Molecular clouds and star formation. I - Observations of the Cepheus OB3 molecular cloud

To determine the connection between newly formed stars and molecular clouds, observations were made in and around the young OB association Cepheus OB3 in the J = 1→0 transition of ^(12)CO. An extended (20 pc x 60 pc) molecular cloud was detected and mapped, and additional observations of ^(13)CO and H_2CO were made at selected positions. Moderately enhanced temperatures and densities were noted in several portions of the cloud. It appears that star formation is still continuing in the Cepheus OB3 complex

The circumstellar environment of the FU Orionis pre-outburst candidate V1331 Cygni

High resolution (~4") aperture synthesis maps of the CO (1→ 0), ^(13)CO (1→0), ^(13)CO (2→1), and asociated continuum emission from the FU Orions candidate V1331 Cygni reveal a massive, 0.5 ± 0.15 M_☉, circumstellar disk surrounded by a flattened gaseous envelope, 6000 x 4400 AU in size, mass >0.32 M_☉. These images and lower resolution measurements also trace a bipolar outflow and gaseous ring, 4.1 by 2.8 x 10^4 AU, mass greater than or equal to 0.07 M_☉, radially expanding at 22 ± 4 kms^(-1). We suggest this ring is a swept-up gaseous torus from an energetic mass ejection stage, possibly an FU Orionis outburst or outburts, ~4 x 10^3 yr ago that imparted >10^(45) ergs into the ambient cloud

The FU Orionis binary system RNO 1B/1C

Observations of CS (7→6) emission reveal a ≥3M_⊙ core, 1.8×10^4 AU in size, surrounding the FU Orionis binary system RNO 1B/1C. Fractional chemical abundances, calculated from LVG and LTE codes, are mostly similar to those in the cold core TMC 1. However, values for Si0/H_2 and CH_(3)0H/H_2 are enhanced, possibly by sputtering reactions or grain-grain collisions in tile outflow associated with the young stars. Aperture syntllesis maps of tile 2.6 and 3.1 mm continuum emission at ~5" and ~9" resolution, respectively, reveal that RNO 1C is surrounded by a flattened, dusty envelope, ~5000 AU in size, with mass ≥1.1 M_⊙. High spatial resolution (~3") interferometer observations of CS (2→1) emission may trace the dense walls of ail outflow cavity comprised of two concentric arcs with dynamical ages of 4×10^3 and 1×10^4 yr. The velocity structure of lower density gas imaged in the CO (1→0) transition is consistent with the arcs being formed by two energetic FU Orionis outbursts. Each event may have imparted more than 4 M_⊙km s^(-1) to the outflow, implying outburst mass loss rates of ~10^(-4) M_⊙ yr^(-1). It appears that RNO 1C is probably the driving source for the outflow and tllat, while pre-main sequence stars are in tile FU Orionis stage, outbursts may dominate both outflow morphology and energetics

Techniques of mm/sub-mm Interferometry in Star Formation

Since the earliest stages of star formation occur deep in clouds of gas and dust, they are hidden from optical view. Nevertheless, infrared and millimeter wavelength observations over the last twenty years have enabled increasingly detailed studies of the processes involved in stellar birth. High resolution, aperture synthesis imaging of the millimeter-wave radiation from dust and molecular gas in star-forming clouds has proven particularly effective. On the other hand, there can be pitfalls to be avoided in the use of mm/sub-mm interferometry techniques. Here, we consider what has been learned from currently-operating mm interferometers and the potential of the next generation of arrays

The Nature of Faint Blue Stars in the Halo. II

Spectra and colors of 189 hot (FB) stars selected colorimetrically and mostly within 30° of the galactic poles are analyzed quantitatively for surface gravity and effective temperatures. Palomar spectra give line intensities and Balmer line profiles, found in the Appendix tables. Using a network of model atmospheres, the g and θ so determined give directly the light-to-mass ratio, and eventually the luminosities. The high-latitude hot stars cover a range of 10^6 in luminosity, and are classifiable into various groups. Spectroscopically normal B stars make up 26 percent of the sample; they rotate and have nearly normal line spectra. Interpreted as Population I stars, on the mass-luminosity relation, they have relatively high luminosities and large z-coordinates. For some, the nuclear lifetimes at present luminosities are less than transit times from the plane. Their high velocities present a problem for galactic kinematics. A few are slightly helium rich, while others are highly evolved Population II stars, from details of the spectroscopy. Some of the Population I stars may be in not-understood, short-lived evolutionary phases of high luminosity and low mass. The (g, θ)-relation can be transformed into luminosity-temperature diagrams if masses are known. Many fall along a relation log gθ^4 = 2.35, have common properties of halo stars, and T_(eff) from 9500° to 40,000° K. The light-to-mass ratio for these is 68 (in solar units) ; most HB and sdB stars have weak metal and weak helium lines, i.e., are halo stars. We call this the extended horizontal branch (EHB). Quantitative classification gives 17 percent of the FB stars as horizontal-branch B and early A (HB) stars, and 16 percent subdwarf B (sdB) stars. There are 16 percent subdwarf O (sdO) stars, including planetary nuclei; these have strong, or very strong, helium lines. Assuming a constant mass, 0.66 m_⊙, the observed L/m gives Mb = +0.5. The EHB for field stars fits the globular cluster HB, and extends it to high T_(eff). The bolometric correction produces the change of M_v with color. The hot white dwarfs are 11 percent of the FB stars and appear as a bridge between the sdO stars and the ordinary DA white dwarfs. Composite stars are unresolved, noninteracting binaries with strange UBV colors; they require a faint Mv for the primary (e.g., sdO or sdB), and an unevolved G companion. The helium deficiency and the evolutionary problems are sharpest for the EHB stars. In a few, slightly stronger helium lines are accompanied by lines of peculiar elements (S III, P II seen at high dispersion. While the L/m ratio is closely the same for sdO as for sdB and HB, the He lines become strong. Evolutionary tracks avoid the region of the hot EHB stars; the hottest sdO stars approach the helium main sequence. The low surface helium is almost certainly not cosmological in origin. Gravitational diffusion in a nonrotating star in the absence of convection is plausible; in the sdO and the peculiar sdB stars convection and helium flashes may have occurred. Numerous radial velocities were measured. Where possible, proper motions, luminosities, and radial-velocity dispersions were used for space motions. The Population I normal stars have abnormally high space motions, not greatly different from Population II stars. The luminosities derived from spectroscopy are consistent with those from peculiar motions

Chemistry in circumstellar disks: CS toward HL Tauri

High-resolution millimeter-wave aperture synthesis images of the CS J = 2 → 1 and dust continuum emission toward the young star HL Tauri have been combined with single-dish spectra of the higher J CS transitions in order to probe the chemical and physical structure of circumstellar material in this source. We find that the extended molecular cloud surrounding HL Tau is similar to other Taurus dark cloud cores, having T_(kinetic) ≈ 10-20 K, n_(H2) ≈ 10^4-10^5 cm^(-3) , and x(CS) = N(CS)/N(H_2) ≈ (1-2) x 10^(-8). In contrast, the gas-phase CS abundance in the circumstellar disk is depleted by factors of at least 25-50, and perhaps considerably more. These results are consistent with substantial depletion onto grains, or a transition from kinetically controlled chemistry in the molecular cloud to thermodynamically controlled chemistry in the outer regions of the circumstellar disk. Dust continuum emission at 3.06 mm, although unresolved in a 3".0 beam, appears centered on the stellar position; combined with other millimeter-wave measurements its intensity indicates an emissivity index of β = 1.2 ± 0.3. This β may reflect grain growth via depletion and aggregation or compositional evolution, and suggests that the 3.06 mm dust opacity exceeds unity within 8-10 AU of HL Tauri. Even at millimeter and submillimeter wavelengths, observational studies of other high dipole moment molecules in circumstellar disks may also be hampered by the combination of grain mantle depletion and dust opacity structure in sources viewed nearly edge-on

Kwok-Yung Lo

Radio astronomer and physicist Kwok-Yung “Fred” Lo was born on 19 October 1947 in Nanjing, China. The third of six sons, he grew up in Hong Kong, where his father had moved his antique business in 1949 in search of better opportunities. Fred came to the US in 1965 to attend MIT, where he earned his BS in 1969 and a PhD in 1974, both in physics. His thesis, titled “Interstellar microwave radiation and early stellar evolution,” was supervised by radio astronomer Bernard Burke