Science at ARO

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Science Overview at ARO

With a diameter of 10m the Sub-Millimeter Telescope  (SMT) is the most accurate astronomical telescope in the world (15 microns rms) designed specifically to operate in the sub-millimeter wavelength region of the spectrum. 

The major areas of millimeter/sub-millimeter science pursued at Steward Observatory include studies of the structure and dynamics of late-type stars and planetary nebulae, molecular cloud morphology, star formation, as well as astro-chemistry of interstellar and circumstellar material. These investigations require heterodyne receivers for both sensitive molecular line searches and large-scale mapping of molecular emission. The ARO provides the broad frequency coverage required for many of the scientific studies. The 12m receivers cover the 65-183 GHz range (2 and 3 mm windows), and the SMT supports 130-500 GHz receivers. Future instrumentation is planned to operate up to 1 THz. Many of these receivers are dual polarization and single sideband. The ARO also supports array receivers, at present, the 345 GHz, seven-pixel Desert Star array, to be followed by SuperCam. ARO is also actively involved in millimeter-wave VLBI, in collaboration with M.I.T. Haystack.

Forefront Scientific Research Programs

The unique capabilities of the ARO telescopes include a broad frequency coverage and many stable, sensitive receiver systems and back-ends that allow for deep line searches, complemented by an active supporting laboratory spectroscopy group that focuses on potential interstellar molecules - a prime combination for astro-chemical studies; also an innovative instrument lab. Routine remote observing has allowed for 24-hrs a day operations from October – July from Asia, Europe and South Africa. ARO focuses on three major areas of astronomical research:

Star Formation and the Evolution of Molecular Clouds

Understanding the detailed physics of molecular cloud formation, core formation, and cloud destruction is the subject of an Observatory Project led by J. Bieging which involves primarily the mapping of molecular could complexes using the J=2->1 and 3->2 transitions of CO. What are the life-cycles of molecular clouds? How do they form, evolve, and become disrupted? How does matter cycle between atomic and molecular phases? How and under what conditions do molecular clouds form stars? How do outflows, shocks, and UV radiation regulate star formation? What are the strengths of magnetic fields in molecular clouds ? To shed new light on these long-standing questions, researchers use the excellent capabilities of the SMT for high-fidelity molecular line imaging in the mm/submm bands to do CO mapping of molecular clouds in J=2->1 and 3->2 transitions over large areas with focal plane arrays. This endeavor is coupled closely to heterodyne array developments of the Steward Observatory Radio Astronomy Laboratory (SOLAR), efforts led by C. Walker, and will initially employ the current DesertSTAR array at 345 GHz, and SuperCam in the future.

Astrochemistry

At present, more than 120 chemical species have been detected in the interstellar space, primarily in giant gas clouds scattered throughout our Galaxy. ARO is in a unique situation where efforts in a molecular spectroscopy laboratory, and the ARO telescopes are combined providing a unique opportunity nation wide to research molecules in space. The effort is led by L. Ziurys, and the millimeter/sub-millimeter spectroscopy laboratory, which currently has four fully-functional lab instruments available for frequency measurements in the range between 60-600GHz, A. Apponi.

The astrochemistry program provides the main impetus for the development of sensitive, dual polarization, single-sideband receivers and stable filter-bank systems.

Of particular interest are studies of Refractory Chemistry, Ion Chemistry, as well as Chemistry of Large Organic Species and Isotopic Gradients on Galactic Scale.

• Refractory Chemistry: How do metal-bearing species form in circumstellar envelopes? How much refractory material remains in the gas-phase? How does circumstellar chemistry change with stellar evolution? How much molecular material survives the final stages of AGB mass loss? What are the constraints on dust grain composition?  What are the carriers of the refractory elements in dense clouds? Are these elements all depleted into dust grains? Are we "missing" some potential interstellar species?

• Ion Chemistry: What are the important molecular ions in PDR regions? How is their chemistry linked? Do PDR conditions produce some unique species? How does ion chemistry vary from PDR regions to dense clouds? What is the ionization balance in these objects?

• Chemistry of large organic species: Are there limits on the complexity of interstellar species? Can gas-phase interstellar chemistry produce the precursors to molecules of true biological significance (sugars, amino acids, etc)? Can we actually conclusively identify complex species using radio and millimeter astronomy? What are the criteria for identification of large organic molecules? How are complex species formed in the ISM? Does life really begin in molecular clouds as opposed to planet surfaces? What is the role of comets in the transport of interstellar material to planet surfaces? How complex are molecules in comets?

• Isotope ratios: Are there really isotope gradients in the Galaxy? What are the relative contributions of Galactic chemical evolution vs. chemical fractionation? What are the "true" tracers of 12C/13C ratio? What stars contribute to 12C and 13C?

Late Stages of Stellar Evolution

Investigations of evolved stars include single-dish measurements of molecules tracing shocks and isotopes and are led by J. Bieging, N. Woolf and L. Ziurys. How do dust formation rates depend on stellar properties and composition? What chemical processes produce the rich variety of molecular species detected in such envelopes? How does the mass loss affect or depend on the evolution of the star? Specific observational programs include:

• Studies of vibrationally-excited HCN lines in a larger sample of carbon stars to try to determine excitation mechanisms and why some stars are masers, others are not

• Chemistry of  elements in circumstellar envelopes: especially compounds containing Si, S, Mg, Al, Na, Fe; abundances as shock diagnostics; relationship to mass loss rates, photospheric composition, stellar properties, and evolutionary state

Dust emission properties by continuum observations and polarimetry with bolometer arrays Searches for new molecules containing refractory elements, based on new laboratory data acquired "in-house"

• Evaluation of gas-phase vs. solid-state refractory components

• Comparison of gas-phase compositions with meteoritic material

• Detailed modeling of circumstellar chemistry and contribution of shocks

• Studies of heavy element isotope ratios to constrain nuclear processes

Science Highlights

Major Discoveries with the ARO telescopes

• Detections of new interstellar molecules (vinyl alcohol, glycolaldehyde, KCN, AlNC, tentative detection of CrH and more)
• Measurement of 12C/13C and D/H ratios at outer edges of Galaxy (DGC> 14Kpc) via CN, HCN, HCO⁺, etc.
• Detection of fringes at 1mm towards 3C454.3 using SMT-IRAM 30m transatlantic baseline: highest angular resolution in astronomy achieved : < 32 micro-arcseconds
• Detection of the J=9->8 line of CO at 1.04 THz at the SMT
• Discovery of strong J=3->2 CO emission in nearby face-on galaxies

 Recent Scientific Highlights

Among other interesting research conducted during the past five years at ARO representative highlights are presented here:

• Evolved Stars:

   

  • One of the areas of major focus at the ARO facilities has been the study of physical and chemical properties of evolved stars. Hrivnak and Bieging (2005), for example, have used the SMT to measure the J=2->1(230 Ghz) and J=4->3(460GHz) transitions of CO towards a sample of 22 protoplanetary nebulae. Modeling of these data has shown that the stellar envelope density is much steeper than r-2, implying a sharp increase in mass loss as the stars evolve on the AGB. The data yield the quantitative time-dependence of mass loss for several post-AGB stars.

   

  • Detection of submm lines of HCN and SiO in AGB star envelopes

    • Strong submm emission in high-excitation lines of HCN in M-type AGB stars, and of SiO in carbon stars implies high abundances far from LTE chemistry model predictions

    • Strong evidence for non-equilibrium chemistry produced by pulsation-driven shocks in stellar atmospheres, and definitely NOT by photochemistry in outer circumstellar envelope

     

  • Discovery of two new HCN maser lines in optically bright carbon stars

    • In vibrationally-excited J=3->2 and 4->3 states in stars of moderate mass loss rates and modest visual extinctions

    • Masing action not detected in high-mass loss objects in these lines

     

• Astrochemistry:

   

  • Graduate student J. Highberger, conducted several successful searches for metal-bearing species in CSE's other than the carbon-star IRC+10216, as part of her thesis work (Highberger et al. 2003; Highberger & Ziurys 2003; Young et al. 2003). MgNC was detected towards the highly evolved PPN CRL618 for the first time using the 12m at 2 and 3mm. MgNC has also been detected towards the C-rich AGB star CIT6 (Halfen & Ziurys, 2005) using the 12-m; a previous identification of this radical had been made by Highberger et al. 2001 in CRL 2688, another CSE in the PPN phase. MgNC thus appears to be a common molecule in circumstellar gas, indicating that refractory elements have a significant gas phase component.

  • Multiple transitions of KCN have been detected towards IRC+10216 confirming its presence in IRC+10216. KCN is the fifth metal cyanide/isocyanide identified in circumstellar gas, along with MgNC, MgCN, NaCN, and AlNC; the most common molecular form of these elements in circumstellar gas is therefore bonded to the CN-moiety –a result that currently defies chemical modelers.

   

  • New advances in understanding chemistry of refractory molecules in interstellar / circumstellar gas include:

    • Detections of new metal-bearing molecules (in chemist's dense) in circumstellar gas

    • Evaluation of the amount of refractory material in the gas phase

    • Studies of magnesium isotope ratios

    • Modeling of circumstellar LTE chemistry

  • Studies of Ion Chemistry include:

    • Detection of HOC⁺, the metastable isomer of HCO⁺, and CO⁺ towards PDR regions

    • Evaluation of CO⁺ and HOC⁺ chemistry

     

  • Studies of Organic Chemistry in Molecular clouds leading to "Bio-Molecules" include:

    • Absolute confirmation of glycolaldehyde in SgrB2 (> 50 transitions observed

    • Detection of vinyl alcohol in SgrB2

    • Observations to confirm interstellar glycine

     

• Molecular Clouds:

   

  • Numerous investigations of Molecular Clouds with the ARO facilities when compared with the near-infrared emission, seem to indicate that cavities created in the cloud, as traced by the IR emission, which lies along the molecular region. This geometry suggests that warm dust is being swept from the HII region into the cloud. The edge of the molecular cloud is coincident with the boundary to the HII region as determined by the free-free radio continuum, suggesting strong interaction between the cloud and the HII region and associated with warm dust.

     

    • the Ophiuchi/PDR complex (Kulesa et al. 2005),

    • the R Coronae Australis (Groppi et al. 2005),

    • M 17 (Wilson, Hansen & Muders, 2003)

    • S140 and NGC 2023 (Savage & Ziurys, 2004)

    • Gem OB1/Sharpless 254-258(Bieging, Peters, Vila-Vilaro, in prep)

     

• Isotope Ratios:

   

  • Millimeter transitions of the CN radical can be a powerful tool in measuring 12C/13C ratio, a valuable chemical barometer in molecular clouds. Milam, Wyckoff and Ziurys (2005) using the ARO telescopes have acquired and compiled the most extensive data set of carbon isotope ratios to date for molecular clouds, building on the initial work by Savage et al. 2001. Studying sources that lie in the range of 0.09-16.4 kpc from the Galactic Center Milam et al. 2005 have demonstrated that the CN ratios result in a gradient similar to that found from CO and H₂CO observations. The agreement among the CO, H₂CO and CN observations suggests that chemical fractionation does not significantly influence the values of these ratios.

• Galaxies

   

  • The characterization of the molecular gas of external galaxies via CO, HCN and CS measurements has been the primary tool used here at the ARO facilities for extragalactic studies of molecular clouds. Studies of early type galaxies (Vila-Vilaro, Cepa & Butner 2003), Ultra Luminous Infrared Galaxies (ULIRGs), and nearby spirals (Walsh et al. 2002) have shown that many of these sources contain a warm and dense interstellar medium, indicative of an efficient star formation rate. Towards the spiral galaxy NGC6946 the molecular component has been found to be unusually massive and much of this material lies in warm gas of the spiral arms (Walsh et al. 2002).

   

  • Detections of CO in central regions of 110 galaxies in ARO surveys resulted in:

    • Universal ISM properties found in disks of spiral galaxies

    • Very high density gas detected in several nearby Seyfert galaxies

  • Detection of strong CO: J=3->2 emission in near-by face-on spiral galaxies

   

  • C I at 490 GHz detected in Seyfert/LINERS for the first time

   

• VLBI

   

  • Successful VLBI measurements from a 3-element array consisting of the SMT, the 12-m and the IRAM 30 m telescopes (Doeleman et al. 2002 and Krichbaum et al. 2004) at 129 GHz have detected SiO maser sources that represent the highest frequency spectral line VLBI detections to date. The J=3->2 transition of SiO in its v=1 state has been observed towards the red hyper-giant VY Canis Majoris, using 4 milli-arcsecond resolution. Maps reconstructed from these data suggest that SiO masers from a ring near the star. In addition, fringes had been detected at 1.3 mm in continuum using the SMT-IRAM 30 m transatlantic baseline towards the quasar 3C454.2; a resolution of 32 micro-arcsec had been achieved-the highest ever attained in radio astronomy.

   

• Solar System Objects

   

  • Numerous molecular line observations and subsequent analysis have been conducted towards various comets, including Linear, Neat and Hale-Bopp, using both telescopes (Milam et al. 2004). These measurements (Milam, Wyckoff and Ziurys 2005) have demonstrated that H₂CO is a common species associated with comets, and may be at least partially produced from paraformaldhyde frozen in the comet ice matrix. Observations at 345 GHz continuum emission from the asteroids Ceres, Pallas and Vesta by Barrera-Pineda et al. 2005, have yield the first sub-millimeter thermal light curves. Surprisingly, the emission from all three asteroids had been found to vary 15-20 % over the course of a single rotation, well in excess of the flux variations (< 5%) expected from shape projection effects. Physical differences must exist across the surface of these objects, a topic of ongoing NASA planetary astronomy investigations.

  • Monitoring of molecules in planetary atmospheres

    • Evaluation of mm spectra of the Venus mesospheric HCO revealed that between 1998 and 2004 conditions were drier than previously thought.

Synergies with other major forefront facilities

• Participation in International Millimeter VLBI
• Provided backup observations for INTEGRAL and ODIN satellites
• Participation in comet campaigns, including NASA Deep Impact Ground - Based Radio Science Team
• Provided both zero-spacing data and target sources for millimeter interferometers, in particular, Berkeley interferometer group.
• Galactic Plane surveys described above designed to complement existing efforts (e.g. the Molecular Ring Survey, the AST/RO Survey, and Spitzer Legacy Programs).
• Heterodyne focal plane array technology being developed for the SMT is a collaborative effort between many institutions.