Disks and outflows around intermediate-mass stars and protostars

Abstract

In order to study the existence and evolution of circumstellar disks around intermediate-mass stars (M 3M), we have obtained single-dish and interferometric continuum images at 2.6 mm and 1.3 mm of the intermediate-mass protostar NGC 7129 FIRS 2 and of the Herbig Be stars LkH 234 and HD 200775. These objects are representative of the dierent stages of the pre-main sequence evolution with ages ranging from a few 10 3 to 8 10 6 years. Single-dish and interferometric observations of the outflows associated with these sources are also presented. In NGC 7129 FIRS 2, two millimeter sources are required to t the interferometric 1.3 mm continuum emission. Only the most intense of these millimeter objects, FIRS 2 { MM1, seems to be associated with the CO outflow. The second and weaker source, FIRS 2 { MM2, does not present any sign of stellar activity. The single-dish map of the CO outflow presents an unusual morphology with the blue and red lobes separated by an angle of 82 .T he COJ =1 ! 0 interferometric image shows that this unusual morphology is the result of the superposition of two outflows, one of them associated with FIRS 2 { MM1 (the blue lobe in the single-dish map) and the other (the red lobe) with a new infrared source (FIRS 2 { IR) which is not detected in the millimeter continuum images. The interferometric 1.3 mm continuum image of NGC 7129 FIRS 1 reveals that LkH 234 is a member of a cluster of embedded objects. Two millimeter clumps are detected in this far-infrared source. The strongest is spatially coincident with the mid-infrared companion of LkH 234, IRS 6. A new millimeter clump, FIRS 1 { MM1, is detected at an oset ( 3:23 00 ,3 .0 00 )f rom LkH 234. We have not detected any compact source towards LkH 234 with a limit for the mass of a circumstellar disk, MD < 0.1 M. The comparison of the interferometric CO J =1 ! 0 and continuum images reveals that IRS 6 very likely drives the energetic molecular outflow detected towards NGC 7129 FIRS 1 and the (SII) jet. The extremely young object FIRS 1 { MM1 (it has not been detected in the near-and mid-infrared) turns out to be the driving source of the H2 jet. There is no evidence for the existence of a bipolar outflow associated with LkH 234. We have not detected 1.3 mm continuum emission towards HD 200775. Our observations imply a 3- upper limit of < 0.002 M for the mass of a circumstellar disk. This is the lowest upper limit obtained so far in a Herbig Be star. Thus our observations provide new important information on three protostars (IRS 6, FIRS 1 { MM1 and FIRS 2 { MM1), one infrared star (FIRS 2 { IR) and two Herbig Be stars. The luminosities of the protostars are consistent with being intermediate-mass objects (M 3:5{4:5 M). They are surrounded by thick envelopes with masses ranging between2 3:5 M and drive energetic outflows. Circumstellar disks and bipolar outflows are not detected toward the Herbig Be stars. We have obtained an upper limit for the disk/stellar mass ratio, MD=M ,o f< 0.02 in LkH 234 and of < 0.0002 in HD 200775. Our limit in HD 200775 implies that in evolved Herbig Be stars the MD=M ratio is more than two orders of magnitude lower than in T Tauri and Herbig Ae stars. We propose that in massive stars (M 5 M )b oth the dispersal of the outer disk and the energetic mass-loss, occur early in the stellar evolution before the star becomes visible. Some mechanisms for the dispersal of the outer disk are discussed.