A Photometric Study of Stars in the MBM 12 Association

We have monitored four fields containing nine previously identified membersof the MBM 12 association to search for photometric variability and periodicityin these pre-main sequence stars. Seven of the nine are found to be variableand definite periodicity (of 1.2, 2.6 and 6.2 days) is found for three of them,including the classical T Tauri star LkH-alpha 264. Two other members arepossibly periodic but each requires confirmation. In addition, a "field" starthat is associated with the X-ray source RX J0255.9+2005 was discovered to be avariable with a period of 4.2 days. Our results indicate that the photometricvariability characteristics of the known MBM 12 association members are typicalof what is found in ~few My old stellar groups such as IC 348, supportingarguments for a similar age. In particular, there is a mix of periodic andnon-periodic variables with typical amplitudes (in Cousins I) of 0.1-0.5 mag,in addition to a small number of larger amplitude variables. The periods, as agroup, are somewhat shorter than in IC 348 but when allowance is made for theknown dependence of period on mass in pre-main sequence stars the differencemay not be significant. Our data confirm and illustrate the value ofphotometric monitoring as a tool for identifying likely association members andfor studying rotation in extremely young stellar groups.Comment: 26 pages, 5 figure