- ID:
- ivo://CDS.VizieR/J/MNRAS/463/296
- Title:
- PanSTARRS-1 slow-blue nuclear hypervariables
- Short Name:
- J/MNRAS/463/296
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We discuss 76 large amplitude transients ({Delta}m>1.5) occurring in the nuclei of galaxies, nearly all with no previously known active galactic nucleus (AGN). They have been discovered as part of the Pan-STARRS1 (PS1) 3{pi} survey, by comparison with Sloan Digital Sky Survey (SDSS) photometry a decade earlier, and then monitored with the Liverpool Telescope, and studied spectroscopically with the William Herschel Telescope (WHT). Based on colours, light-curve shape, and spectra, these transients fall into four groups. A few are misclassified stars or objects of unknown type. Some are red/fast transients and are known or likely nuclear supernovae. A few are either radio sources or erratic variables and so likely blazars. However the majority (~66 per cent) are blue and evolve slowly, on a time-scale of years. Spectroscopy shows them to be AGN at z ~0.3-1.4, which must have brightened since the SDSS photometry by around an order of magnitude. It is likely that these objects were in fact AGN a decade ago, but too weak to be recognized by SDSS; they could then be classed as 'hypervariable' AGN. By searching the SDSS Stripe 82 quasar database, we find 15 similar objects. We discuss several possible explanations for these slow-blue hypervariables - (i) unusually luminous tidal disruption events; (ii) extinction events; (iii) changes in accretion state; and (iv) large amplitude microlensing by stars in foreground galaxies. A mixture of explanations (iii) and (iv) seems most likely. Both hold promise of considerable new insight into the AGN phenomenon.
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- ID:
- ivo://CDS.VizieR/J/ApJ/794/23
- Title:
- Pan-STARRS1 transients optical photometry
- Short Name:
- J/ApJ/794/23
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In the past decade, several rapidly evolving transients have been discovered whose timescales and luminosities are not easily explained by traditional supernovae (SNe) models. The sample size of these objects has remained small due, at least in part, to the challenges of detecting short timescale transients with traditional survey cadences. Here we present the results from a search within the Pan-STARRS1 Medium Deep Survey (PS1-MDS) for rapidly evolving and luminous transients. We identify 10 new transients with a time above half-maximum (t_1/2_) of less than 12 days and -16.5>M>-20 mag. This increases the number of known events in this region of SN phase space by roughly a factor of three. The median redshift of the PS1-MDS sample is z=0.275 and they all exploded in star-forming galaxies. In general, the transients possess faster rise than decline timescale and blue colors at maximum light (g_P1_-r_P1_<~-0.2). Best-fit blackbodies reveal photospheric temperatures/radii that expand/cool with time and explosion spectra taken near maximum light are dominated by a blue continuum, consistent with a hot, optically thick, ejecta. We find it difficult to reconcile the short timescale, high peak luminosity (L>10^43^ erg/s), and lack of UV line blanketing observed in many of these transients with an explosion powered mainly by the radioactive decay of ^56^Ni. Rather, we find that many are consistent with either (1) cooling envelope emission from the explosion of a star with a low-mass extended envelope that ejected very little (<0.03 M_{sun}_) radioactive material, or (2) a shock breakout within a dense, optically thick, wind surrounding the progenitor star. After calculating the detection efficiency for objects with rapid timescales in the PS1-MDS we find a volumetric rate of 4800-8000 events/yr/Gpc^3^ (4%-7% of the core-collapse SN rate at z=0.2).
- ID:
- ivo://CDS.VizieR/J/AJ/151/160
- Title:
- 1103 parallaxes and proper motions from URAT
- Short Name:
- J/AJ/151/160
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present 1103 trigonometric parallaxes and proper motions from the United States Naval Observatory Robotic Astrometric Telescope (URAT) observations taken at the Naval Observatory Flagstaff Station (NOFS) over a three-year period from 2012 April to 2015 June covering the entire sky north of about -10{deg} decl. We selected two samples: previously suspected nearby stars from known photometric distances and stars showing a large, significant parallax signature in URAT epoch data without any prior selection criteria. All systems presented in this paper have an observed parallax {>=}40mas with no previous published trigonometric parallax. The formal errors on these weighted parallax solutions are mostly between 4 and 10mas. This sample gives a significant (of the order of 50%) increase to the number of known systems having a trigonometric parallax to be within 25pc of the Sun (without applying Lutz-Kelker bias corrections). A few of these are found to be within 10pc. Many of these new nearby stars display a total proper motion of less than 200mas/yr. URAT parallax results have been verified against Hipparcos and Yale data for stars in common. The publication of all significant parallax observations from URAT data is in preparation for CDS.
- ID:
- ivo://CDS.VizieR/I/129
- Title:
- Parallaxes and Proper Motions near SGP
- Short Name:
- I/129
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The catalog contains 6125 stars brighter than B=17.5, V=17.0 in the south galactic cap. The data have been obtained using the UK Schmidt telescope between 1975 and 1981. The plates were measured on the GALAXY machine at RGO. External errors of the parallaxes range between -/+0.012 and -/+0.017 arcsec according to magnitude. Internal proper motion errors range from -/+0.006 and -/+0.008 arcsec. In addition to the positions, proper motions, and parallaxes, the photometric data include B and V.
- ID:
- ivo://CDS.VizieR/J/A+A/616/L15
- Title:
- Parallaxes and Proper Motions of OB stars
- Short Name:
- J/A+A/616/L15
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Gaia mission has released the second data set (Gaia DR2), which contains parallaxes and proper motions for a large number of massive, young stars. We investigate the spiral structure in the solar neighborhood revealed by Gaia DR2 and compare it with that depicted by VLBI maser parallaxes. We examined three samples with different constraints on parallax uncertainty and distance errors and stellar spectral types: (1) all OB stars with parallax errors less than 10%; (2) only O-type stars with 0.1mas errors imposed and with parallax distance errors less than 0.2kpc; (3) only O-type stars with 0.05 mas errors imposed and with parallax distance errors less than 0.3kpc. In spite of the significant distance uncertainties for stars in DR2 beyond 1.4kpc,the spiral structure in the solar neighborhood demonstrated by Gaia agrees well with that illustrated by VLBI maser results. The O-type stars available from DR2 extend the spiral arm models determined from VLBI maser parallaxes into the fourth Galactic quadrant, and suggest the existence of a new spur between the Local and Sagittarius arms.
- ID:
- ivo://CDS.VizieR/J/ApJ/784/156
- Title:
- Parallaxes for 1507 nearby mid-to-late M dwarfs
- Short Name:
- J/ApJ/784/156
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The MEarth survey is a search for small rocky planets around the smallest, nearest stars to the Sun as identified by high proper motion with red colors. We augmented our planetary search time series with lower cadence astrometric imaging and obtained two million images of approximately 1800 stars suspected to be mid-to-late M dwarfs. We fit an astrometric model to MEarth's images for 1507 stars and obtained trigonometric distance measurements to each star with an average precision of 5mas. Our measurements, combined with the Two Micron All Sky Survey photometry, allowed us to obtain an absolute K_s_ magnitude for each star. In turn, this allows us to better estimate the stellar parameters than those obtained with photometric estimates alone and to better prioritize the targets chosen to monitor at high cadence for planetary transits. The MEarth sample is mostly complete out to a distance of 25pc for stars of type M5.5V and earlier, and mostly complete for later type stars out to 20pc. We find eight stars that are within 10pc of the Sun for which there did not exist a published trigonometric parallax distance estimate. We release with this work a catalog of the trigonometric parallax measurements for 1507 mid-to-late M dwarfs, as well as new estimates of their masses and radii.
- ID:
- ivo://CDS.VizieR/J/ApJ/783/130
- Title:
- Parallaxes of high mass star forming regions
- Short Name:
- J/ApJ/783/130
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Over 100 trigonometric parallaxes and proper motions for masers associated with young, high-mass stars have been measured with the Bar and Spiral Structure Legacy Survey, a Very Long Baseline Array key science project, the European VLBI Network, and the Japanese VLBI Exploration of Radio Astrometry project. These measurements provide strong evidence for the existence of spiral arms in the Milky Way, accurately locating many arm segments and yielding spiral pitch angles ranging from about 7{deg} to 20{deg}. The widths of spiral arms increase with distance from the Galactic center. Fitting axially symmetric models of the Milky Way with the three-dimensional position and velocity information and conservative priors for the solar and average source peculiar motions, we estimate the distance to the Galactic center, R_0_, to be 8.34+/-0.16kpc, a circular rotation speed at the Sun, {Theta}_0_, to be 240+/-8km/s, and a rotation curve that is nearly flat (i.e., a slope of -0.2+/-0.4km/s/kpc) between Galactocentric radii of {approx}5 and 16kpc. Assuming a "universal" spiral galaxy form for the rotation curve, we estimate the thin disk scale length to be 2.44+/-0.16kpc. With this large data set, the parameters R_0_ and {Theta}_0_are no longer highly correlated and are relatively insensitive to different forms of the rotation curve. If one adopts a theoretically motivated prior that high-mass star forming regions are in nearly circular Galactic orbits, we estimate a global solar motion component in the direction of Galactic rotation, V_{sun}_=14.6+/-5.0km/s. While {Theta}_0_and V_{sun}_are significantly correlated, the sum of these parameters is well constrained, {Theta}_0_+V_{sun}_=255.2+/-5.1km/s, as is the angular speed of the Sun in its orbit about the Galactic center, ({Theta}_0_+V_{sun}_)/R_0_=30.57+/-0.43km/s/kpc. These parameters improve the accuracy of estimates of the accelerations of the Sun and the Hulse-Taylor binary pulsar in their Galactic orbits, significantly reducing the uncertainty in tests of gravitational radiation predicted by general relativity.
- ID:
- ivo://CDS.VizieR/J/ApJS/240/19
- Title:
- Parallaxes of late-T and Y dwarfs
- Short Name:
- J/ApJS/240/19
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present preliminary trigonometric parallaxes of 184 late-T and Y dwarfs using observations from Spitzer (143), the U.S. Naval Observatory (18), the New Technology Telescope (14), and the United Kingdom Infrared Telescope (9). To complete the 20pc census of >=T6 dwarfs, we combine these measurements with previously published trigonometric parallaxes for an additional 44 objects and spectrophotometric distance estimates for another 7. For these 235 objects, we estimate temperatures, sift into five 150K wide T_eff_ bins covering the range 300-1050K, determine the completeness limit for each, and compute space densities. To anchor the high-mass end of the brown dwarf mass spectrum, we compile a list of early- to mid-L dwarfs within 20pc. We run simulations using various functional forms of the mass function passed through two different sets of evolutionary code to compute predicted distributions in T_eff_. The best fit of these predictions to our L, T, and Y observations is a simple power-law model with {alpha}~0.6 (where dN/dM{propto}M^-{alpha}^), meaning that the slope of the field substellar mass function is in rough agreement with that found for brown dwarfs in nearby star-forming regions and young clusters. Furthermore, we find that published versions of the log-normal form do not predict the steady rise seen in the space densities from 1050 to 350K. We also find that the low-mass cutoff to formation, if one exists, is lower than ~5M_Jup_, which corroborates findings in young, nearby moving groups and implies that extremely low-mass objects have been forming over the lifetime of the Milky Way.
- ID:
- ivo://CDS.VizieR/J/A+A/493/L27
- Title:
- Parallaxes of 10 ultracool subdwarfs
- Short Name:
- J/A+A/493/L27
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We measure absolute trigonometric parallaxes and proper motions with respect to many background galaxies for a sample of ten ultracool subdwarfs. Observations were taken in the H-band with the OMEGA2000 camera on the 3.5 m-telescope at Calar Alto, Spain during a time period of 3.5-years. For the first time, the reduction of the astrometric measurements was carried out directly with respect to background galaxies. We obtained absolute parallaxes with mean errors ranging between 1 and 3mas.
- ID:
- ivo://CDS.VizieR/J/A+A/645/L8
- Title:
- Parallaxes & proper motions of OB stars from Gaia EDR3
- Short Name:
- J/A+A/645/L8
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The astrometric satellite Gaia is expected to significantly increase our knowledge as to the properties of the MilkyWay. The Gaia Early Data Release 3 (Gaia EDR3) provides the most precise parallaxes for many OB stars, which can be used to delineate the Galactic spiral structure. We investigate the local spiral structure with the largest sample of spectroscopically confirmed young OB stars available to date, and we compare it with what was traced by the parallax measurements of masers. A sample consisting of three different groups of massive young stars, including O-B2 stars, O-B0 stars and O-type stars with parallax accuracies better than 10% was compiled and used in our analysis. The local spiral structures in all four Galactic quadrants within ~5kpc of the Sun are clearly delineated in detail. The revealed Galactic spiral pattern outlines a clear sketch of nearby spiral arms, especially in the third and fourth quadrants where the maser parallax data are still absent. These O-type stars densify and extend the spiral structure constructed by using the Very Long Baseline Interferometry (VLBI) maser data alone. The clumped distribution of O-type stars also indicates that the Galaxy spiral structure is inhomogeneous.