cores has a range of properties (line width, column density, size, mass, embedded stars) similar to those of past studies.

Detection of the 205 μm [N ii ] Line from the Carina Nebula

We report the first detection of the 205 μm 3P1 P0 [N II] line from a ground-based observatory using a direct detection spectrometer. The line was detected from the Carina star formation region using the South Pole Imaging Fabry-Perot Interferometer (SPIFI) on the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) at the South Pole. The [N II] 205 μm line strength indicates a low-density (n ~ 32 cm-3) ionized medium, similar to the low-density ionized halo previously reported in its [O III] 52 and 88 μm line emission. When compared with the Infrared Space Observatory [C II] observations of this region, we find that 27% of the [C II] line emission arises from this low-density ionized gas, but the large majority (~73%) of the observed [C II] line emission arises from the neutral interstellar medium. This result supports and underpins prior conclusions that most of the observed [C II] 158 μm line emission from Galactic and extragalactic sources arises from the warm, dense photodissociated surfaces of molecular clouds. The detection of the [N II] line demonstrates the utility of Antarctic sites for THz spectroscopy.

C I] 809 GHz Imaging of the NGC 6334 Complex

We present Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) observations of submillimeter emissions from the NGC 6334 complex for both the $^{12}[\mathrm{C {I}}] \; {{3\atop} \mathrm{P}}_2 \rightarrow {{3\atop} \mathrm{P}}_1$ fine-structure line of atomic carbon and the ${{12\atop} \mathrm{CO}} J = 4 \rightarrow 3$ rotational transition of CO. We detected strong $^{12}[\mathrm{C {I}}] \; {{3\atop} \mathrm{P}}_2 \rightarrow {{3\atop} \mathrm{P}}_1$ emission from the entire star-forming NGC 6334 complex, and present maps of $^{12}[\mathrm{C {I}}] \; {{3\atop} \mathrm{P}}_2 \rightarrow {{3\atop} \mathrm{P}}_1$ emission and ${{12\atop} \mathrm{CO}} J = 4 \rightarrow 3$ emission. Comparing these maps with archival ASCA data, we show that, to the south of NGC 6334, the [C I] emissions are likely to be associated with strong X-ray emissions from the FIR sources. However, the calculation using the X-ray dissociation region model shows that the majority of the [C I] emission can not arise from X-ray dissociation of the cloud. Alternatively, far-ultraviolet radiation produced in the photodissociation regions is expected to contribute to dissociate the carbon monoxide in the NGC 6334 complex.

A Multitransition CO Study in the 30 Doradus Complex in the Large Magellanic Cloud

ABSTRACTWe have made a multitransition CO study of the largest H ii complex, the 30 Doradus nebula, in the Large Magellanic Cloud (LMC). This is the most luminous example of a starburst region in the Local Group. We have searched for12CO J = 7→6 emission toward the 30 Doradus complex with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO), located at 2847 m altitude at the Amundsen‐Scott South Pole Station. As a result, we have detected a12CO J = 7→6 emitting cloud near the 30 Doradus complex. The12CO J = 7→6/12CO J = 4→3 line temperature ratio in this region is approximately a factor of 2 higher than that observed near the Sgr B2 complex. A radiative transfer calculation using the line ratios shows that the core of massive star formation in the LMC is much warmer and denser than that of the Milky Way.

Large‐Scale CO and [Ci] Emission in the ρ Ophiuchi Molecular Cloud

We present a comprehensive study of the ρ Ophiuchi molecular cloud that addresses aspects of the physical structure and condition of the molecular cloud and its photodissociation region (PDR) by combining far-infrared and submillimeter-wave observations with a wide range of angular scale and resolution. We present 1600 arcmin2 maps (2.3 pc2) with 0.1 pc resolution in submillimeter CO (4 → 3) and [C I] (3P1 → 3P0) line emission from the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) and pointed observations in the CO (7 → 6) and [C I] ( 3P2 → 3P1) lines. Within the large-scale maps, smaller spectral line maps of 3000 AU resolution over ~90 arcmin2 (0.2 pc2) of the cloud in CO, CS, HCO+, and their rare isotopomers are made at the Heinrich Hertz Telescope (HHT) in Arizona. Comparison of CO, HCO+, and [C I] maps with far-infrared observations of atomic and ionic species from the Infrared Space Observatory (ISO) far-infrared and submillimeter continuum emission and near-infrared H2 emission allows clearer determination of the physical and chemical structure of the ρ Oph PDR, since each species probes a different physical region of the cloud structure. Although a homogeneous plane-parallel PDR model can reproduce many of the observations described here, the excitation conditions needed to produce the observed HCO+ and [O I] emission imply inhomogeneous structure. Strong chemical gradients are observed in HCO+ and CS; the former is ascribed to a local enhancement in the H2 ionization rate, and the latter is principally due to shocks. Under the assumption of a simple two-component gas model for the cloud, we find that [C II] and [C I] emission predominantly arises from the lower density envelopes (103-104 cm-3) that surround denser cloud condensations, or clumps. The distribution of [C I] is very similar to that of C18O and is generally consistent with illumination from the far side of the cloud. A notable exception is found at the western edge of the cloud, where UV photons create a PDR viewed edge-on. The abundance of atomic carbon is accurately modeled using a radiation field that decreases with increasing projected distance from the exciting star HD 147889 and a total gas column density that follows that of C18O, decreasing toward the edges of the cloud. In contrast to the conclusions of other studies, we find that no nonequilibrium chemistry is needed to enhance the [C I] abundance. Each spectral line is traced to a particular physical component of the cloud and PDR. Although CO rotational line emission originates from both dense condensations and diffuse envelopes, the millimeter-wave transitions mostly find their origins in envelope material, whereas the high-J submillimeter lines stem more from the dense clumps. Submillimeter HCO+ and infrared [O I] and [C II] emission indicate clump surface temperatures of 50-200 K, an ultraviolet radiation field with IUV ≈ 10-90, densities of 105-106 cm-3, and interior temperatures of ≤20 K. This study highlights the value of large-scale infrared and submillimeter mapping for the interpretation of molecular cloud physical and chemical structure, and important future observations are highlighted.

An AST/RO Survey of the Coalsack

AbstractSelected regions of the southern molecular cloud complex, the Coalsack are being imaged at sub-millimeter wavelengths using the Antarctic Sub-millimeter Telescope and Remote Observatory (AST/RO) located at the South Pole.

CO J = 7 → 6 Emission in the Large Magellanic Cloud

AbstractWe present the first detection of 12CO (J = 7 → 6) emission in the Magellanic Clouds toward the 30 Doradus region using the Antarctic Sub-millimeter Telescope and Remote Observatory (AST/RO).

Antarctic Submillimeter Telescope and Remote Observatory Observations of12CO \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $J=4arrow 3$ \end{document} Emission from the N44 Complex in the Large Magellanic Cloud

We present Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) observations of 12CO J = 4 → 3 and [C I] emission in the N44 H II complex in the Large Magellanic Cloud. We detected strong 12CO J = 4 → 3 emission toward the H II region known as N44BC, which is located on the rim of an expanding giant shell in the N44 region. Analysis with a photodissociation region model showed that the 12CO J = 4 → 3 emitting cloud is very dense, with n0 ≈ 105 cm-3. We also note that there is a high-velocity component associated with the 12CO J = 4 → 3 emission. This probably originates from molecular material accelerated as a result of the motion induced by the expanding giant shell surrounding LH 47 in the N44 complex. We found that the kinetic energy of this high-velocity gas observed in the 12CO J = 4 → 3 emission toward the rim of the expanding H II shell is at least a factor of 4 higher than that derived for the H I and H II gas in this region.

Gas Density, Stability, and Starbursts near the Inner Lindblad Resonance of the Milky Way

A key project of the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) reported by Martin et al. (2004) is the mapping of CO J=4-3 and J=7-6 emission from the inner Milky Way, allowing determination of gas density and temperature. Galactic center gas that Binney et al. (1991) identify as being on x_2 orbits has a density near 10^3.5 cm ^-3, which renders it only marginally stable against gravitational coagulation into a few Giant Molecular Clouds, as discussed by Elmegreen (1994). This suggests a relaxation oscillator mechanism for starbursts in the Milky Way, where inflowing gas accumulates in a ring at 150 pc radius for approximately 20 million years, until the critical density is reached, and the resulting instability leads to the sudden formation of giant clouds and the deposition of 4 x 10^7 solar masses of gas onto the Galactic center.

Millimeter and Submillimeter Survey of the R Coronae Australis Region

Using a combination of data from the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO), the Arizona Radio Observatory Kitt Peak 12 m telescope, and the Arizona Radio Observatory 10 m Heinrich Hertz Telescope, we have studied the most active part of the R CrA molecular cloud in multiple transitions of carbon monoxide, HCO+, and 870 μm continuum emission. Since R CrA is nearby (130 pc), we are able to obtain physical spatial resolution as high as 0.01 pc over an area of 0.16 pc2, with velocity resolution finer than 1 km s-1. Mass estimates of the protostar driving the millimeter-wave emission derived from HCO+, dust continuum emission, and kinematic techniques point to a young, deeply embedded protostar of ~0.5-0.75 M☉, with a gaseous envelope of similar mass. A molecular outflow is driven by this source that also contains at least 0.8 M☉ of molecular gas with ~0.5 L☉ of mechanical luminosity. HCO+ lines show the kinematic signature of infall motions, as well as bulk rotation. The source is most likely a Class 0 protostellar object not yet visible at near-IR wavelengths. With the combination of spatial and spectral resolution in our data set, we are able to disentangle the effects of infall, rotation, and outflow toward this young object.

The AST/RO Survey of the Galactic Center Region. I. The Inner 3 Degrees

We present fully-sampled maps of 461 GHz CO (4-3), 807 GHz CO (7-6), and 492 GHz [CI] (3P1-3P0) emission from the inner 3 degrees of the Galactic Center region taken with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) in 2001--2002. The data cover -1.3 70K) at cloud edges to low values (<50K) in the interiors. Typical gas pressures in the Galactic Center gas are n(H_2) T_kin approx 10^5.2 K cm^-3. We also present an (l,b) map of molecular hydrogen column density derived from our LVG results.

The Inner 200pc: Hot Dense Gas

We present fully-sampled maps of 461GHz CO J = 4 → 3, 807GHz CO J = 7 → 6, and 492GHz [CI]3P1 → 3P0 emission from the inner 3 degrees of the Galactic Center region taken with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) in 2001–2002. The data cover −1°.3 < 𝓁 < 2°, −0°.3 < b < 0°.2 with 0.5′ spacing, resulting in spectra in 3 transitions at over 24,000 positions on the sky. The CO J = 4 → 3 emission is found to be essentially coextensive with lower-J transitions of CO. The CO J = 7 → 6 emission is spatially confined to a far smaller region than the lower-J CO lines. The [CI] 3P1 → 3P0 emission has a spatial extent similar to the low-J CO emission, but is more diffuse. Bright CO J = 7 → 6 emission is detected in the well-known Galactic Center clouds SgrA and SgrB. Analyzing our CO J = 7 → 6 and CO J = 4 → 3 data in conjunction with J = 1 → 0 12CO and 13CO data previously observed with the Bell Laboratories 7-m antenna, we apply a Large Velocity Gradient (LVG) model to estimate the kinetic temperature and density of molecular gas in the inner 200 pc of the Galactic Center region. Typical pressures in the Galactic Center gas are n(H2) · Tkin ∼ 105.2 Kcm−3. We present an (𝓁, b) map of molecular hydrogen column density derived from our LVG results.

CO (J = 4→3) and [Ci] Observations of the Carina Molecular Cloud Complex

We present large area, fully-sampled maps of the Carina molecular cloud complex in the CO (J = 4 - 3) and neutral carbon [CI] 3P1 - 3P0 transitions. These data were obtained using the 1.7 meter diameter Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). The maps cover an area of approximately 3 square degrees with a uniform 1' spatial sampling. Analysis of these data, in conjunction with CO (J = 1 - 0) data from the Columbia CO survey and the IRAS HIRES continuum maps for the same region, suggests that the spiral density wave shock associated with the Carina spiral arm may be playing an important role in the formation and dissociation of the cloud complex, as well as in maintaining the internal energy balance of the clouds in this region. Massive stars form at the densest regions of the molecular cloud complex. The winds and outflows associated with these stars have a disrupting effect on the complex and inject mechanical energy into the parent clouds, while the UV radiation from the young stars also heat the parent clouds. The present set of data suggests, however, that massive stars alone may not account for the energetics of the clouds in the Carina region. The details of the data and the correlation among the various data sets hint at the possible role that the spiral density wave shock plays in feeding interstellar turbulence and in heating molecular clouds.

AST/RO Observations of Atomic Carbon near the Galactic Center

We present a coarsely-sampled map of the region |l| 3P0 fine structure transition of neutral carbon, observed with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). The distribution of C[I] emission is similar on the large scale to that of CO J=1-->0. On average, the ratio of the integrated intensities, I_{C I}/I_{12CO}, is higher in the Galactic disk than in the Galactic Center region. This result is accounted for by the absorption of 12CO within the clouds located in the outer Galactic disk. I_{C I}/I_{12CO} is surprisingly uniform over the variety of environments near the Galactic Center. On average, C[I] is optically thin (or as optically thin as 13CO(J=1-->0)) even in the dense molecular clouds of the Galactic Center region.

Optical and mechanical design of the Antarctic Submillimeter Telescope and Remote Observatory

Antarctic Submillimeter Telescope and Remote Observatory (AST/RO), a 1.7 m diameter telescope for astronomy and aeronomy studies at wavelengths between 200 and 3000 μm, was installed at the South Pole during the 1994–95 Austral summer. The optical design is Gregorian, offset in both azimuth and elevation, with the exit pupil at the chopping tertiary mirror: this arrangement provides for consistent illumination of the primary mirror even when the beam is thrown one degree or more on the sky. Aberrations are minimized by the choice of secondary mirror offset angle. Alignment is accomplished by mechanical means. There is a Coudé focus in a warm, spacious receiver room and also a Nasmyth focus. Both the elevation and azimuth axes are driven by two pinion gears with opposed torques to eliminate backlash. The encoders are unusually robust but have high friction, necessitating a stiff coupling. The azimuth limit switch scheme permits 1.5 rotations, but the switches will operate under extreme conditions with no single point of failure. The instrument is now operational with four heterodyne receivers and three acousto-optical spectrometers.

The 492 GHz Atmospheric Opacity at the Geographic South Pole

We present narrow-bandwidth submillimeter-wave sky opacity measurements made from the South Pole between 1995 February 9 and November 17, a period that includes an entire Austral winter. These measurements were made with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) using a heterodyne receiver tuned to a band that includes the 492 GHz fine-structure line of neutral atomic carbon. The zenith optical depth was below 0.72 half the time during the Austral winter and spring, and it reached values as low as 0.34 on day 232. The stability was also remarkably good: the opacity remained below 1.0 for weeks at a time. The South Pole is therefore an excellent site for submillimeter astronomy throughout the Austral winter and spring. The functional relationship between 492 GHz opacity and measured precipitable water vapor shows that a significant fraction of the opacity is caused by atmospheric constituents other than water vapor, indicating the need for accurate, site-dependent atmospheric modeling when opacity measurements at lower frequencies are extrapolated into the submillimeter.

The Antarctic Submillimeter Telescope and Remote Observatory (AST/RO)

AbstractThe Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) is a highly automated 1·7 m diameter telescope aimed at exploiting the superb submillimetre skies of the Antarctic Plateau for astronomy and aeronomy studies. It was recently installed at the Amundsen-Scott South Pole Station during the 1994/95 Austral season and is currently undergoing its first winter-over of operation. In this paper we briefly outline the capabilities of the instrument and describe some recent achievements culminating in the telescope’s first observations of the South Polar submillimetre sky.