Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.atnf.csiro.au/whats_on/workshops/mm_science2002/talks/cpurcell.pdf Дата изменения: Tue Nov 26 05:49:39 2002 Дата индексирования: Wed Dec 26 10:27:35 2007 Кодировка: Поисковые слова: star formation

Hot Molecular Cores at Mopra
Molecular line observations of star-forming regions 2000­2002
Cormac Purcell, Ramesh Balasubramanyam, Lucyna KedzioraChudczer, Vincent Minier, Tracey Hill, Michael Burton, Maria Hunt, Peter Barnes, Jill Rathborne.

Cormac Purcell, Star Formation Group, UNSW.


What is a `Hot Molecular Core'?
~104 yr Molecular Cold Core Cloud
Hot Core Phase Methanol Maser

~105 yr
UCHII Phase

Massive Star

~104 yr ?

Walsh et al 1998

· Protostellar source at an evolutionary stage characterised by rich chemistry. · High abundances of saturated hydrocarbons fuelled by evaporation from icy grainmantles · n(cm-3)~107 100K Cormac Purcell, Star Formation Group, UNSW.


What is a `Hot Molecular Core'?
· Temperature gradient: the volatile non-polar ices evaporate first leading to an `onion layer' effect.

From Van Dishoeck et al 1998, after Tielens et al 1991

· Short lifetimes lead to non-equilibrium chemistry.
Cormac Purcell, Star Formation Group, UNSW.


What is a `Hot Molecular Core'?
· Time dependent chemical models (Rodgers & Charnley) · Depending on the initial abundances, the chemistry can be N-rich or O-rich
N-Rich Chemistry O-Rich Chemistry

Rodgers & Charnley, 2001

· Chemical fingerprints for different evolutionary stages
Cormac Purcell, Star Formation Group, UNSW.


UNSW HMC Project
· Project Objectives:
­ Identify HMCs through their CH3CN emission. ­ Undertake a mm line survey of candidate hot molecular cores and establish their chemical and physical characteristics. ­ Constrain current chemical models of hot cores with observations, leading to the development of a time dependent `chemical fingerprint'.
Cormac Purcell, Star Formation Group, UNSW.


Source Selection
· 82 sources towards the inner galaxy. · Source selection criteria:
­ Red IRAS colours- difference between 12, 25 & 60µm fluxes. ­ Association with methanol maser at 6.67GHz. ­ Association with a radio continuum source signifying a UCHII region.

· Sample split into two groups with strong and weak radio continuum.
Cormac Purcell, Star Formation Group, UNSW.


The Molecules- CH3CN
· Confined to hot core phase of star formation.
­ Formed by reactions involving species desorbed from dust grains

· Symmetric top- each (J+1) to J transition has K=0 to K=J components.
J(5-4), K=3, =91.987 GHz

K=0 K=1

K=2

K=3

K=4

Cormac Purcell, Star Formation Group, UNSW.


The Molecules- CH3CN
· Derived Parameters: rotational temperature, T and beam-averaged column density.
rot

J.M. Hollis, ApJ 160:159-162

­ Slope of line fitted through integrated fluxes of Kcomponents ~ Trot ­ Y-axis intercept ~ beam-averaged column densities.

· Assumes LTE conditions and optically thin lines
­ May not always be valid.

Cormac Purcell, Star Formation Group, UNSW.


The Molecules- HCN
· Large dipole moment & high abundance
­ good tracer of dense gas.

· Each rotational transition split into F = 0-1, F=1-1 & F=2-1 levels, with angular momentum J-1, J & J+1
J(1-0), F=(1-1) =90.665 GHz

F(1-1) F(2-1) F(0-1)

· Ratios of hyperfine components sensitive to optical depth effects.
­ At LTE predicted integrated fluxes in the ratio of 1:3:5
Cormac Purcell, Star Formation Group, UNSW.


The Molecules- HCO+
· Large dipole moment & high abundance
­ Abundance enhanced near central source where the ionisation fraction is highest. J(1-0), =89.189 GHz

· Exhibits saturated and self-absorbed line profiles in regions of massive star-formation.
­ Signatures of infall / outflow in line profile asymmetries.

Cormac Purcell, Star Formation Group, UNSW.


The Molecules- Others
· N2 H
+

­ Distinguishes between the two primary reaction networks- N-rich and O-rich.

· HNC & H13CO+
­ Complimentary to the HCN and HCO+ observations, may reveal optical depth effects. ­ HCN / HNC ratio depends on a number of key reactions.

· CH3OH
­ Compare line strenghts to methanol maser fluxes and the number of maser spots.
Cormac Purcell, Star Formation Group, UNSW.


Observations & Data Reduction
· Position switching ­ 10` / 10 nod · Bandwidth: 64MHz · Typical Tsys ~290K · Sensitivities: ­ CH3CN: 25 mK ­ Others: 50mK · SPC coupled with in-house scripts used to form quotients and average scans. · TCL/TK script available at : ­ www.phys.unsw.edu.au/astro/mopra
Cormac Purcell, Star Formation Group, UNSW.


Initial Results
· 600+ spectra taken from 2000 to 2002 · Initial reduction on the 2000 & 2001 data:
Molecule:
CH3CN HCN HCO+ HNC H13CO+

Reduced:
79 / 82 79 / 82 79 / 82 68 / 82 45 / 82

Detection Rate:
32% 85% 91% 95% 88%

· 25 detections of CH3CN from our source list.
Cormac Purcell, Star Formation Group, UNSW.


IRAS 17470-2853 (G0.54-0.85)
CH3CN

HCN

· Strong candidate Hot Core · Strongest CH3CN emission of the 25 detections ~ 0.2 K. · Trot derived to be 108.2 K. · Strong methanol maser emission ~ 19 Jy.
­ Strong methanol line at 96.7 GHz ~ 1 K.

HCO+

CH3OH

· Weak associated radio flux at 8.7 GHz ~ 87 mJy
­ Supports hypothesised evolutionary scenario.
Cormac Purcell, Star Formation Group, UNSW.


Looking Forward
· Next 3 months:
­ Reduce and analyse current data.

· 2003
­ Any other interesting lines?- Thermal SiO ­ Additional sources- cold cores seen in 1.2mm SIMBA maps (Tracey Hill).

· High-resolution molecular maps of selected sources using ATCA
­ See Vincent Minier's presentation.

Cormac Purcell, Star Formation Group, UNSW.