Disks and outflows around intermediate-mass stars and protostars^*

¹ Observatorio Astronómico Nacional (IGN), Campus Universitario, Apdo. 1143, 28800 Alcalá de Henares (Madrid), Spain
² Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, Domaine Universitaire, 38406 St Martin d'Hères Cedex, France
³ Universidad Complutense de Madrid, Av. Arcos de Jalón s/n, 28037 Madrid, Spain
⁴ Nobeyama Radio Observatory, Nobeyama, Minamimaki, Minamisaku, Nagano, 384-1305, Japan
⁵ Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy

A. Fuente

Received: 28 July 2000
Accepted: 14 November 2000

Abstract

In order to study the existence and evolution of circumstellar disks around intermediate-mass stars (), 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 LkHα 234 and HD 200775. These objects are representative of the different stages of the pre-main sequence evolution with ages ranging from a few 10³ to 8 10⁶ 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 fit 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. The CO 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 LkHα 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 α 234, IRS 6. A new millimeter clump, FIRS 1 -MM1, is detected at an offset (, 3.0'') from LkHα 234. We have not detected any compact source towards LkHα 234 with a limit for the mass of a circumstellar disk, M_D < 0.1 . The comparison of the interferometric CO 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 LkHα 234. We have not detected 1.3 mm continuum emission towards HD 200775. Our observations imply a 3-σ upper limit of < 0.002 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 ( -). They are surrounded by thick envelopes with masses ranging between ∼ 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, , of < 0.02 in LkHα 234 and of < 0.0002 in HD 200775. Our limit in HD 200775 implies that in evolved Herbig Be stars the ratio is more than two orders of magnitude lower than in T Tauri and Herbig Ae stars. We propose that in massive stars ( ≥ 5 ) both 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.