Weighing a high mass protostellar candidate

Coordinator: D. Nuernberger (ESO-Chile)

Co-Is: R. Chini, R. Siebenmorgen


We propose both high angular resolution (AO supported) and high spectral resolution observations of a large circumstellar disk, seen (almost edge-on) in silhouette against the diffuse background emission of the M17 HII region. The CRIRES data will allow us to investigate in so far unmatched detail the kinematics of gas and dust within the disk plane and, by comparison of the derived (Keplerian?) rotation curve to theoretical models, to significantly constrain the mass of the central protostellar source.

Allocated Time: 4 hours

Targets list

NameRA(2000)DEC(2000)Range (nm)Wavelength IDF_nu or F_lineDIT(s),NDIT
M17 disk18 20 26.3-16 12 102166 +/- 126/-1/n10^-15 erg/s/cm^2900, 18
HII ref18 20 25.0-16 12 102166 +/- 126/-1/n10^-15 erg/s/cm^2900, 2
SAO161357 (AO-ref)18 20 27.64-16 12 01.10------V=9.7, K5V---

Project description/scientific objective:

High mass protostars are usually deeply embedded in their natal environment which can be penetrated only at wavelengths beyond the mid infrared. However, in the neighbourhood of early type main sequence stars, strong stellar winds and energetic/ionizing photons might help to lift the curtains, by evaporating and dispersing remnant material of the molecular cloud core and thus making the protostars accessible at a relatively early stage to today's state-of-the-art near and mid infrared instruments.

Such premises are given, e.g., at the interface between the M17 HII region and its adjacent molecular cloud. There, Chini et al. (2004) have discovered and characterized a tremendous opaque silhouette against the diffuse homogeneous background emission of the M17 HII region. This silhouette, unambiguously seen in near infrared data taken with ISAAC and NACO, is associated with an hourglass shaped nebula and surrounded by a large disrupting envelope.

Based on CO data obtained with the Plateau de Bure interferometer (PdBI), Chini et al. (2004) estimated that the entire disk + envelope system might contain a gas and dust mass of up to 110 M_sol. Although the kinematics of the innermost part of the disk is not properly traced by the PdBI CO data due to the limited spatial resolution (~7 arcsec) as well as due to the potential confusion by emission from outflowing and infalling material, the millimeter spectroscopy indicates that the disk + envelope system slowly rotates at a velocity of about 0.85 km/s. Dynamical considerations concerning the (Keplerian) rotation of the outer disk yield a first (but rather uncertain) mass estimate of about 15 M_sol for the central protostar.

In the framework of the SINFONI Science Verification programme, Nuernberger et al. (2006) performed AO supported IFU spectroscopy of the M17 silhouette and discovered an associated H_2 jet. Because ejection of material through a jet / outflow is always linked to accretion of gas and dust either onto the circumstellar disk or onto the central (protostellar) source, the presence of the H_2 jet provides indirect but unquestionable evidence for ongoing accretion processes. As derived from the SINFONI data, high mass outflow and mass accretion rates of > 10^(-5) M_sol/yr suggest that indeed a star of rather high mass is forming.

The SINFONI data reveal that the image contrast between the disk silhouette and the diffuse background emission of the HII region is enhanced in lines (Br gamma, Br delta and HeI) compared to the neighbouring continuum. Therefore, if the absorbing gas and dust follows any kinematics (rotation) within the circumstellar disk, it will leave corresponding (blue/redshifted) signatures in these line profiles. If one follows the (Keplerian) rotation curve derived from the PdBI CO data, one would expect to measure rotation velocities of the order of a few km/s at the innermost parts of the disk. This is exactly the point where CRIRES comes into play! CRIRES mounted at UT1 provides angular resolution, spectral resolution and sensitivity sufficiently high enough to investigate in unmatched detail the kinematics of gas and dust within the M17 silhouette disk. Subsequent comparison of the derived (Keplerian?) rotation curve to simple theoretical models will then allow us to unambigously nail down the mass of the central protostar.

Therefore, we propose to observe the M17 silhouette disk with CRIRES in the wavelength range 2166 +/- 1 nm (i.e., Br gamma line) which will provide the best image contrast [about 10^(-15) erg/s/cm^2 as shown by our SINFONI data; Nuernberger et al. 2006] between the opaque disk silhouette and the diffuse background emission of the HII region. The undisturbed line profile of the latter has to be determined at a slightly offsetted reference position, because the full diameter of the circumstellar envelope (up to 40 arcsec; although the densest part of the disk itself measures less than 5 arcsec in diameter) is comparable to the length of the CRIRES slit. To avoid confusion from gas and dust above/below the disk midplane, the slit width should be chosen as small as possible, i.e., 0.4 arcsec as recommended by the instrument team for science verification programmes.

Observing strategy

Following the most recent CRIRES ETC, version 3.0.8, we can expect to achieve a S/N of 10 for a line of flux = 10^(-15) erg/s/cm^2 and FWHM = 1 nm with a setting of DIT = 900 seconds and NDIT = 19 in the instrument mode 26/-1/n (as selected by the ETC) and in standard observing conditions (seeing = 0.8 arcsec, airmass = 1.2). Taking into account 2 separate integrations at the HII reference position, we request a total on-source integration time of 5 hours to trace the gas and dust kinematics even in those parts of the disk where the image contrast is rather faint.

A suitable star for AO referencing (SAO 161357) is available about 20 arcsec towards the east of the M17 silhouette disk (see FCs). It has already been used successfully for AO supported observations of the M17 silhouette disk with NACO and SINFONI.

The M17 silhouette (disk + envelope) is seen almost edge-on; the inclination between the disk midplane and the line-of-sight is about 10 deg. The overall (midplane) diameter of the envelope is up to 40 arcsec, though the densest part of the disk itself measures less than 5 arcsec in diameter.

The disk midplane has a position angle of about 148 deg and the CRIRES slit (width = 0.4 arcsec) should be oriented accordingly to trace this midplane. The disk midplane goes through the position of the central protostar, i.e., RA = 18 20 26.30 and DEC = -16 12 10.0 (J2000.0); please note that the central protostar is very faint (K=19.5 mag) and won't be seen in any acquisition image.

Spectra of the diffuse background emission of the HII region have to be taken at a reference position, as the full diameter of the circumstellar envelope (up to 40 arcsec) is comparable to the length of the CRIRES slit. To avoid straylight from the AO reference star, we suggest a reference position offsetted by about 20 arcsec towards the west of the silhouette disk (see coordinates for HII ref given above).


Chini R., Hoffmeister V., Kimeswenger S., Nielbock M., Nuernberger D., Schmidtobreick L. & Sterzik M., 2004, Nature 429, 155
Nuernberger D.E.A., Chini R., Eisenhauer F., Kissler-Patig M., Modigliani A., Siebenmorgen R., Sterzik M.F. & Szeifert Th., 2006, A&A, submitted