Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.atnf.csiro.au/management/atuc/2005jun/docs/4.2_ATUC_7mm_workshop.doc Дата изменения: Wed Jun 8 09:34:03 2005 Дата индексирования: Sun Jun 27 16:04:00 2010 Кодировка: Поисковые слова: annular solar eclipse

ATNF ATUC Memorandum

To: ATUC
From: Enno Middelberg
Date: 6 June 2005
Subject Summary of 7 mm workshop on 31 May 2005

A workshop was held to extend the science case for the upgrade of the
Compact Array with receivers in the range 25 GHz to 50 GHz. 15 talks were
given by members of ATNF, ANU, UNSW, USYD and Swinburne, covering a wide
range of galactic and extragalactic astronomy. The talks indicated that
this system will be a considerable benefit to the Compact Array and the
Australian astronomy community.

The workshop began with a description of the receiver design and the
presentation of recent results from tests of the cooled LNAs. The expected
system temperatures match the requirements by NASA, who is a major sponsor
of the system to use it as a backup tracking station (10 h / week), and
hence promise an excellent sensitivity for astronomical observations.

The astronomical contributions can be loosely divided into four topics.

1. Star formation and stellar evolution

The transition from molecular cloud to high-mass (M>8 Msun) star is
poorly understood, in contrast to the formation of lower-mass stars.
Radio jets, an indicator for accretion disks in low-mass stars, have
barely been observed in high-mass stars, and the question remains open
whether massive stars form via the same processes as low-mass stars or
via coalescence.

When high-mass stars have finally formed, they ionize their surroundings,
forming HII regions of various degrees of compactness. These regions tend
to be small and very dense. CA observations at 7 mm will be able to probe
these regions as the optical depths of free-free and dust emission are
about the same, and the then almost continuous frequency coverage of the
CA will allow one to precisely model SEDs from 1 GHz to 100 GHz.

In the later stages of stellar lifecycles, SiO masers can be observed at
43 GHz in stellar atmospheres. With the CA, they would be visible almost
throughout the Milky Way and could be pinned down with high accuracies,
yielding valuable insights about stellar dynamics in our Galaxy. Finally,
7 mm offers the opportunity to study SN1987A in more detail. The CA's
resolution at 7 mm will be much better matched to HST and Chandra, it
will allow to resolve the interactions of the supernova's expanding shell
with the ISM and may help to study diffuse shock acceleration.


2. Molecular lines and masers

In the 7 mm band exists a wealth of molecular lines, many of them yet
unexplored, which can be used as tracers of various physical parameters.
Prominent lines include SiO, OH, methanol, and ammonia, but especially
the range 35 GHz to 50 GHz provides access to many more astrophysically
interesting lines.

Methanol masers have been studied predominantly at lower frequencies,
because the 25 GHz to 50 GHz range is not commonly accessible. The higher-
frequency transitions, however, can provide density and temperature
information, data for statistical analysis and appear to be the most
suitable for a blind survey.

3. VLBI

The mechanism by which gas in AGN is fed into the central supermassive
black holes is still unconstrained by imaging observations, because the
regions are very small and very distant. VLBI at 7 mm is an important
contribution in the quest for higher angular resolution, because it still
provides very good sensitivity. Spectral index studies of radio jets
combining 7 mm ATCA observations with 20 cm LBA observations seem
plausible because the resolutions are similar. Also SiO masers in stellar
envelopes could be targeted, possibly in collaboration with the Japanese
VERA network. VLBI observations with a combined VLBA+CA network would
yield some of the highest-resolution images available today.

As Mopra will also get a 7 mm receiver and Tidbinbilla may get improved 7
mm capabilities in the future, an Australian 7 mm network would be
formed, yielding images with a resolution of a few milli-arcseconds.

After the upgrade, the CA could also participate in the next-generation
ICRF, which will use observations at 24 GHz and 43 GHz.

4. The high-redshift universe

The most important single molecule to study is CO at high redshifts. CO
traces the pools of gas from which stars formed throughout the universe,
and have been found in quasars, ULIRGs, sub-mm galaxies, and radio
galaxies. Various selection effects influence the conclusions drawn from
the few existing observations, and a more general approach is needed to
study CO and star formation at high redshifts. The CA at 7 mm is the
ideal instrument to do this: at almost any one redshift above z=1.5, the
7 mm band will be the best choice in detection experiments, and in the
same range will provide crucially important information about lower
transitions which are needed to model the gas in these galaxies. A blind
CO survey in the 7 mm band would be much more efficient than at any other
frequency.

A 7 mm-upgraded CA could be used to study the Sunyaev-Zeldovich effect
which provides galaxy cluster masses independent of redshift. In a
compact configuration, a typical cluster would be detected in 12 h, and
with the new correlator in 1 h.

It was generally felt that the 7 mm system will fill an astrophysically
valuable gap hitherto left open by the millimetre upgrade, allowing
astronomers to almost freely choose their observing frequency at the CA.
Given that only few radio telescopes around the world can observe in the 7
mm band at all, this capability emphazises the CA's unique position.

Technical limitations allow only to observe either the 25 GHz to 40 GHz
range or the 30 GHz to 50 GHz range, but not the entire range of 25 GHz to
50 GHz where the atmosphere is sufficiently transparent. A poll among the
speakers and the audience at the workshop showed that a majority of more
than 90% opted for the higher frequency range.