Circumstellar disks around massive Young Stellar Objects

Coordinator: A. Bik (ESO-Garching) and H. Linz (MPIA Heidelberg, Germany)

Co-Is: H. Beuther, M. Goto, T. Henning, E. Puga Antolin, B. Stecklum, W-.F. Thi, M. van den Ancker, L.B.F.M. Waters


We propose R=50,000 observations of two massive Young Stellar object to characterise the warm molecular gas and its dynamics in the inner parts of a massive circumstellar disk - a region that is in general still poorly constrained to date.

Allocated Time: 2 hours (1h per target)

Targets list

NameRA(2000)DEC(2000)Range (nm)Wavelength IDF_nu or F_lineDIT(s),NDIT
18006nr76618 03 40.23-24 22 39.62253-235625/ 1/n100 mJy60, 8
............25/1/i...60, 8
............24/-1/n...60, 8
............24/-1/i...60, 8
.........2115-217426/-1/n...60, 5
............26/-1/i...60, 5
AO Ref. Star: Her3618 03 40.20-24 22 43.0------V=9.1dist.=4", PA=185.5
Telluric Std. Star: HD 17589218 58 24.83-22 31 46.2------V=6.1, Sp.Type=A1V---
M 8E IR18 04 53.177-24 26 41.42122-217526/-1/n10400 mJy30, 2
............26/-1/i...30, 2
.........2253-230425/ 1/n10400 mJy30, 8
............25/ 1/i...30, 8
............24/-1/n...30, 8
............24/-1/i...30, 8

Project description/scientific objective:

Disks are found around many young low-mass stars as a natural byproduct of their formation process. Observational evidence around their more massive counterparts, however, remain sparse and many details of massive star formation are not yet clarified. Extended disks have been found around very young massive stars (e.g. Chini et al 2004). In Bik et al (2006) we have identified a sample of objects which likely already have finished their formation, but are still surrounded by remnants of their accretion disk.

Only a few attempts to find massive disk kinematics via 2.3 micron CO bandhead emission have been undertaken to date. They were successful in a few cases, which proves the general concept of using CO rovibrational lines to trace (massive) disks. The main goal of this programme is to scrutinize two well-known massive YSOs with high-resolution IR spectroscopy where (indirect) hints for circumstellar disks are already compelling, based on previous studies.

With the high resolution of CRIRES we will achieve the following:
- Resolve the Bracket gamma line, look for the double peaked profile expected from a Keplerian disk.
- Seperate out the other components in the line, e.g. the diffuse HII region and possible outflows.
- Resolve the CO ro-vibrational lines to detect the double peaked profile which allows a model free measurement of the rotation velocity.
- Search for possible H2O (and OH) emission bluewards of the CO bandheads (see Thi & Bik, 2005) which will provide us with information about the chemistry of the inner disk regions.
- Additionally, the long slit of CRIRES can be used to study the kinematics and ionization of the diffuse HII region close to the YSO.
- Confirm the presence of CO ro-vibrational lines associated with the deeply embedded massive young stellar object M 8E IR.
- Reveal the kinematics of the molecular gas within the potential circumstellar disk associated with the two targets.
- Trace differences of the gas kinematics along the jet/outflow direction compared to the data obtained along the disk major axis.
- Attempt a preliminary analysis of the excitation conditions in the molecular gas by analysing the CO line ratios and by showing the (non-)existence of higher excitation CO lines within our chosen spectral band.

The data will be reduced using the IRAF long-slit package, the ESO pipeline (if available), and customised scripts and routines we have alread tested on previous data sets taken at different telescopes (VLT, UKIRT, Gemini South, and Subaru). In this way, we will be able to compare our independently reduced data with the outcome of the beta version of the pipeline provided by ESO.

Observing strategy

For target 18006nr766:
Slit position and rotation angle (PA=90) are indicated on the finding chart. The angle is chosen such that the bright star south of the object does not fall in the slit.

Please observe the telluric immediately after the science observation and with an airmass difference 0.1 .

For target M 8E IR:
Telescope guiding itself should be possible on the target itself (R=16.88 mag). Switch off the AO system, since potential AO reference stars are too distant.

The Brackett gamma settings 26/-1/n and 26/-1/i need to be repeated four times, each at a different slit orientation: -40 degrees, +50 degrees, +140 degrees, and +230 degrees. This is because we want to apply the spectro-astrometry technique to the Br gamma line of M8E-IR. Therefore, we have to investigate if asymmetries in the slit shape exist, which is done here by cross-checking the results for one slit orientation with the corresponding results at a slit orientation of 180 degrees difference (anti-parallel). We chose the general starting point of -40 degrees for the slit orientation, since the large scale bipolar CO outflow is known to be oriented in this direction.

The CO bandhead settings should be done just in one slit orientation, namely -40 degrees (orientation always counted from the north over east).

For the telluric standard stars, we chose bright objects in order to have at least the same SNR for the standard star as for the science target. We list here HR 6879 and HR 7121 as possible choices, that are relatively near to the science target. If possible, observe one of these at a similar airmass to M8E-IR. For these, one slit orientation is enough (-40 degrees). Because we have to do the science target without the AO, it would be desireable to do also the telluric standard star observations with AO switched off. Otherwise, the spectral resolution might differ between these observations and a telluric line cancelation by dividing just the science target by the standard star would be very difficult (an AO-corrected point source is almost totally within the slit of 0.4" width, while a seeing-limited object is not).

HR 7121: K = 2.43 mag, RA=18 55 15.92, DEC=-26 17 48.2, J2000, SpT=B2.5V HR 6879: K = 1.80 mag, RA=18 24 10.33, DEC=-34 23 04.6, J2000, SpT=B9.5III

For these standard stars, 4 x 5s for each of the necessary wavelengths settings is sufficient to reach an even better SNR that for the science target, which is necessary to not degrade the quality of the science target results.

All the objects are quite bright at K band. Use only short DIT settings for the slit viewer!


Chini, R., Hoffmeister, V., Kimeswenger, S., Nielbock, M., N�rnberger, D., Schmidtobreick, L., Sterzik, M. 2004, Nature, 429,155
Bik, A., & Thi, W.F. 2004, A&A 427, L13
Thi, W.F., & Bik, A. 2005, A&A, 438,557
Bik, A., Kaper, L., Waters, L.B.F.M. 2006, A&A, 455,561