The Serpens Star Forming Region in HCO+, HCN, and N2H+

Michiel R. HogerheijdeSteward ObservatoryThe University of Arizona

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

Molecular clouds and star formation

Stars form in condensations in interstellar clouds

Cloud structure determines stellar masses

Jets, outflows affect cloud chemistry and structure

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)

JCMT/SCUBA images of Serpens

Davis et al. (1999)

SMM4

SMM11

SMM6

SMM2

SMM3SMM1/FIRS1

SMM8 SMM9/S68N

SMM10

SMM5

Many outflows ‘Tangle’ of outflows

Intermediatevelocities:±7 km s-1

Extreme velocities:±11 km s-1

Davis et al. (1999)

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)

Single-dish maps

HCO+ 1–0 HCN 1–0 N2H+ 1–0

Resolution 1 = 0.12 pc = 21,000 AU

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)

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

Interferometer mosaics: N2H+

Resolution 17 = 0.033 pc = 7000 AU

Interferometer mosaics: N2H+

Color: N2H+

Contours: 850 µm

Interferometer mosaics: HCNResolution 21 = 0.041 pc = 8500 AU

Interferometer mosaics: HCO+

Resolution 12 = 0.022 pc = 4600 AU

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’

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

Combined BIMA and KP12m

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

Abundances

Are HCO+ and HCN enhanced by outflow action?

Throughout core, or only locally?

Abundances

Olmi & Testi (2002) C18O 1–0 map FCRAO: 1 arcmin

resolution Tex

N(H2) Use to derive

abundances on 1 arcmin scales

Abundances

HCO+ HCN N2H+

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

Shock model for HCN and N2H+

Offsets between HCN (color) and N2H+ (contours)

SMM3’s jet Shock position

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)

Two models for shock position: 1

Jet driven by SMM4, deflected by dense material. N2H+ in magnetic precursor of C-type shock.

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.

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?

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

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