A spectroscopic study of U Mon an RV Tau-type post-AGB star

Osamu Hashimoto1, Satoshi Honda2, Hikaru Taguchi3, & Kazuo Yoshioka4

1 Gunma Astronomical Observatory, Japan 2 Kwasan Observatory, Kyoto University , Japan 3 Gunma Lifelong Learning Center, Japan 4 The Open University of Japan

Abstract

We have made spectroscopic observations of an RV Tauri-type variable star U Monocerotis ( U Mon ) since 2008 with the use of the high resolution echelle spectrograph GAOES on the 150 cm reflector at Gunma Astronomical Observatory ( GAO ) in Japan. The profile of the Hα line varies in a complicated manner through the observation period. There are some absorption components with a narrow line width less than a few angstroms in a broad emission component which disappears sometimes. It is probable that some narrow emission components may also exist over the broad emission in some occasions.

GAOES on the Gunma 150cm reflector

U Monocerotis ( HD13775 )

RV Tauri-type variable star P ~ 92 days mv ~ 5.1 － 7.1 mag Sp. type : F8eIb ( max ) － K0pIb ( min ) [ group A] RVb ~ 2475 days Post-AGB Low mass star ( m ~ 1 Mo ) AGB( asymptotic giant branch ) ➔ PN( planetary nebula )

Observations

GAOES ( Gunma Astronomical Observatory Echelle Spectrograph ) Gunma 150cm reflector R = λ / δλ ~ 70000 wavelengths : 482 nm － 660 nm ( one exposure coverage ) exposure : ~ 60 min － 120 min S/N > 100 14 January 2008 － 16 March 2010 ( JD 2454480 － 2455271 ) 9 cycles ( P ~ 92 days )

Light curve ( from AAVSO )

1 2 3 4 5 6 7 *cycle

8 0

*cycle : counted as phase = 1.0 at the primary minimum in February 2008

Hα feature time variation

1. Broad emission component FWHM ~ 0.5 nm ? very weak or almost disappears sometimes ( cf. 20080308 phase 1.42 )2. Narrow absorption components FWHM < 0.2 nm a few independent components independent of the broad emission component3. Narrow emission components ? may appear in some occasions observed only in the period of phase 4.75 － 5.84 ( see 20090402 phase 5.65 )

Tim

e ( phase )

Variation of Hα feature of U Mon

phas

e

Variation of Hα feature of U Mon

- Broad emission component - Dependence on phase

- Absorption components - Dependence on phase ( sum of all the absorption lines )

- Radial velocity of the broad emission component - Dependence on phase

- Broad emission component - Dependence on phase ( absorption corrected )

- Velocity of the main absorption component - Dependence on phase ( against the broad emission )

1. Broad emission component

strong around the phase ~ 0.0 ( primary minimum ) at the phases of ~ 0.6 － 0.8 with a rapid increase at ~ 0.7 and a rapid decline after the peak weak at the phases of ~ 0.2 － 0.6 sometimes very weak or almost disappearing mild variation around the phase of ~ 0.0

2. Narrow absorption components

a number of absorption lines ( independent each other ? ) no strong correlation with the phase of RV variation no strong correlation with the broad emission components but, tend to be weak at the phases around 0.6 － 0.8 which may be correlated with the rapid increase of the broad emission component

Summary

3. Radial velocities of the Hα components

the brad width of the emission component corresponds to a velocity of the order of magnitude of 100 km/s

some variations depending on the phase are seen some variations for a longer time scale than the RV period may exist

but more detailed and careful analysis of the data is required

4. How to explain or how to understand

complicated very much we need more studies