The Serpens Star Forming Region in HCO + , HCN, and N 2 H +
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Transcript of The Serpens Star Forming Region in HCO + , HCN, and N 2 H +
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The Serpens Star Forming Region in HCO+, HCN, and N2H+
Michiel R. HogerheijdeSteward ObservatoryThe University of Arizona
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Outline
Molecular clouds and star formation The Serpens star-forming region Single-dish images Interferometer images Combination single-dish and interferometer Abundances A shock model for HCN and N2H+
Conclusions
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Molecular clouds and star formation
Stars form in condensations in interstellar clouds
Cloud structure determines stellar masses
Jets, outflows affect cloud chemistry and structure
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The Serpens star-forming region
Dense cluster of pre–main-sequence stars
Two condensations, NW and SE
~Dozen submm continuum peaks
Many associated with YSOs, some starless cores
SCUBA 850 µm on DSS imageDavis et al. (1999)
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JCMT/SCUBA images of Serpens
Davis et al. (1999)
SMM4
SMM11
SMM6
SMM2
SMM3SMM1/FIRS1
SMM8 SMM9/S68N
SMM10
SMM5
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Many outflows ‘Tangle’ of outflows
Intermediatevelocities:±7 km s-1
Extreme velocities:±11 km s-1
Davis et al. (1999)
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How do outflows affect structure and chemistry in Serpens? J=1–0 transitions (≈3 mm) of HCO+, HCN, N2H+
Common tracers of dense [n(H2)≈105–6 cm-3] and cool (Tkin≈30 K) gas
Morphology chemistry
Single-dish on-the-fly maps from Kitt Peak 12-meter telescope
Interferometer mosaics from the Berkeley-Illinois-Maryland array (SE region only)
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Single-dish maps
HCO+ 1–0 HCN 1–0 N2H+ 1–0
Resolution 1 = 0.12 pc = 21,000 AU
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Single-dish maps
N2H+ equally strong in NE and SE– Follows submillimeter continuum
HCO+ and HCN peak in SE – Where most embedded YSOs and their
outflows are E-W velocity gradient
– Solid-body rotation, also noted by Olmi & Testi (2002)
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Interferometer mosaics
Higher resolution: 10–20 arcsec; 140 arcsecs primary field of view
Filters out large-scale emission ~ 115 arcsec 13-point ‘mosaic’: overlapping pointings
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Interferometer mosaics: N2H+
Resolution 17 = 0.033 pc = 7000 AU
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Interferometer mosaics: N2H+
Color: N2H+
Contours: 850 µm
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Interferometer mosaics: HCNResolution 21 = 0.041 pc = 8500 AU
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Interferometer mosaics: HCO+
Resolution 12 = 0.022 pc = 4600 AU
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Interferometer mosaics
N2H+:– At continuum peaks– Ahead of SMM3’s jet– North of ‘shock position’
HCO+ and HCN:– near YSOs, outflows
Strong blue-shifted HCN west of SMM4:– ‘shock position’
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Combining single-dish and interferometer data Interferometers filter out emission on
scales larger ~ shortest antenna spacing
Missing ‘zero-spacing’ flux KP12m well matched to 6-m BIMA
antennas Method: joint deconvolution
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Combined BIMA and KP12m
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Combined BIMA and KP12m
Combined maps qualitatively look as expected
High resolution of BIMA brings out velocity details on small scales
‘Washed out’ in KP12m map BIMA recovers ~30% of line flux
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Abundances
Are HCO+ and HCN enhanced by outflow action?
Throughout core, or only locally?
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Abundances
Olmi & Testi (2002) C18O 1–0 map FCRAO: 1 arcmin
resolution Tex
N(H2) Use to derive
abundances on 1 arcmin scales
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Abundances
HCO+ HCN N2H+
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AbundancesAverageSerpens
Peakpositions
Dark cloud valuesa
N2H+ 3.8x10-10 7.8x10-10 (5–10)x10-10
HCO+ 3.0x10-10 6.5x10-10 (2–8)x10-9
HCN 5.4x10-10 9.6x10-10 (0.5–5)x10-8
Factor 2 enhancement near YSOs (outflows)Abundances HCN, HCO+<< dark clouds: depletion; Tex?
a) van Dishoeck et al. 1993; Ohishi et al. 1992; Turner 2000
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Shock model for HCN and N2H+
Offsets between HCN (color) and N2H+ (contours)
SMM3’s jet Shock position
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Shock model for HCN and N2H+
C-type shock Magnetic precursor Ions accelerate, compress,
and heat before neutrals N2H+ emission up in
precursor HCN abundance up in
warm region: evaporation of ices
Accompanying H2O destroys N2H+
Draine & Katz (1986)
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Two models for shock position: 1
Jet driven by SMM4, deflected by dense material. N2H+ in magnetic precursor of C-type shock.
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Two models for shock position: 2
Jet driven by SMM1, hitting dense matter. HCN at bow shock, N2H+ along the sides where shock speeds are lower.
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Conclusions
HCN, HCO+ locally enhanced by shocks Depleted in rest of cloud compared to dark-cloud
values Unresolved observations would trace outflow-
affected material preferrentially
N2H+ undepleted: traces condensations N2H+ emission ahead of shocks: enhancement in
magnetic precursor, destruction in warm region?
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Future work
C18O on 10–20 arcsec scales, fully sampled
Higher-J lines: excitation HCN, HCO+, and N2H+
High-resolution interferometry shock region; additional species
Time-dependent shock-chemistry model
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Many thanks to…
Staff of the Kitt Peak 12 meter Radio Telescope Staff of the Berkeley-Illinois-Maryland Association
millimeter array Chris Davis and Luca Olmi for making their data
available electronically