Smithsonian Astrophysical Observatory

We have discovered a detached pair of white dwarfs (WDs) with a 12.75 min orbital period and a 1,315 km s −1 radial velocity amplitude. We measure the full orbital parameters of the system using its light curve, which shows ellipsoidal variations, Doppler boosting, and primary and secondary eclipses. The primary is a 0.25 M ⊙ tidally distorted helium WD, only the second tidally distorted WD known. The unseen secondary is a 0.55 M ⊙ carbon-oxygen WD. The two WDs will come into contact in 0.9 Myr due to loss of energy and angular momentum via gravitational wave radiation. Upon contact the systems may merge yielding a rapidly spinning massive WD, form a stable interacting binary, or possibly explode as an underluminous supernova type Ia. The system currently has a gravitational wave strain of 10 −22 , about 10,000 times larger than the Hulse-Taylor pulsar; this system would be detected by the proposed LISA gravitational wave mission in the first week of operation. This system's rapid change in orbital period will provide a fundamental test of general relativity.

We present early observations of the afterglow of the Gamma-Ray Burst (GRB) 030329 and the spectroscopic discovery of its associated supernova SN2003dh. We obtained spectra of the afterglow of GRB 030329 each night from March 30.12 (0.6 days after the burst) to April 8.13 (UT) ...

We present multi-band optical photometry of 94 spectroscopically-confirmed Type Ia supernovae (SN Ia) in the redshift range 0.0055 to 0.073, obtained between 2006 and 2011. There are a total of 5522 light curve points. We show that our natural system SN photometry has a precision of roughly 0.03 mag or better in BVr'i', 0.06 mag in u', and 0.07 mag in U for points brighter than 17.5 mag and estimate that it has a systematic uncertainty of 0.014, 0.010, 0.012, 0.014, 0.046, and 0.073 mag in BVr'i'u'U, respectively. Comparisons of our standard system photometry with published SN Ia light curves and comparison stars reveal mean agreement across samples in the range of ~0.00-0.03 mag. We discuss the recent measurements of our telescope-plus-detector throughput by direct monochromatic illumination by Cramer et al (in prep.). This technique measures the whole optical path through the telescope, auxiliary optics, filters, and detector under the same conditions...

We present extensive optical and infrared photometry of the afterglow of gamma-ray burst (GRB) 030329 and its associated supernova (SN) 2003dh over the first two months after detection (2003 March 30-May 29 UT). Optical spectroscopy from a variety of telescopes is shown and, when combined with the photometry, allows an unambiguous separation between the afterglow and supernova contributions. The optical afterglow of the GRB is initially a power-law continuum but shows significant color variations during the first week that are unrelated to the presence of a supernova. The early afterglow light curve also shows deviations from the typical power-law decay. A supernova spectrum is first detectable ∼ 7 days after the burst and dominates the light after ∼ 11 days. The spectral evolution and the light curve are shown to closely resemble those of SN 1998bw, a peculiar Type Ic SN associated with GRB 980425, and the time of the supernova explosion is close to the observed time of the GRB. It is now clear that at least some GRBs arise from core-collapse SNe.

As the closest example of a galactic nucleus, the Galactic center presents an exquisite laboratory for learning about supermassive black holes (SMBH) and their environs. In this document, we describe how detailed studies of stellar dynamics deep in the potential well of a galaxy offer several exciting directions in the coming decade. First, it will be possible to obtain precision measurements of the Galaxy's central potential, providing both a unique test of General Relativity (GR) and a detection of the extended dark matter distribution that is predicted to exist around the SMBH. Tests of gravity have not previously been possible on scales larger than our solar system, or in regimes where the gravitational energy of a body is >∼1% of its rest mass energy. Similarly, only upper limits on the extended matter distribution on small scales currently exist; detection of dark matter on these scales is an important test of Lambda-CDM and the detection of stellar remnants would reveal a population that may dominate the stellar dynamics on the smallest scales. Second, our detailed view of the SMBH and its local gas and stellar environment provides insight into how SMBHs at the centers of galaxies form, grow and interact with their environs as well as on the exotic processes at work in the densest stellar clusters in the Universe. The key questions, still unanswered, of when and how SMBHs formed in the early universe, and the myriad ways in which feedback from SMBHs can affect structure formation, can be informed by directly observing the physical processes operating at the SMBH. The keys to realizing these science objectives are (1) observing modes and program support on telescopes that will allow for long time-baseline observations and careful monitoring of orbits during closest approach, (2) larger field of view (FOV) Adaptive Optics fed Integral Field Units, (3) improved strehl ratio (SR) performance for adaptive optics facilities on existing large ground-based telescopes, (4) more realistic multi-scale and multi-physics numerical modeling of the stellar dynamics, stellar evolution, and star formation taking place in the dense star cluster around a SMBH, and (5) higher angular resolution from high sensitivity interferometry, and, most importantly, a future extremely large (30-m) ground-based telescope (ELT), which is critical to several opportunities outlined herein.

Accurate positions are need to compare observations of objects made at various wavelengths, such as are to be found in the Virtual Observatory. New instruments for ground-based observing, such as multifiber spectrographs, also need very accurate positions for objects fainter than those already catalogued. Recent large catalogs have revolutionized our ability to do astrometry with CCD images. The recently published FITS World Coordinate System standard has provided a standard way of parameterizing that astrometry, and the WCSTools and SExtractor software packages allow the automation of the "plate-fitting" process. As part of a survey to be conducted with one of these new spectrographs, we have amassed 1728 15 by 30 arcminute CCD images of a portion of the northern sky. After matching image point sources to objects in each of the catalogs and fitting world coordinate systems to them using the IMWCS program, we find mean residuals between observed and catalog star positions of between 0.09 and 0.25 arcseconds for the latest catalogs.

We use 2MASS photometry to select blue horizontal-branch (BHB) candidates covering the sky |b| > 15 •. A 12.5 < J 0 < 15.5 sample of BHB stars traces the thick disk and inner halo to d ⊙ < 9 kpc, with a density comparable to that of M giant stars. We base our sample selection strategy on the Century Survey Galactic Halo Project, a survey that provides a complete, spectroscopically-identified sample of blue stars to a similar depth as the 2MASS catalog. We show that a −0.20 < (J − H) 0 < 0.10, −0.10 < (H − K) 0 < 0.10 color-selected sample of stars is 65% complete for BHB stars, and is composed of 47% BHB stars. We apply this photometric selection to the full 2MASS catalog and see no spatial overdensities of BHB candidates at high Galactic latitude, |b| > 50 •. We insert simulated star streams into the data and conclude that the high Galactic latitude BHB candidates are consistent with having no ∼5 • wide star stream with density greater than 0.33 objects deg −2 at the 95% confidence level. The absence of observed structure suggests there have been no major accretion events in the inner halo in the last few Gyrs. However, at low Galactic latitudes a two-point angular correlation analysis reveals structure on angular scales θ 1 •. This structure is apparently associated with stars in the thick disk, and has a physical scale of 10-100 pc. Interestingly, such structures are expected by cosmological simulations that predict the majority of the thick disk may arise from accretion and disruption of satellite mergers.

We identify SDSS J010657.39−100003.3 (hereafter J0106−1000) as the shortest period detached binary white dwarf (WD) system currently known. We targeted J0106−1000 as part of our radial velocity program to search for companions around known extremely low-mass (ELM, ∼ 0.2M ⊙) WDs using the 6.5m MMT. We detect peak-to-peak radial velocity variations of 740 km s −1 with an orbital period of 39.1 min. The mass function and optical photometry rule out a main-sequence star companion. Follow-up high-speed photometric observations obtained at the McDonald 2.1m telescope reveal ellipsoidal variations from the distorted primary but no eclipses. This is the first example of a tidally distorted WD. Modeling the lightcurve, we constrain the inclination angle of the system to be 67 • ± 13 •. J0106−1000 contains a pair of WDs (0.17M ⊙ primary + 0.43M ⊙ invisible secondary) at a separation of 0.32R ⊙. The two WDs will merge in 37 Myr and most likely form a core He-burning single subdwarf star. J0106−1000 is the shortest timescale merger system currently known. The gravitational wave strain from J0106−1000 is at the detection limit of the Laser Interferometer Space Antenna (LISA). However, accurate ephemeris and orbital period measurements may enable LISA to detect J0106−1000 above the Galactic background noise.

We report the discovery of a new detached, double white dwarf (WD) system with an orbital period of 39.8 min. We targeted SDSS J163030.58+423305.8 (hereafter J1630) as part of our radial velocity program to search for companions around low-mass WDs using the 6.5m MMT. We detect peak-to-peak radial velocity variations of 576 km s −1. The mass function and optical photometry rule out main-sequence companions. In addition, no millisecond pulsar companions are detected in radio observations. Thus the invisible companion is most likely another white dwarf. Unlike the other 39 min binary SDSS J010657.39−100003.3, follow-up high speed photometric observations of J1630 obtained at the McDonald 2.1m telescope do not show significant ellipsoidal variations, indicating a higher primary mass and smaller radius. The absence of eclipses constrain the inclination angle to i 82 •. J1630 contains a pair of WDs, 0.3M ⊙ primary + 0.3M ⊙ invisible secondary, at a separation of 0.32R ⊙. The two WDs will merge in less than 31 Myr. Depending on the core composition of the companion, the merger will form either a single core-He burning subdwarf star or a rapidly rotating massive WD. The gravitational wave strain from J1630 is detectable by instruments like the Laser Interferometer Space Antenna (LISA) within the first year of operation.

Context. Hyper-velocity stars are suggested to originate from the dynamical interaction of binary stars with the supermassive black hole in the Galactic centre (GC), which accelerates one component of the binary to beyond the Galactic escape velocity. Aims. The evolutionary status and GC origin of the hyper-velocity star SDSS J113312.12+010824.9 (aka HVS 7) is constrained from a detailed study of its stellar parameters and chemical composition. Methods. High-resolution spectra of HVS 7 obtained with UVES on the ESO VLT were analysed using state-of-the-art NLTE/LTE modelling techniques that can account for a chemically-peculiar composition via opacity sampling. Results. Instead of the expected slight enrichments of α-elements and near-solar iron, huge chemical peculiarities of all elements are apparent. The helium abundance is very low (<1/100 solar), C, N and O are below the detection limit, i.e they are underabundant (<1/100, < ∼ 1/3 and <1/10 solar). Heavier elements, however, are overabundant: the iron group by a factor of ∼10, P, Co and Cl by factors ∼40, 80 and 440 and rare-earth elements and mercury even by ∼10 000. An additional finding, relevant also for other chemically peculiar stars are the large NLTE effects on abundances of Ti ii and Fe ii (∼0.6−0.7 dex). The derived abundance pattern of HVS 7 is characteristic for the class of chemical peculiar magnetic B stars on the main sequence. The chemical composition and high projected rotation velocity v sin i = 55 ± 2 km s −1 render a low mass nature of HVS 7 as a blue horizontal branch star unlikely. Conclusions. Such a surface abundance pattern is caused by atomic diffusion in a possibly magnetically stabilised, non-convective atmosphere. Hence all chemical information on the star's place of birth and its evolution has been washed out. High precision astrometry is the only means to validate a GC origin for HVS 7.

We present optical and near-infrared photometry and spectroscopy of the type Ia SN 1998bu in the Leo I Group galaxy M96 (NGC 3368). The data set consists of 356 photometric measurements and 29 spectra of SN 1998bu between UT 1998 May 11 and July 15. The well-sampled light curve indicates the supernova reached maximum light in B on UT 1998 May 19.3 (JD 2450952.8±0.8) with B = 12.22 ± 0.03 and V = 11.88 ± 0.02. Application of a revised version of the Multicolor Light Curve Shape (MLCS) method yields an extinction toward the supernova of A V = 0.94 ± 0.15 mag, and indicates the supernova was of average luminosity compared to other normal type Ia supernovae. Using the HST Cepheid distance modulus to M96 (Tanvir et al. 1995) and the MLCS fit parameters for the supernova, we derive an extinction-corrected absolute magnitude for SN 1998bu at maximum, M V = −19.42 ± 0.22. Our independent results for this supernova are consistent with those of Suntzeff et al. (1999). Combining SN 1998bu with three other well-observed local calibrators and 42 supernovae in the Hubble flow yields a Hubble constant, H 0 = 64 +8 −6 km s −1 Mpc −1 , where the error estimate incorporates possible sources of systematic uncertainty including the calibration of the Cepheid period-luminosity relation, the metallicity dependence of the Cepheid distance scale, and the distance to the LMC.

We report the detection of a radial velocity companion to SDSS J091709.55+463821.8, the lowest mass white dwarf currently known with M~0.17Msun. The radial velocity of the white dwarf shows variations with a semi-amplitude of 148.8 km/s and a period of 7.5936 hours, which implies a companion mass of M &gt; 0.28Msun. The lack of evidence of a companion in the optical photometry forces any main-sequence companion to be smaller than 0.1Msun, hence a low mass main sequence star companion is ruled out for this system. The companion is most likely another white dwarf, and we present tentative evidence for an evolutionary scenario which could have produced it. However, a neutron star companion cannot be ruled out and follow-up radio observations are required to search for a pulsar companion.

We measure the radial velocity curve of the eclipsing detached white dwarf binary NLTT 11748. The primary exhibits velocity variations with a semiamplitude of 273 km s −1 and an orbital period of 5.641 hr. We do not detect any spectral features from the secondary star, or any spectral changes during the secondary eclipse. We use our composite spectrum to constrain the temperature and surface gravity of the primary to be T eff = 8690 ± 140 K and log g = 6.54 ±0.05, which correspond to a mass of 0.18 M ⊙. For an inclination angle of 89.9 • derived from the eclipse modeling, the mass function requires a 0.76 M ⊙ companion. The merger time for the system is 7.2 Gyr. However, due to the extreme mass ratio of 0.24, the binary will most likely create an AM CVn system instead of a merger.

2048 charge-couple device (CCD) with 15 Ȑm pixels, 7 electrons per pixel readout noise, and a 0.64 arcsec/pixel We present new CCD photometry of the Centaur object 1995 GO obtained in April 1996, including a lightcurve, a V ؊ R image scale (in 2 ϫ 2 binning mode). A Kron-Cousins R color, and a surface brightness profile. The apparent brightness filter was used for most of the observations; a few observaimplies a circular effective radius of 53.5 km, assuming a 4% tions were also made with a Johnson V filter in order to albedo. The shape of the lightcurve is compatible with an determine the V Ϫ R color. The target had a sufficiently aspherical rotating body, with a rotation period of 8.870 ؎ small angular motion (9 arcsec/hr) that the telescope was 0.020 hr. No evidence of cometary activity was found in the tracked at sidereal rate at all times. Integration times (700observations, allowing us to place an upper limit of 3 kg sec ؊1 800 sec) were chosen so that the trailing was less than on the mass loss rate in 1995 GO. We measure the colors V ؊ the average seeing disk of ȁ1.8 arcsec full width at half R ‫؍‬ 0.73 ؎ 0.04 and B ؊ V ‫؍‬ 0.78 ؎ 0.05, which fall within maximum (FWHM). the previously reported color range for Centaurs and Kuiper Belt objects.