Gas-dust thermal coupling in proto-planetary disks

Coordinator: A. Carmona (ESO-Garching and MPIA Heidelberg) and D. Fedele (ESO-Garching)

Co-Is: M. Goto, T. Henning, H. Linz, B. Stecklum, M. van den Ancker, M. Wittkowski


We propose to obtain observations of three bright pre-main sequence stars in the CO overtone band at 2.3 microns and in the water emission region around 2.2 micron in order to study the gast/dust coupling in their protoplanetary disks. The primary scientific objective is to spectrally resolve the emission of warm and hot molecular gas in the the planet-formation region of the disks (R 50 AU). These observations will permit to constrain the dynamics, temperature and mass of the inner disk. Furthermore, the observations proposed here offer an excellent opportunity to test the maximum achievable spectral resolution and S/N with CRIRES.

Allocated Time: 3 hours

Targets list

NameRA(2000)DEC(2000)Range (nm)Wavelength IDF_nu or F_lineDIT(s),NDIT
DoAr 2116 26 03.03-24 23 36.42087-213327/ 1/n1e-15 erg/s/cm230, 10
.........2200-225525/-1/n...30, 10
.........2253-231025/ 1/n...30, 10
............25/-1/n...30, 10
HD 13534415 15 48.44-37 09 16.02087-213327/ 1/n1e-15 erg/s/cm230, 10
.........2200-225525/-1/n...30, 10
.........2253-231025/ 1/n...30, 10
............25/ 1/i...30, 10
WL 1616 27 02.5-24 37 30.02087-213327/ 1/n1e-15 erg/s/cm230, 10
.........2200-225525/-1/n...30, 5
.........2253-231025/ 1/n...30, 10
............25/ 1/i...30, 10
T CrA19 01 58.78-36 57 49.92087-213327/ 1/n1e-15 erg/s/cm230, 10
.........2200-225525/-1/n...30, 10
.........2253-231025/ 1/n...30, 12
............25/ 1/i...30, 12
HD14156915 49 57.74-03 55 16.32253-231025/ 1/n1e-15 erg/s/cm230, 12
............25/ 1/i...30, 12

Project description/scientific objective:

Planets are thought to form in a (protoplanetary) disk rotating around the star during its pre-main sequence phase. The disk, consisting of gas and dust, evolves until it dissipates all its material and disappears leaving a planetary system orbiting around the central star. In theoretical models usually the assumption is made that gas and dust are thermally coupled in such disks. This allows to solve the equation of hydrostatic equilibrium once the properties (temperature and density) of the dust are known. There are however concerns about the validity of this assumption. If these doubts are confirmed, this might have important consequences for planet formation theory, e.g. the de-coupled dust might easily settle and condense on the disk mid-plane. This may enable the formation of giant planets in the core-accretion scenario for planet formation.

We propose to obtain high (R > 50,000) spectral resolution and high S/N (systematics-limited; > ~50) CRIRES spectra of three bright (M = 4.2-5.1) pre-main sequence stars (PMSs), where to search simultaneously for the CO overtone at 2.3 micron and water emission at 2.1218 (v=1-0 S(0)) and 2.2214 (v=1-0 S(1)) micron.

Our top priority candidates are three intermediate-mass (Herbig Ae/Be) pre-main sequence stars: DoAr 21, for which Bary et al. (2003) already detected H2; HD 135344, which has a flared disk; and WL16, a Herbig star, for which Najita et al. (1996) reported the detection of CO overtone emission. These new observations will reject or confirm the above tentative results and will expand the above work to a wider selection of lines. In addition, we have also included in our targets list one backup target (T CrA), only visible from the south and for which no such data exist in the literature, and one optional target (HD141569), in which Goto et al. (2006) already detected CO.

We believe that this project is ideal for the CRIRES SV for 3 main reasons: (1. scientific) spectrally resolved NIR gas lines are a powerful tool of investigations of protoplanetary disks (one of the main science drivers for CRIRES); (2. competitive) similar projects are right now being pursued with Keck/NIRSPEC; the best way to ensure a high visibility of CRIRES results in this field is to obtain and present some initial results on YSO disks now (3. technical) these observations provide a good verification of the maximum achievable spectral resolution and S/N for bright sources in these wavelength regions.

Members of our team have extensive experience in reducing and analyzing infrared spectroscopic data. We plan to analyze our data using the CRIRES pipeline (if available), augmented by our own custom-built IDL procedures for optimal removal of telluric features by matching the exact wavelength calibration and achieved spectral resolution between science spectrum and the spectrum of the telluric standard.

With these data we can
1) explore the molecular gas within 10AU from the stars, which is little understood to date, but has direct consequences for planet formation.
2) reveal the disk kinematics at the inner edge of the gas disk. With the high spectral and spatial resolution of CRIRES, the assumption of Keplerian rotation of matter in the disk can be tested directly. One exciting possibility is that this method may directly reveal the presence of gaps in the radial distribution of matter in the disk, thereby providing indirect evidence for the presence of planets.
3) by comparing the observations in CO and H2 with different tracers (e.g. ionized gas/dust grains) we can attempt to understand the chemistry in the disks during its evolution.

Methods: H2 is the most abundant gas-phase molecule. To determine the excitation mechanism measurements of two lines are needed. The H2 2.12 and 2.22 micron emission would give us constraints on the hot inner part of the disk. CO is the second most abundant molecule and the gas molecule with the largest Einstein coefficient. The combination of H2 and CO data will give us a handle on the chemistry of the disk.

Observing strategy

The proposed observations consist of high resolution spectroscopy of three young stars. No preferential setting of slit width and position angle are requested, and the 0.4" slit width recommended is fine for the project. All three sources are expected to be point-like in the near-IR continuum.

The minimum data set that must be obtained for this project to be scientifically useful is a complete set of spectra of a single source. A smaller set of wavelength settings than requested here will not allow a meaningful analysis of these data. Observations of more than one source will allow us to investigate the variation of disk gas and gas/dust ratio with disk parameters.

Telluric standard star observations should be carried out one per science target, immediately following the science observation, and with airmass identical (within 0.1) to the median airmass of the science target.


[1] Goto, M., Usuda, T., Dullemond, C. P., Henning, Th., Linz, H., & Stecklum, B. 2006, ApJ, in press. (astro-ph/0606362)
[2] Carmona, A., van den Ancker, M.E., Thi, W-.F., Goto, M., Henning, Th. 2005, A&A, 436, 977
[3] Brittain, S. et al. 2003 ApJ, 588,535
[4] Blake, G. & Boogert, C.A 2004 ApJ, 606 L73
[5] Bary, J.S., Weintraub, D.A and Kastner, J.H., 2003, ApJ, 586,1136
[6] Najita, J., Carr, J.S., Glassgold, A.E., Shu, F.H., Tokunaga, A.T., 1996, ApJ, 462,919