Intra-day Variability of Sagittarius A* at 3 Millimeters

We report observations and analysis of flux monitoring of Sagittarius A* at 3-mm wavelength using the OVRO millimeter interferometer over a period of eight days (2002 May 23-30). Frequent phase and flux referencing (every 5 minutes) with the nearby calibrator source J1744-312 was employed to control for instrumental and atmospheric effects. Time variations are sought by computing and subtracting, from each visibility in the database, an average visibility obtained from all the data acquired in our monitoring program having similar uv spacings. This removes the confusing effects of baseline-dependent, correlated flux interference caused by the static, thermal emission from the extended source Sgr A West. Few-day variations up to ~20% and intra-day variability of \~20% and in some cases up to ~40% on few-hour time scales emerge from the differenced data on SgrA*. Power spectra of the residuals indicate the presence of hourly variations on all but two of the eight days. Monte Carlo simulation of red-noise light curves indicates that the hourly variations are well described by a red-noise power spectrum with P(f) ~ f^(-1). Of particular interest is a ~2.5 hour variation seen prominently on two consecutive days. An average power spectrum from all eight days of data reveals noteworthy power on this time scale. There is some indication that few-hour variations are more pronounced on days when the average daily flux is highest. We briefly discuss the possibility that these few-hour variations are due to the dynamical modulation of accreting gas around the central supermassive black hole, as well as the implications for the structure of the SgrA* photosphere at 3 mm. Finally, these data have enabled us to produce a high sensitivity 3-mm map of the extended thermal emission surrounding SgrA*.Comment: Accepted for publication in The Astrophysical Journal Letters, 8 pages, 4 figure

Isolated Wolf-Rayet Stars and O Supergiants in the Galactic Center Region Identified via Paschen-alpha Excess

We report the discovery of 19 hot, evolved, massive stars near the Galactic center region (GCR). These objects were selected for spectroscopy owing to their detection as strong sources of Paschen-alpha emission-line excess, following a narrowband imaging survey of the central 0.65 x 0.25 degress (l, b) around Sgr A* with the Hubble Space Telescope. Discoveries include 5 carbon-type (WC) and 6 nitrogen-type (WN) Wolf-Rayet stars, 6 O supergiants, and 2 B supergiants. Two of the O supergiants have X-ray counterparts, the properties of which are broadly consistent with colliding-wind binaries and solitary O stars. The infrared photometry of 17 stars is consistent with the Galactic center distance, but 2 of them are located in the foreground. Several WC stars exhibit a relatively large infrared excesses, which is possibly the signature of thermal emission from hot dust. Most of the stars appear scattered throughout the GCR, with no relation to the three known massive young clusters; several others lie near the Arches and Quintuplet clusters and may have originated within one of these systems. The results of this work bring the total sample of Wolf-Rayet stars in the GCR to 92. All sources of strong P-alpha excess have been identified in the area surveyed with HST, which implies that the sample of WN stars in this region is near completion, and is dominated by late (WNL) types. The current WC sample, although probably not complete, is almost exclusively dominated by late (WCL) types. The observed Wolf-Rayet subtype distribution in the GCR is a reflection of the intrinsic rarity of early subtypes (WNE and WCE) in the inner Galaxy, an effect that is driven by metallicity.Comment: Correction to Fig 4,5,6,10, and Tables 3, 5. The spectrum of the previously known WC9 star WR101q had been mistakenly labeled as Star #19 in Fig. 5 of the original manuscript. Star #19 is actually a newly discovered WN5b star, whose spectrum was accidentally not included in the original paper. The correct WN5b spectrum is now included in Fig. 4. An erratum was submitted to ApJ on Aug 15, 201

HST Palpha Survey of the Galactic Center -- Searching the missing young stellar populations within the Galactic Center

We present preliminary results of our \hst Pa$\alpha$ survey of the Galactic Center (\gc), which maps the central 0.65$\times$0.25 degrees around Sgr A*. This survey provides us with a more complete inventory of massive stars within the \gc, compared to previous observations. We find 157 Pa$\alpha$ emitting sources, which are evolved massive stars. Half of them are located outside of three young massive star clusters near Sgr A*. The loosely spatial distribution of these field sources suggests that they are within less massive star clusters/groups, compared to the three massive ones. Our Pa$\alpha$ mosaic not only resolves previously well-known large-scale filaments into fine structures, but also reveals many new extended objects, such as bow shocks and H II regions. In particular, we find two regions with large-scale Pa$\alpha$ diffuse emission and tens of Pa$\alpha$ emitting sources in the negative Galactic longitude suggesting recent star formation activities, which were not known previously. Furthermore, in our survey, we detect $\sim$0.6 million stars, most of which are red giants or AGB stars. Comparisons of the magnitude distribution in 1.90 $\mu$m and those from the stellar evolutionary tracks with different star formation histories suggest an episode of star formation process about 350 Myr ago in the \gc .Comment: 10 pages, 6 figures, Proceedings of the Galactic Center Workshop 2009, Shangha

On the Progenitor System of the Type Iax Supernova 2014dt in M61

We present pre-explosion and post-explosion Hubble Space Telescope images of the Type Iax supernova (SN Iax) 2014dt in M61. After astrometrically aligning these images, we do not detect any stellar sources at the position of the SN in the pre-explosion images to relatively deep limits (3 sigma limits of M_F438W > -5.0 mag and M_F814W > -5.9 mag). These limits are similar to the luminosity of SN 2012Z's progenitor system (M_F435W = -5.43 +/- 0.15 and M_F814W = -5.24 +/- 0.16 mag), the only probable detected progenitor system in pre-explosion images of a SN Iax, and indeed, of any white dwarf supernova. SN 2014dt is consistent with having a C/O white-dwarf primary/helium-star companion progenitor system, as was suggested for SN 2012Z, although perhaps with a slightly smaller or hotter donor. The data are also consistent with SN 2014dt having a low-mass red giant or main-sequence star companion. The data rule out main-sequence stars with M_init > 16 M_sun and most evolved stars with M_init > 8 M_sun as being the progenitor of SN 2014dt. Hot Wolf-Rayet stars are also allowed, but the lack of nearby bright sources makes this scenario unlikely. Because of its proximity (D = 12 Mpc), SN 2014dt is ideal for long-term monitoring, where images in ~2 years may detect the companion star or the luminous bound remnant of the progenitor white dwarf.Comment: 5 pages, 3 figures, submitted to ApJ

HST Paschen-alpha Survey of the Galactic Center: Data Reduction and Products

Our HST/NICMOS Pa survey of the Galactic center (GC) provides a uniform, panoramic, high-resolution map of stars and ionized diffuse gas in the central 416 arcmin^2 of the Galaxy. This survey was carried out with 144 HST orbits using two narrow-band filters at 1.87 and 1.90 micron in NICMOS Camera 3. In this paper, we describe in detail the data reduction and mosaicking procedures followed, including background level matching and astrometric corrections. We have detected ~570,000 near-IR sources and are able to quantify photometric uncertainties of the detections. The source detection limit varies across the survey field but the typical 50% completion limit is ~17th mag (Vega System) in the 1.90 micron band. A comparison with the expected stellar magnitude distribution shows that these sources are primarily Main-Sequence massive stars (>7M) and evolved lower mass stars at the distance of the GC. In particular, the observed source magnitude distribution exhibits a prominent peak, which could represent the Red Clump stars within the GC. The observed magnitude and color of these RC stars support a steep extinction curve in the near-IR toward the GC. The flux ratios of our detected sources in the two bands also allow for an adaptive and statistical estimate of extinction across the field. With the subtraction of the extinction-corrected continuum, we construct a net Pa emission map and identify a set of Pa-emitting sources, which should mostly be evolved massive stars with strong stellar winds. The majority of the identified Pa point sources are located within the three known massive GC stellar clusters. However, a significant fraction of our Pa-emitting sources are located outside the clusters and may represent a new class of `field' massive stars, many of which may have formed in isolation and/or in small groups. The maps and source catalogues presented here are available electronically.Comment: Accepted for publication in MNRA

High-frequency VLBI observations of SgrA* during a multi-frequency campaign in May 2007

In May 2007 the compact radio source Sgr A* was observed in a global multi-frequency monitoring campaign, from radio to X-ray bands. Here we present and discuss first and preliminary results from polarization sensitive VLBA observations, which took place during May 14-25, 2007. Here, Sgr A* was observed in dual polarization on 10 consecutive days at 22, 43, and 86 GHz. We describe the VLBI experiments, our data analysis, monitoring program and show preliminary images obtained at the various frequencies. We discuss the data with special regard also to the short term variability.Comment: 6 pages, 5 figures;necessary style files included; contribution for the conference "The Universe under the Microscope" (AHAR 2008), held in Bad Honnef (Germany) in April 2008, to be published in Journal of Physics: Conference Series by Institute of Physics Publishing, R. Schoedel, A. Eckart, S. Pfalzner, and E. Ros (eds.

Near-infrared counterparts to Chandra X-ray sources toward the Galactic Center. II. Discovery of Wolf-Rayet stars and O supergiants

We present new identifications of infrared counterparts to the population of hard X-ray sources near the Galactic center detected by the Chandra X-ray Observatory. We have confirmed 16 new massive stellar counterparts to the X-ray population, including nitrogen-type (WN) and carbon-type (WC) Wolf-Rayet stars, and O supergiants. For the majority of these sources, the X-ray photometry is consistent with thermal emission from plasma having temperatures in the range of kT=1-8 keV or non-thermal emission having power-law indices in the range of -1<gamma<3, and X-ray luminosities in the range of Lx~1e32-1e34 erg/s. Several sources have exhibited X-ray variability of several factors between separate observations. The X-ray properties are not a ubiquitous feature of single massive stars but are typical of massive binaries, in which the high-energy emission is generated by the collision of supersonic winds, or by accretion onto a compact companion. However, the possibility of intrinsic hard X-ray generation from single stars cannot be completely ruled out. The spectral energy distributions of these sources exhibit significant infrared excess, attributable to free-free emission from ionized stellar winds, supplemented by hot dust emission in the case of the WC stars. With the exception of one object located near the outer regions of the Quintuplet cluster, most of the new stars appear isolated or in loose associations. Seven hydrogen-rich WN and O stars are concentrated near the Sagittarius B HII region, while other similar stars and more highly evolved hydrogen-poor WN and WC stars lie scattered within ~50 pc, in projection, of Sagitarrius A West. We discuss various mechanisms capable of generating the observed X-rays and the implications these stars have for massive star formation in the Galaxy's Central Molecular Zone.Comment: Accepted to ApJ on December 5, 2009. 61 pages, including 17 figure

V5856 Sagittarii/2016: Broad Multi-Epoch Spectral Coverage of a Sustained High Luminosity Nova

Nova V5856 Sagittarii is unique for having remained more than nine magnitudes above its pre-outburst brightness for more than six years. Extensive visible and IR spectra from the time of outburst to the present epoch reveal separate emitting regions with distinct spectral characteristics. Permitted emission lines have both broad and narrow components, whereas the forbidden line profiles are almost entirely broad. The permitted line components frequently display P Cygni profiles indicating high optical depth, whereas the broad components do not show detectable absorption. The densities and velocities deduced from the spectra, including differences in the O I 7773 and 8446 lines, are not consistent with an on-going wind. Instead, the prolonged high luminosity and spectral characteristics are indicative of a post-outburst common envelope that enshrouds the binary, and is likely the primary source of the visible and IR emission.Comment: 27 pages; 13 figures. Accepted for publication in the Ap

Discovery of a Luminous Blue Variable with an Ejection Nebula Near the Quintuplet Cluster

We report on the discovery of a luminous blue variable (LBV) lying ≈7 pc in projection from the Quintuplet cluster. This source, which we call LBV G0.120 – 0.048, was selected for spectroscopy owing to its detection as a strong source of Paschen-α (Pα) excess in a recent narrowband imaging survey of the Galactic center region with the Hubble Space Telescope/Near-Infrared Camera and Multi-Object Spectrometer. The K-band spectrum is similar to that of the Pistol Star and other known LBVs. The new LBV was previously cataloged as a photometric variable star, exhibiting brightness fluctuations of up to ≈1 mag between 1994 and 1997, with significant variability also occurring on month-to-month timescales. The luminosity of LBV G0.120 – 0.048, as derived from Two-Micron All Sky Survey photometry, is approximately equivalent to that of the Pistol Star. However, the time-averaged brightness of LBV G0.120 – 0.048 between 1994 and 1997 exceeded that of the Pistol Star; LBV G0.120 – 0.048 also suffers more extinction, which suggests that it was intrinsically more luminous in the infrared than the Pistol Star between 1994 and 1997. Pα images reveal a thin circular nebula centered on LBV G0.120 – 0.048 with a physical radius of ≈0.8 pc. We suggest that this nebula is a shell of ejected material launched from a discrete eruption that occurred between 5000 and 10,000 years ago. Because of the very short amount of time that evolved massive stars spend in the LBV phase, and the close proximity of LBV G0.120 – 0.048 to the Quintuplet cluster, we suggest that this object might be coeval with the cluster, and may have once resided within it