Hongchi Wang

A Northern Sky Survey for Steady Tera-Electron Volt Gamma-Ray Point Sources Using the Tibet Air Shower Array

Results of a steady TeV γ-ray point-source search using data taken from the Tibet HD (1997 February-1999 September) and Tibet III (1999 November-2001 October) arrays are presented. From 0° to 60° in declination, significant excesses from the well-known steady source Crab Nebula and the high state of the flare-type source Markarian 421 are observed. Because the levels of significance from other positions are not sufficiently high, 90% confidence level upper limits on the flux are set assuming different power-law spectra. To allow cross-checking, two independently developed analyses are used in this work.

Multi-TeV Gamma-Ray Flares from Markarian 421 in 2000 and 2001 Observed with the Tibet Air Shower Array

Several strong TeV γ-ray flares were detected from Mrk 421 in the years 2000 and 2001 by the Tibet III air shower array at a level of statistical significance of 5.1 σ. Mrk 421 was unprecedentedly active at X-ray and TeV γ-ray energies during this period, and a positive correlation was found between the change of the all-sky monitor Rossi X-Ray Timing Explorer X-ray flux and the Tibet TeV γ-ray flux. When a power-law energy spectrum for γ-rays from this source is assumed, the spectral index is calculated to be -3.24 ± 0.69 at the most active phase in 2001. The spectral index observed by the Tibet air shower array is consistent with those obtained via imaging air Cerenkov telescopes.

A Large-Scale Molecular Line Survey for Cold IRAS Sources in the Galaxy. I. The CO (J = 1-0) Data

We conducted a large-scale survey for the cold infrared sources along the northern Galactic plane in the CO (J = 1-0) line. There are 1912 IRAS sources selected on the basis of their color indices over the 12, 25, and 60 l m wave bands and their association with regions of recent star formation. A quick single-point survey was made toward all of the sources, which results in a detection of 1331 sources with significant CO emission above the detection limit of 0.7 K, inferring a CO detection rate of 70%. Located over a wide range of the Galactocentric distances, the CO sources show high concentration toward the spiral arms. Among the detected sources, there are 351 sources found to have high-velocity CO wing emission. A search for the latest catalog of high-velocity CO flows (HVFs) from young stellar objects indicates that 289 sources are beyond the present lists of HVFs. These high-velocity wing sources provide us with a comprehensive database for the study of HVFs from young stellar objects. Using the known outflow sources as an effective indicator, we found that the detection rate for high-velocity wings during the quick survey is 62%, moderately sensitive in searching for new outflow sources. The CO detection rate of the IRAS sources, combined with the ratio of high-velocity wing, suggests that 41% of the CO sources are undergoing the HVF phase. In this paper, the CO spectra are presented along with the preliminary statistics of the data.

A search for optical outflows from brown dwarfs in the Chamaeleon I molecular cloud

A deep and wide-field HH object survey has been carried out toward the Chamaeleon I (Cha I) molecular cloud with a sky coverage of 1 degrees x 2 degrees, which embraces the entire Cha I molecular cloud. In total, 18 HH objects have been detected, including the previously known objects HH 48-51. The newly discovered HH objects are named HH 905-918. The possible exciting sources of the detected HH objects are discussed based on the morphology and locations of the HH objects and on results of previous surveys. Although more than 40 brown dwarfs or brown dwarf candidates have been detected in Cha I, no compelling evidence for optical outflows from brown dwarfs has been found in our images at the detection limit of 2.3 x 10(-20) W m(-2) arcsec(-2) (3 sigma) in the [S II] lambda lambda 6717, 6731 emission. The typical peak brightness of HH objects in Cha I is about 20 times above our detection limit. We conclude that the brightness of optical outflows from substellar objects should be about 20 times lower than that of outflows from young solar-type stars if all the HH objects detected in Cha I are driven by young stars. In three cases - i.e., HH 51, 908, and 909 - however, there are hints of a connection between HH objects and the nearby brown dwarfs or very low mass stars.

THE PREDICTIVE POWER OF OHL'S PRECURSOR METHOD

Using linear regression techniques and correlation analysis, the predictive power of Ohls method is shown to satisfy one of the following relationships. Either a successful prediction of cycle amplitude can be obtained if the correlation between the minimum aa geomagnetic index in the declining phase of a solar cycle and the sunspot maximum of the succeeding cycle becomes stronger, or the prediction error exceeds the expected prediction error if the correlation becomes weaker. The correlation coefficient has a declining secular variation, which leads to a weakening trend in predictive power, as well as a 44 year periodicity that may explain why the prediction method did not work well for solar cycle 23. As this finding only emerged twice in the ~100 year period studied, this 44 year periodicity may occur with a degree of uncertainty and may therefore need to be checked in the future. Two quantities, namely a prediction parameter and the prediction index, are proposed to analyze predictive power. Using the above properties, the success probability of a prediction result can be analyzed in advance.

HERBIG-HARO OBJECTS IN THE LUPUS I AND III MOLECULAR CLOUDS

We performed a deep search for Herbig-Haro (HH) objects toward the Lupus I and III clouds, covering a sky area of similar to 1 and similar to 0.5 deg(2), respectively. In total, 11 new HH objects, HH 981-991, are discovered. The HH objects both in Lupus I and in Lupus III tend to be concentrated in small areas. The HH objects detected in Lupus I are located in a region of radius 0.26 pc near the young star Sz 68. The abundance of HH objects shows that this region of the cloud is active in on-going star formation. HH objects in the Lup III cloud are concentrated in the central part of the cloud around the Herbig Ae/Be stars HR 5999 and 6000. HH 981 and 982 in Lupus I are probably driven by the young brown dwarf SSTc2d J154457.9-342340 which has a mass of 50 M(J). HH 990 and 991 in Lup III align well with the HH 600 jet emanating from the low-mass star Par-Lup3-4, and are probably excited by this low-mass star of spectral type M5. High proper motions for HH 228 W, E, and E2 are measured, which confirms that they are excited by the young star Th 28. In contrast, HH 78 exhibits no measurable proper motion in the time span of 18 years, indicating that HH 78 is unlikely part of the HH 228 flow. The HH objects in Lup I and III are generally weak in terms of brightness and dimension in comparison to HH objects we detected with the same technique in the R CrA and Cha I clouds. Through a comparison with the survey results from the Spitzer c2d program, we find that our optical survey is more sensitive, in terms of detection rate, than the Spitzer IRAC survey to high-velocity outflows in the Lup I and III clouds.

OUTFLOWS IN rho OPHIUCHI AS SEEN WITH THE SPITZER INFRARED ARRAY CAMERA

Using the IRAC images from the Spitzer c2d program, we have made a survey of mid-infrared outflows in the rho Ophiuchi molecular cloud. Extended objects that have prominent emission in IRAC channel 2 (4.5 mu m) compared to IRAC channel 1 (3.6 mu m) and stand out as green objects in the three-color images (3.6 mu m in blue, 4.5 mu m in green, 8.0 mu m in red) are identified as mid-infrared outflows. As a result, we detected 13 new outflows in the rho Ophiuchi molecular cloud that have not been previously observed in either the optical or the near-infrared. In addition, at the positions of previously observed HH objects or near-infrared emission, we detected 31 mid-infrared outflows, among which seven correspond to previously observed HH objects and 30 to near-infrared emission. Most of the mid-infrared outflows detected in the rho Ophiuchi cloud are concentrated in the L1688 dense core region. In combination with the survey results for young stellar objects (YSOs) and millimeter and submillimeter sources, the distribution of mid-infrared outflows in the rho Ophiuchi molecular complex hints a propagation of star formation in the cloud in the direction from the northwest to the southeast, as suggested by previous studies of the region.

Newly discovered Herbig-Haro objects in Barnard 1 and NGC 1333

We have carried out a wide-held survey for Herbig-Haro (HH) objects in nearby star-forming regions using the Beijing Astronomical Observatory 60/90 cm Schmidt telescope. The survey covered 56 deg(2) in Perseus, Taurus, Orion, Monoceros, and other regions. Here we report our discovery of seven HH objects (HH 427-433) in the Barnard 1 dark cloud and one HH object (HPI 426) in NCC 1333. The newly discovered HH objects demonstrate a great variety of morphological structures: HH 426, 429, and 432 each consist of two knots, HH 427 is a single knot, HH 428 is a bar, HH 430 is a jet, HH 331 is a bow shock-shaped patch, and HH 433 is a complex of four knots. HH 432A, 432B, and 433 constitute a giant, highly collimated jet with a projected linear size of similar to 0.5 pc and a collimation ratio of similar to 11. The jet aligns well with a nearby star, IRAS 03304+3100 (LkH alpha 327), and may be driven by this source. Two optically visible stars are supposed to be the exciting sources for HH 426 and 428. For HH 427, 429, and 430, no exciting source can be identified.

How do stars gain their mass? : A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star Forming Regions

Most protostars have luminosities that are fainter than expected from steady accretion over the protostellar lifetime. The solution to this problem may lie in episodic mass accretion—prolonged periods of very low accretion punctuated by short bursts of rapid accretion. However, the timescale and amplitude for variability at the protostellar phase is almost entirely unconstrained. In A James Clerk Maxwell Telescope/SCUBA-2 Transient Survey of Protostars in Nearby Star-forming Regions, we are monitoring monthly with SCUBA-2 the submillimeter emission in eight fields within nearby (<500 pc) star-forming regions to measure the accretion variability of protostars. The total survey area of ~1.6 deg^2 includes ~105 peaks with peaks brighter than 0.5 Jy/beam (43 associated with embedded protostars or disks) and 237 peaks of 0.125–0.5 Jy/beam (50 with embedded protostars or disks). Each field has enough bright peaks for flux calibration relative to other peaks in the same field, which improves upon the nominal flux calibration uncertainties of submillimeter observations to reach a precision of ~2%–3% rms, and also provides quantified confidence in any measured variability. The timescales and amplitudes of any submillimeter variation will then be converted into variations in accretion rate and subsequently used to infer the physical causes of the variability. This survey is the first dedicated survey for submillimeter variability and complements other transient surveys at optical and near-IR wavelengths, which are not sensitive to accretion variability of deeply embedded protostars.

Herbig-Haro objects in the Monoceros OB1 molecular cloud

We carried out a wide-field [S II]-emission imaging of the Monoceros OB1 molecular cloud (NGC 2264) to investigate the star formation and outflow activity of the region. Our narrowband [S II] imaging covers an area of sky of similar to2 deg(2), including the entire Mon OB1 cloud. In the northern part of the cloud two new Herbig-Haro (HH) objects, HH 572 and HH 575, are discovered. All four of the previously known HH objects, HH 124, 125, 225, and 226, are also detected in our imaging. Low-dispersion spectroscopic observations of HH 572A, 575B, and 575C1 further confirmed the HH-object nature of these objects. Molecular hydrogen v = 1-0 S(1) narrowband imaging revealed a bright H-2 emission knot in each of the HH 572 and HH 575 regions. By comparing the results of our optical [S II] emission survey with those of the CO molecular outflow survey, it is found that the occurrence frequency of HH objects is similar to that of CO outflows in the Mon OB1 cloud. However, HH objects and CO outflows display different spatial distributions.