Stellar Systems Group

Mugrauer, M., Neuhäuser, R., Guenther, E. W., Hatzes, A. P., Huélamo, N., Fernández, M., Ammler, M., Retzlaff, J., König, B., Charbonneau, D., Jayawardhana, R., and Brandner, W.
Astronomy and Astrophysics, v.417, p.1031-1038 (2004)
04/2004

ABSTRACT

We present the first results from our search for close stellar and sub-stellar companions to young nearby stars on the northern sky. Our infrared imaging observations are obtained with the 3.5 m Calar Alto telescope and the AO system ALFA. With two epoch observations which were separated by about one year, we found two co-moving companion candidates, one close to HD 77407 and one close to GJ 577. For the companion candidate near GJ 577, we obtained an optical spectrum showing spectral type M 4.5; this candidate is a bound low-mass stellar companion confirmed by both proper motion and spectroscopy. We estimate the masses for HD 77407 B and GJ 577 B to be ˜0.3 to 0.5 M_sun and ˜0.16 to 0.2 M_sun, respectively. Compared to Siess et al. (\cite{Siess2000}) models, each of the two pairs appears co-eval with HD 77407 A, B being 10 to 40 Myrs and GJ 577 A, B being ≥100 Myrs old. We also took multi-epoch high-resolution spectra of HD 77407 to search for sub-stellar companions, but did not find any with 3 M_Jup as upper mass (m sin i) limit (for up to 4 year orbits); however, we detected a long-term radial velocity trend in HD 77407 A, consistent with a ˜0.3 M_sun companion at ˜50 AU separation, i.e. the one detected by the imaging. Hence, HD 77407 B is confirmed to be a bound companion to HD 77407 A. We also present limits for undetected, but detectable companions using a deep image of HD 77407 A and B, also observed with the Keck NIRC2 AO system; any brown dwarfs were detectable outside of 0.5 arcsec (17 AU at HD 77407), giant planets with masses from ˜6.5 to 12 M_Jup were detectable at ≥1.5 arcsec.