HCN Near IK Tau and TX Cam

Kevin MarvelAmerican Astronomical Society

Waikoloa Beach Outrigger ResortKona-Kailua, HawaiiMonday July 1, 2002

Evolved Stars by Radio

• AAVSO members have long aided professional astronomers by observing long period variable stars

• Radio Astronomers tuned their early receivers to laboratory predicted frequencies of molecules

• One type of source that shows interesting molecular emission are the long period variables (Miras, AGB Stars etc.)

AAVSO Data

Evolved Stars

• Have moved beyond the hydrogen burning stage (and into a pulsating stage of He burning)

• Have large luminosities due to their large radii (measured in AUs!)

• Are cool in temperature (~ 2000K )• Have extended “shells” around them

formed after dust condenses and is slowly (5-30 km/s) pushed away from the star

Dust Emission - CSO

Dust Emission - Keck

Molecular Shells

• Due to the cold temperatures and large quantities of molecules, chemistry can take place

• The particular chemistry that does exist in the shells is governed by whether there is more carbon or oxygen in the shell

• Carbon-rich sources have many different molecules, oxygen-rich have fewer

Different Shells Example

BIMA Data

A View of CO near TT Cyg

• Image made with the IRAM telescope, a millimeter array

• Shows the CO gas• Diameter of shell is

half a light-year (!)• CO “puffed” out from

central star about 6,000 years ago

• Still expanding…

The IRAM Telescope

HCN in O-rich Stars

• Early single dish work showed that there was often HCN associated with O-rich stars

• This shouldn’t happen because all available C bonds with all available O and the leftover determines the chemistry of the star

• Chemical models were made that claimed that methane could be formed near the central star, blown outward and transformed to HCN through exposure to UV light and some chemical reactions

My task: test the models…

…with the OVRO telescope

(that has some interesting art)

The model

• Methane condenses relatively near the star and is blown outwards

• When exposed to more UV light, it breaks up and makes other molecules

• HCN is produced along with methanol and CCH

• We know that HCN is there, but nobody had ever looked for the other molecules

The Observations

• Done in winter/spring 1997-1998• Several configurations of the OVRO

telescopes• Observed transitions of HCN, methanol

and CCH• Basic result: no CCH, no methanol• Advanced result: first ever maps of the

HCN distribution in the star, could help future model-makers

HCN Maps

• Radio data is often presented as contour maps

• Higher contour levels mean more light coming from the source

• Since molecular emission is emitted at only particular frequencies, the Doppler effect shifts the received frequency for gas that is moving towards or away from us

TX Cam

HCN Shell Size

• To determine the shell size, I fit 2-dimensional Gaussians (elliptically-shaped hills) to the emission at the “zero” velocity

• The “zero” velocity gas is expanding directly in the plane of the sky and represents the true diameter of the distribution

• Also placed upper limits on the abundances for methanol and CCH, which are much better than with a single dish telescope (resolution effect)

TX Cam Shell Size

What else is going on near these Stars?

• Masers…SiO, water and OH

• With the VLBA they can be tracked over time

• Also, the kinematics of the gas shells themselves can be studied

• New results seem to indicate that the shells are rotating or at least have interesting dynamics going on

R Aqr – Boboltz et al.

TX Cam – SiO masers Diamond et al.

Maser Animation

Conclusions

• A millimeter wavelength search for CCH and methanol around TX Cam and IK Tau turned up nada, forcing model-makers to think again

• First ever HCN maps of TX Cam and IK Tau gave size of distribution and that it is centrally condensated, again important for model-makers