A massive disk around

the intermediate-mass young star

AFGL 490 ?

Katharina Schreyer (AIU Jena, Germany)

Thomas Henning (MPIA Heidelberg, Germany)

Floris van der Tak (MPIfR Bonn, Germany)

Annemieke Boonman (Univ. Leiden, NL)

Ewine F. van Dishoeck (Univ. Leiden, NL)

Ø100´´

Introduction – Motivation

formation of high-mass stars – one of the unresolved mysteries of the present research

dominant formation process: disk accretion or coalesence ?

recent detections of disks around massive protostars: IRAS 20126+4104 (1.7kpc, Cesaroni et al. 1999, Zhang et al. 1998, B2) & G 192.16-3.82 (2.0kpc, Shepherd et al. 2001, B2...3) disks are more massive and larger than disks around T Tauri and Herbig Ae stars

Search for high-mass objects in early evolutionary states survey of bright IRAS sources (Klein, Posselt, Henning, Schreyer: Poster) one of these targets: AFGL 490

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AFGL 490 — General Properties

optical: diffuse nebulosity, NIR: luminous source

D = 1 kpc, L= 1.4 – 4·103 L early B2..3 star, M= 8...10 M

typical properties of a Becklin Neugebauer Object: - weak continuum flux at 1cm - broad & strong Br and Br (Bunn et al. 1995)

ionized region 100 AU(Simon et al. 1981, 1983)

AFGL 490

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K-band image

AFGL 490 — General Properties

embedded in a dense cloud core (e.g Kawabe et al. 1984, Snell et al. 1984)

poorly collimated high-velocity outflow (e.g. Lada & Harvey 1981)

previous interferom. observations presence of a huge disk ? (Mundy & Adelmann 1988, Nakamura et al. 1991)

- 3mm cont.: 2500 x 1500 AU- 13CO 1–0: 45000 x 14000 AU

Motivation:study of this disk-like structure

Our Observations:used JCMT, IRAM 30m, PdBI

13CO 1 – 0

Texas telescope

NMA

OVRO

4/9

2500

0 AU

box: 55´´x 55´´

AFGL 490 — Observational Results: CS 2–1 PdBI

bar-like structure(2.5x0.4)104AU

different outflowsystems

an unvisiblejet enters the denser cloud material ?

disk-like system aroundAFGL 490

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AFGL 490 — Comparison of the CS 2–1 line wings with : 6/9

Model of a typical diskof a Herbig Ae star (R = 400 AU)

4000

AU

large-scale high-

velocity CO outflow

(a) VLA 2cm continuum map (b) Speckle H-band image (Campbell et al. 1986) (Hoare et al. 1996)

repeated 2cm + H band observations by Hoare (2001): at the moment a point source

-

A disk around AFGL 490 ? 7/9

Mass - from a Keplerian model – fit to the outer line wings:estimates Mdisk = 7...9 M inside R = 4000 AU (M= 8 M, i = 20°)

- from the 3mm continuum (deconvolved point source): Mgas = 3...6 M inside R = 500 AU (Tkin = 100...150 K)

M Mdisk dynamical / self-gravitational stability ? lifetime ?

dynamical stability Toomre´s Q parameter (e.g. Stone et al. 2000):

with epicylce frequency = (GM/r3)0.5 & surface density = Mdisk/R2disk

when Q < 1: disk locally graviationally unstable, fragmentation

AFGL 490: Rdisk= 300…4000 AU, Tdisk= 50…200 K, Mdisk= 3…10 M Q < 0.5

AFGL 490 — Estimate of the lifetime against: 8/9

Its known more evolved Be stars (tlife = 105…106 yrs) have no disks anymore (Natta et al. 1997) speculation about the destruction mechanism:

(a) photoevaporation: (weak wind model by Hollenbach et al. 1994): M= 8 M, Mdisk= 6 M

Ly continuum flux = 3x1044 s-1 tdestruction = 108 yrs

(b) accretion onto the star: tacc = M/M, with M = 10-5 M/yr (Palla & Stahler 1992),

M= 8 M taccrection = 8x105 yrs

- large compared with tdyn(outflow) = 2x104 yrs (Churchwell 1999)

- to build up a star with 8 M M must have been larger in the past

(c) self-gravitation: e.g. Adams et al. (1989), Laughlin & Bodenheimer (1994) – evolution of disk with Q 1 : fragmentation within the time of the orbital period ( tdestruction = 103–104 yrs) most important destruction mechanism

• •

•

AFGL 490 — Model Conclusions

inner free zoneR = 50..100 AU

a larger gas torus R 4000 AU feeds an inner(accrection) diskR 500 AU

remnant of the flattened innercloud core R 25000 AU

further high-resolution observationsand theoretical work are needed

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25 00

0 AU

END

END

AFGL 490 — Our Observations

Single-dish Observations

Mapping in - CS J = 5 – 4, 7 – 6, C18O 2 – 1: JCMT 15m, 1994 - CS 3 – 2, 2 – 1, C18O 2 – 1: IRAM 30m, 1995 - Continuum 450m & 870 m, SCUBA, 1999 - Set of molecular lines at [0,0] position

Plateau de Bure Interferometer Observations

Mapping in - CS 2 – 1 & continuum al = 97.98 GHz

- clean beam size: 2.73´´ 2.22´´ - primary beam 51´´

AFGL 490 — Observational Results

CS 2–1 PdB Interferometer

IRAM 30m

primary beam Ø51´´

K-band image (Hodapp 1994) + CS 2–1

AFGL 490 — Observational Results: CS 2–1 PdBI Spectra

AFGL 490 — Observational Results: Single-Dish

AFGL 490:- embedded in a dense cloud core - CS maps: spherically symmetric morphology- C18O maps: extended in north-south similar to the continuum for 800m

AFGL 490 — Observational Results: = 3mm PdBI

Continuum

only one strong mm source

green contours:> 3 rms

white contours:1 rms

AFGL 490 — Position-velocity-maps

A simple model of Keplerian motion (Vogel et al. 1985)

Assumptions:- rotational equilibrium model: a central star + the disk mass lineary increasing with the radius- Rdisk = 4´´ = 4000 AU, M = 8(1) M

Fit parameters: Mdisk = 7...9 M, inclination angle i = 20°