Science Highlight




TW Hya

Science Target Overview

TW Hya is a pre-main sequence classical T Tauri star at a distance of about 52+/-1 pc (Mamajek 2005,2010). It is the most studied member of the TW Hydra association (TWA) of low mass stars. From a wide variety of previous observations from the infrared to submillimeter, TW Hya is known to have a hot inner disk extending to radii < 4 AU, which is optically thin in the IR, and a larger cold dust disk out to about 200 AU (see for example the introduction by Vacca & Sandell 2011, and references therein). Recent optical interferometry finds that TW Hya also contains a hot optically thick disk on even smaller size scales of ~0.5 AU, and suggests that the optically thin disk could be due to gas clearing by a planet (Akeson et al. 2011). TW Hya is apparently still accreting from its disk at a rate of about (4-20) x 10-10 Msun/year and the most recent estimates of its spectral type, mass, and age are M2.5V, 0.4 Msun, and 3 Myr (Vacca & Sandell 2011).

Millimeter and submillimeter observations of the continuum and spectral lines are particularly useful for tracing the outer cold disk. Previous observations by the VLA at 7 mm (Wilner et al. 2000), ATCA at 3 mm (Wilner et al. 2003), and the SMA at 1.3, 0.87, and 0.45 mm (Qi et al. 2004, 2006, 2008 and Hughes et al. 2011) reveal Keplerian rotation in the disk and an inclination angle of about 7 degrees (i.e. almost face-on). Detailed studies of the dust continuum properties from the SMA work suggest that there are centimeter-sized particles within the cold proto-planetary disk.

ALMA Data Overview

ALMA Science Verification data at Band 7 (~345 GHz) was taken for TW Hya on April 22, 2011. A scheduling block about 1.5 hours long was run three times in a row for a total of about 4.5 hours of observing time. Nine antennas were available during these runs, but one has to be flagged (see CASA Guide). All four available basebands were used, resulting in four spectral windows (spws) containing data. Two basebands were placed in the Lower Sideband (LSB) and two basebands in the Upper Sideband (USB). In the LSB the CO(3-2) line at a rest frequency of 345.79599 GHz is located in spw=2. In the USB the HCO+(4-3) line at a rest frequency of 356.7342 GHz is located in spw=0. The other two spectral windows do not contain strong spectral lines and are used for continuum. Each spectral window is 0.5 GHz wide and the channel width is 122 kHz. Because the ALMA correlator was configured to apply Hanning smoothing of the signal, the effective spectral resolution is about twice the channel width, which in this case is about 0.2 km/s. For the antenna configuration in use at the time, the angular resolution is expected to be about 1.5".

The ALMA CO(3-2) data presented here is similar to the Submillimeter Array data presented in Hughes et al. 2011 (ApJ, 727, 85), thoughthe SMA data have ~3 times smaller channel width at 50 kHz.

HCO+(4-3) data has not previously been published, but SMA HCO+(3-2) data (at 267.55762 GHz) is presented in Qi et al. 2008 (ApJ, 681, 1396). These SMA data have comparable angular resolution but a wider 203 kHz channel width.

We thank the following people for suggesting TW Hya for ALMA Science Verification: Meredith Hughes, Stuartt Corder, Karin Oberg, Chunhua Qi, Michiel Hogerheide, Andrea Isella, Dmitry Semenov; with special thanks to Meredith Hughes, Chunhua Qi, and Ken Young for assistance with the Science Verification process.


Using the data for publication

The following statement should be included in the acknowledgment of papers using the datasets listed above:

“This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00001.SV. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ."

Obtaining the Data

The data products are contained in three downloadable  files:

  • Uncalibrated data with tables for reduction
  • Calibrated data
  • Reference images

and can be downloaded here:  TW Hya ALMA Science Verification Data

PLEASE make full use of the CASA Guides provided for this data set: TW Hydra Band 7 CASA Guide (This link will take you to an external web site, hosting the CASA Guides.)

NOTE: This script was developed in CASA version 3.3. It will not run in later version of CASA. For additional information see the following knowledge base article: "If my data were calibrated and imaged in CASA 3.3 and I want to redo it, are there resources to help?".