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From the analysis of sensitive HI 21-cm line observations, we find evidence for vertically extended HI emission (|z, Comment: to appear in the August 20 Astrophysical Journal; 17 pages; version with full resolution figures available at http://cfa-www.harvard.edu/~lmatthew/

We have used the Very Large Array to image the isolated ``superthin'' galaxy UGC7321 in the HI line with a spatial resolution of 16'' and a spectral resolution of 24 kHz (5.2 km/s). We have reached a sensitivity of ~0.38 mJy/bm per channel, which corresponds to a column density of 8 x 10**{18} cm**{-2} (1 sigma). UGC7321 has a gas-rich disk with M_HI = (1.06 +/- 0.01) 10**9 Msun and MHI/LB = 1.0 (solar units), and no detectable radio continuum emission (F_CONT = 0.41 +/- 0.25 mJy). The global \HI distribution of UGC 7321 is rather symmetric and extends to ~1.5 times the optical radius (D_HI = 8.65' +/- 0.15') at n_HI = 3 x 10**{19} atoms cm**{-2}). An ``integral sign'' warp is observed in the HI disk, commencing near the edge of the stellar distribution, and twisting back toward the equatorial plane in the outermost regions. In addition, the position-velocity diagram suggests the presence of a bar or inner arm within ~40'' from the center. The rotation curve of UGC7321 is slowly rising; it reaches its asymptotic velocity of ~110 km/s ~2.5' from the center (about 0.9 optical radii) and declines near the edge of the HI disk. The ratio of the inferred dynamical mass to the mass in gas and stars is ~12, implying that UGC7321 is a highly dark-matter dominated galaxy., Comment: 29 pages (13 figures, one double), Latex), uses emulateapj.sty, to appear in the Astronomical Journal (May 2003). High-resolution versions of the figures can be downloaded from http://www.cv.nrao.edu/~juson/ugc7321/ugc7321.html The continuum image and spectral image cube described in the paper will be available in FITS format at the web address above upon publication of the paper in the journal. Astronomical Journal vol. 125 (in press, May 2003)

We present optical longslit spectroscopic observations of 21 low-luminosity, extreme late-type spiral galaxies. Our sample is comprised of Sc-Sm Local Supercluster spirals with moderate-to-low optical surface brightnesses and with luminosities at the low end for spiral disk galaxies (M_V>-18.8). For each galaxy we have measured high spatial resolution position-velocity (P-V) curves using the H alpha emission line, and for 15 of the galaxies we also derive major axis rotation curves. In ~50% of our sample, the P-V curves show significant asymmetries in shape, extent, and/or amplitude on the approaching and receding sides of the disk. A number of the P-V curves are still rising to the last measured point, or reach a clear turnover on only one side. In most instances we find good agreement between the kinematic centers of extreme late-type spirals as defined by the global HI emission profile and by their optical continuum, although in a few cases we see evidence of possible real offsets. In spite of their shallow central gravitational potentials, at least 6 of the galaxies in our sample possess semi-stellar nuclei that appear to be compact nuclear star clusters; in 5 of these cases we see kinematic signatures in the P-V curves at the location of the nucleus. Finally, we find that like giant spirals, our sample galaxies have higher specific angular momenta than predicted by current cold dark matter models., Comment: 20 pages; accepted to ApJS; higher resolution version of Figure 1 available as .png file

We use a combination of three-dimensional Monte Carlo radiative transfer techniques and multi-wavelength (BRHK,H alpha) imaging data to investigate the nature of the interstellar medium (ISM) in the edge-on, low surface brightness (LSB) galaxy UGC7321. Using realistic models that incorporate multiple scattering effects and clumping of the stars and the interstellar material, we explore the distribution and opacity of the interstellar material (gas+dust), and its effects on the observed stellar disk luminosity profiles, color gradients, and rotation curve shape. We find that UGC7321 contains a small but non-negligible dusty component to its ISM, yielding a B-band optical depth tau_e,B~4.0 from disk edge to center. A significant fraction (~50+/-10%) of the interstellar material in the innermost regions of UGC7321 appears to be contained in a clumpy medium, indicating that LSB galaxies can support a modest, multi-phase ISM structure. In spite of the clear presence of dust, we conclude that the large radial optical color gradients observed in UGC7321 and other similar LSB spiral galaxies cannot be accounted for by dust and must result primarily from significant stellar population and/or metallicity gradients. We show that realistic optical depth effects will have little impact on the observed rotation curves of edge-on disk galaxies and cannot explain the linear, slowly rising rotation curves seen in some edge-on LSB spirals. Projection effects create a far larger uncertainty in recovering the true underlying rotation curve shape of galaxies viewed at inclinations i>85 degrees., accepted to ApJ; 16 pages, 2 tables, and 18 figures; uses emulateapj.sty

Using new and archival spectra from the Goddard High Resolution Spectrograph, we have searched for evidence of chromospheric and transition region emission in six stars of mid- to late-A spectral type. Two of the stars, alpha Aql (A7 IV-V) and alpha Cep (A7 IV-V), show emission in the C~II 1335 A doublet, confirming the presence of hot plasma with temperatures comparable to that of the solar transition region. Using radiative equilibrium photospheric models, we estimate the net surface fluxes in the C II emission line to be 9.4 X 10^4 erg/cm/cm/s for alpha Aql and 6.5 X 10^4 erg/cm/cm/s for alpha Cep. These are comparable to fluxes observed in early to mid-F-type dwarfs, indicating that significant upper atmospheric heating is present in at least some stars as hot as ~8000 K (B-V=0.22). We find no evidence for the blue-shifted emission reported by Simon et al (1994). We estimate the basal flux level to be about 30% of that seen in early~F stars, and that the bulk of the emission is not basal in origin. We conclude that the basal flux level drops rapidly for B-V < 0.3, but that magnetic activity may persist to B-V as small as 0.22., uuencoded, compressed, tarred postscript. 8 figures included. Latex file astars.tex (AAS/WGAS macros version 3.0) or postscript file (astars.ps) and individual postscript figures (astars_fig_*.ps) are available by at ftp://sbast3.ess.sunysb.edu/pub/fred/ . Astrophysical Journal, in press (1 July 1995).

Recently published Spitzer observations of the classical Cepheid archetype delta Cephei revealed an extended dusty nebula surrounding this star and its hot companion. The infrared emission resembles a bow shock aligned with the direction of space motion of the star, indicating that delta Cep is undergoing mass-loss through a stellar wind. Here we report HI 21-cm line observations with the VLA to search for neutral atomic hydrogen associated with this wind. Our VLA data reveal a spatially extended HI nebula (~13' or 1 pc across) surrounding the position of delta Cep. The nebula has a head-tail morphology, consistent with circumstellar ejecta shaped by the interaction between a stellar wind and the ISM. We directly measure a mass of circumstellar hydrogen M_HI\approx0.07M_odot, although the total HI mass may be larger. The HI data imply a stellar wind with an outflow velocity V_o=35.6\pm1.2 km/s and a mass-loss rate of M_dot=(1.0\pm0.8)x10**-6 M_dot/yr. We have computed theoretical evolutionary tracks that include mass loss across the instability strip and show that a mass-loss rate of this magnitude, sustained over the preceding Cepheid lifetime of delta Cep, could be sufficient to resolve a significant fraction of the discrepancy between the pulsation and evolutionary masses for this star. (abridged), ApJ, in press (January 1, 2012). Version with full resolution figures available at http://www.haystack.mit.edu/hay/staff/lmatthew/matthews_deltaCep.pdf

Using the Expanded Very Large Array, we have mapped the vicinity of the Orion BN/KL Hot Core with sub-arcsecond angular resolution in seven metastable inversion transitions of ammonia: (J,K)=(6,6) to (12,12). This emission comes from levels up to 1500 K above the ground state, enabling identification of source(s) responsible for heating the region. We used this multi-transition dataset to produce images of the rotational/kinetic temperature and the column density of ammonia for ortho and para species separately and on a position-by-position basis. We find rotational temperature and column density in the range 160-490 K and (1-4)x10^17 cm^-2, respectively. Our spatially-resolved images show that the highest (column) density and hottest gas is found in a northeast-southwest elongated ridge to the southeast of Source I. We have also measured the ortho-para ratio of ammonia, estimated to vary in the range 0.9-1.6. Enhancement of ortho with respect to para and the offset of hot ammonia emission peaks from known (proto)stellar sources provide evidence that the ammonia molecules have been released from dust grains into the gas-phase through the passage of shocks and not by stellar radiation. We propose that the combined effect of Source I's proper motion and its low-velocity outflow impinging on a pre-existing dense medium is responsible for the excitation of ammonia and the Orion Hot Core. Finally, we found for the first time evidence of a slow (5 km/s) and compact (1000 AU) outflow towards IRc7., Comment: To appear in Astrophysical Journal Letters Special Issue on the EVLA. 8 pages, 4 figures

We present the first spectroscopic images of solar radio transients from the prototype for the Murchison Widefield Array (MWA), observed on 2010 March 27. Our observations span the instantaneous frequency band 170.9-201.6 MHz. Though our observing period is characterized as a period of `low' to `medium' activity, one broadband emission feature and numerous short-lived, narrowband, non-thermal emission features are evident. Our data represent a significant advance in low radio frequency solar imaging, enabling us to follow the spatial, spectral, and temporal evolution of events simultaneously and in unprecedented detail. The rich variety of features seen here reaffirms the coronal diagnostic capability of low radio frequency emission and provides an early glimpse of the nature of radio observations that will become available as the next generation of low frequency radio interferometers come on-line over the next few years., Comment: 15 pages, 5 figures, accepted for publication in Astrophysical Journal Letters. Movies for figures 4 and 5 available at http://www.mwatelescope.org/info/mwa_proto.html

We present new 7mm continuum observations of Orion BN/KL with the VLA. We resolve the emission from the protostar radio Source I and BN at several epochs. Source I is highly elongated NW-SE, and remarkably stable in flux density, position angle, and overall morphology over nearly a decade. This favors the extended emission component arising from an ionized disk rather than a jet. We have measured the proper motions of Source I and BN for the first time at 43 GHz. We confirm that both sources are moving at high speed (12 and 26 km/s, respectively) approximately in opposite directions, as previously inferred from measurements at lower frequencies. We discuss dynamical scenarios that can explain the large motions of both BN and Source I and the presence of disks around both. Our new measurements support the hypothesis that a close (~50 AU) dynamical interaction occurred around 500 years ago between Source I and BN as proposed by Gomez et al. From the dynamics of encounter we argue that Source I today is likely to be a binary with a total mass on the order of 20 Msun, and that it probably existed as a softer binary before the close encounter. This enables preservation of the original accretion disk, though truncated to its present radius of ~50 AU. N-body numerical simulations show that the dynamical interaction between a binary of 20 Msun total mass (I) and a single star of 10 Msun mass (BN) may lead to the ejection of both and binary hardening. The gravitational energy released in the process would be large enough to power the wide-angle flow traced by H2 and CO emission in the BN/KL nebula. Assuming the proposed dynamical history is correct, the smaller mass for Source I recently estimated from SiO maser dynamics (>7 Msun) by Matthews et al., suggests that non-gravitational forces (e.g. magnetic) must play an important role in the circumstellar gas dynamics., 17 pages, 7 figures, 4 tables, accepted by ApJ

Around high-mass Young Stellar Objects (YSOs), outflows are expected to be launched and collimated by accretion disks inside radii of 100 AU. Strong observational constraints on disk-mediated accretion in this context have been scarce, largely owing to difficulties in probing the circumstellar gas at scales 10-100 AU around high-mass YSOs, which are on average distant (>1 Kpc), form in clusters, and ignite quickly whilst still enshrouded in dusty envelopes. Radio Source I in Orion BN/KL is the nearest example of a high-mass YSO, and only one of three YSOs known to power SiO masers. Using VLA and VLBA observations of different SiO maser transitions, the KaLYPSO project (http://www.cfa.harvard.edu/kalypso/) aims to overcome past observational limitations by mapping the structure, 3-D velocity field, and dynamical evolution of the circumstellar gas within 1000 AU from Source I. Based on 19 epochs of VLBA observations of v=1,2 SiO masers over ~2 years, we produced a movie of bulk gas flow tracing the compact disk and the base of the protostellar wind at radii < 100 AU from Source I. In addition, we have used the VLA to map 7mm SiO v=0 emission and track proper motions over 10 years. We identify a narrowly collimated outflow with a mean motion of 18 km/s at radii 100-1000 AU, along a NE-SW axis perpendicular to that of the disk traced by the v=1,2 masers. The VLBA and VLA data exclude alternate models that place outflow from Source I along a NW-SE axis. The analysis of the complete (VLBA and VLA) dataset provides the most detailed evidence to date that high-mass star formation occurs via disk-mediated accretion.

Nature 389, 358–360 (1997) There was a typographical error in the declination of the optical counterpart given at the start of the fourth paragraph: this should be −37 ° 54′35.8″.

We present B and V CCD aperture photometry for a morphologically-selected sample of forty-nine southern, extreme late-type spiral galaxies. All objects are moderate-to-low surface brightness Local Supercluster field galaxies that were detected previously in HI surveys. Our sample features objects that have optical luminosities, optical sizes, and HI masses which are at the low end for spiral galaxies. These objects are not a new class of galaxy, but are examples of a common type of spiral galaxy that has been under-represented in nearby galaxy samples. (Abridged)., Comment: To appear in the Astronomical Journal; 31 pages + 5 tables (LaTex; uses aasms4.sty) + 10 postscript figures; .gif copies of 7 additional figure plates available via anonymous ftp at uwast.astro.wisc.edu/outgoing/jsg/papers/ELT_photom/

Using the Wide Field Planetary Camera 2 on the Hubble Space Telescope, we have obtained a deep color-magnitude diagram in V- and I-band equivalents for more than 2000 stars in a patch of the outer disk of the Large Magellanic Cloud LMC). Aperture photometry is feasible from these data with good signal-to-noise ratio for stars with V ≤ 25, which allows us for the first time to construct a color magnitude diagram for LMC disk stars on the lower main sequence, extending beyond the oldest main sequence turnoff point. We analyze the structure of the main-sequence band and overall morphology of the color-magnitude diagram to obtain a star formation history for the region. A comparison between the distribution of stars across the main-sequence band for M_v ≤ 4 and a stellar population model constrains historical star formation rates within the past 3 Gyr. The stellar populations in this region sample the outer LMC disk for stars with ages of 1 Gyr or older that have had time to spatially mix. The structure of the main-sequence band requires that star formation occurred at a roughly constant rate during most of the past ≈ 3 Gyr. However, the distribution of subgiant stars indicate that a pronounced peak in the star formation rate likely occurred about 2 Gyr ago, prior to which the star formation rate had not been enhanced for several Gyr. Studies over timescales of more than 3 Gyr require a separation of the effects of star formation history and the chemical evolution on the LMC color-magnitude diagrams, which is difficult to achieve without additional constraints. If lower main-sequence stars in the LMC have moderate metallicities, then the age for most LMC disk stars is less than about 8 Gyr.

The Wide Field Planetary Camera 2 (WFPC2) was installed in the Hubble Space Telescope (HST) in 1993 December. Since then, the instrument has been providing high-quality images. A significant among of calibration data has been collected to aid in the understanding of the on-orbit performance of the instrument. Generally, the behavior of the camera is similar to its performance during the system-level thermal vacuum test at JPL in 1993 May. Surprises were a significant charge-transfer-efficiency (CTE) problem and a significant growth rate in hot pixels at the original operating temperature of the CCDs (-76 deg C). The operating temperature of the WFPC2 CCDs was changed to -88 deg C on 1994 April 23, and significant improvements in CTE and hot pixels are seen at this temperature. In this paper we describe the on-orbit performance of the WFPC2. We discuss the optical and thermal history, the instrument throughput and stability, the Point Spread Function (PSF), the effects of undersampling on photometry, the properties of cosmic rays observed on-orbit, and the geometric distortion in the camera. We present the best techniques for the reduction of WFPC2 data, and describe the construction of calibration products including superbiases, superdarks, and flat fields.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Full list of authors: Akiyama, Kazunori; Alberdi, Antxon; Alef, Walter; Algaba, Juan Carlos; Anantua, Richard; Asada, Keiichi; Azulay, Rebecca; Bach, Uwe; Baczko, Anne-Kathrin; Ball, David; Balokovic, Mislav; Barrett, John; Baubock, Michi; Benson, Bradford A.; Bintley, Dan; Blackburn, Lindy; Blundell, Raymond; Bouman, Katherine L.; Bower, Geoffrey C.; Boyce, Hope; Bremer, Michael; Brinkerink, Christiaan D.; Brissenden, Roger; Britzen, Silke; Broderick, Avery E.; Broguiere, Dominique; Bronzwaer, Thomas; Bustamante, Sandra; Byun, Do-Young; Carlstrom, John E.; Ceccobello, Chiara; Chael, Andrew; Chan, Chi-kwan; Chatterjee, Koushik; Chatterjee, Shami; Chen, Ming-Tang; Chen, Yongjun; Cheng, Xiaopeng; Cho, Ilje; Christian, Pierre; Conroy, Nicholas S.; Conway, John E.; Cordes, James M.; Crawford, Thomas M.; Crew, Geoffrey B.; Cruz-Osorio, Alejandro; Cui, Yuzhu; Davelaar, Jordy; De Laurentis, Mariafelicia; Deane, Roger; Dempsey, Jessica; Desvignes, Gregory; Dexter, Jason; Dhruv, Vedant; Doeleman, Sheperd S.; Dougal, Sean; Dzib, Sergio A.; Eatough, Ralph P.; Emami, Razieh; Falcke, Heino; Farah, Joseph; Fish, Vincent L.; Fomalont, Ed; Ford, H. Alyson; Fraga-Encinas, Raquel; Freeman, William T.; Friberg, Per; Fromm, Christian M.; Fuentes, Antonio; Galison, Peter; Gammie, Charles F.; Garcia, Roberto; Gentaz, Olivier; Georgiev, Boris; Goddi, Ciriaco; Gold, Roman; Gomez-Ruiz, Arturo, I; Gomez, Jose L.; Gu, Minfeng; Gurwell, Mark; Hada, Kazuhiro; Haggard, Daryl; Haworth, Kari; Hecht, Michael H.; Hesper, Ronald; Heumann, Dirk; Ho, Luis C.; Ho, Paul; Honma, Mareki; Huang, Chih-Wei L.; Huang, Lei; Hughes, David H.; Ikeda, Shiro; Impellizzeri, C. M. Violette; Inoue, Makoto; Issaoun, Sara; James, David J.; Jannuzi, Buell T.; Janssen, Michael; Jeter, Britton; Jiang, Wu; Jimenez-Rosales, Alejandra; Johnson, Michael D.; Jorstad, Svetlana; Joshi, Abhishek, V; Jung, Taehyun; Karami, Mansour; Karuppusamy, Ramesh; Kawashima, Tomohisa; Keating, Garrett K.; Kettenis, Mark; Kim, Dong-Jin; Kim, Jae-Young; Kim, Jongsoo; Kim, Junhan; Kino, Motoki; Koay, Jun Yi; Kocherlakota, Prashant; Kofuji, Yutaro; Koch, Patrick M.; Koyama, Shoko; Kramer, Carsten; Kramer, Michael; Krichbaum, Thomas P.; Kuo, Cheng-Yu; La Bella, Noemi; Lauer, Tod R.; Lee, Daeyoung; Lee, Sang-Sung; Leung, Po Kin; Levis, Aviad; Li, Zhiyuan; Lico, Rocco; Lindahl, Greg; Lindqvist, Michael; Lisakov, Mikhail; Liu, Jun; Liu, Kuo; Liuzzo, Elisabetta; Lo, Wen-Ping; Lobanov, Andrei P.; Loinard, Laurent; Lonsdale, Colin J.; Lu, Ru-Sen; Mao, Jirong; Marchili, Nicola; Markoff, Sera; Marrone, Daniel P.; Marscher, Alan P.; Marti-Vidal, Ivan; Matsushita, Satoki; Matthews, Lynn D.; Medeiros, Lia; Menten, Karl M.; Michalik, Daniel; Mizuno, Izumi; Mizuno, Yosuke; Moran, James M.; Moriyama, Kotaro; Moscibrodzka, Monika; Muller, Cornelia; Mus, Alejandro; Musoke, Gibwa; Myserlis, Ioannis; Nadolski, Andrew; Nagai, Hiroshi; Nagar, Neil M.; Nakamura, Masanori; Narayan, Ramesh; Narayanan, Gopal; Natarajan, Iniyan; Nathanail, Antonios; Fuentes, Santiago Navarro; Neilsen, Joey; Neri, Roberto; Ni, Chunchong; Noutsos, Aristeidis; Nowak, Michael A.; Oh, Junghwan; Okino, Hiroki; Olivares, Hector; Ortiz-Leon, Gisela N.; Oyama, Tomoaki; Palumbo, Daniel C. M.; Paraschos, Georgios Filippos; Park, Jongho; Parsons, Harriet; Patel, Nimesh; Pen, Ue-Li; Pesce, Dominic W.; Pietu, Vincent; Plambeck, Richard; PopStefanija, Aleksandar; Porth, Oliver; Potzl, Felix M.; Prather, Ben; Preciado-Lopez, Jorge A.; Pu, Hung-Yi; Ramakrishnan, Venkatessh; Rao, Ramprasad; Rawlings, Mark G.; Raymond, Alexander W.; Rezzolla, Luciano; Ricarte, Angelo; Ripperda, Bart; Roelofs, Freek; Rogers, Alan; Ros, Eduardo; Romero-Canizales, Cristina; Roshanineshat, Arash; Rottmann, Helge; Roy, Alan L.; Ruiz, Ignacio; Ruszczyk, Chet; Rygl, Kazi L. J.; Sanchez, Salvador; Sanchez-Arguelles, David; Sanchez-Portal, Miguel; Sasada, Mahito; Satapathy, Kaushik; Savolainen, Tuomas; Schloerb, F. Peter; Schonfeld, Jonathan; Schuster, Karl-Friedrich; Shao, Lijing; Shen, Zhiqiang; Small, Des; Sohn, Bong Won; SooHoo, Jason; Souccar, Kamal; Sun, He; Tazaki, Fumie; Tetarenko, Alexandra J.; Tiede, Paul; Tilanus, Remo P. J.; Titus, Michael; Torne, Pablo; Traianou, Efthalia; Trent, Tyler; Trippe, Sascha; Turk, Matthew; van Bemmel, Ilse; van Langevelde, Huib Jan; van Rossum, Daniel R.; Vos, Jesse; Wagner, Jan; Ward-Thompson, Derek; Wardle, John; Weintroub, Jonathan; Wex, Norbert; Wharton, Robert; Wielgus, Maciek; Wiik, Kaj; Witzel, Gunther; Wondrak, Michael F.; Wong, George N.; Wu, Qingwen; Yamaguchi, Paul; Yoon, Doosoo; Young, Andre; Young, Ken; Younsi, Ziri; Yuan, Feng; Yuan, Ye-Fei; Zensus, J. Anton; Zhang, Shuo; Zhao, Guang-Yao; Zhao, Shan-Shan; Agurto, Claudio; Araneda, Juan Pablo; Arriagada, Oriel; Bertarini, Alessandra; Berthold, Ryan; Blanchard, Jay; Brown, Ken; Cardenas, Mauricio; Cantzler, Michael; Caro, Patricio; Chuter, Tim C.; Ciechanowicz, Miroslaw; Coulson, Iain M.; Crowley, Joseph; Degenaar, Nathalie; Dornbusch, Sven; Duran, Carlos A.; Forster, Karl; Geertsema, Gertie; Gonzalez, Edouard; Graham, Dave; Gueth, Frederic; Han, Chih-Chiang; Herrera, Cristian; Herrero-Illana, Ruben; Heyminck, Stefan; Hoge, James; Huang, Yau-De; Jiang, Homin; John, David; Klein, Thomas; Kubo, Derek; Kuroda, John; Kwon, Caleb; Laing, Robert; Liu, Ching-Tang; Liu, Kuan-Yu; Mac-Auliffe, Felipe; Martin-Cocher, Pierre; Matulonis, Callie; Messias, Hugo; Meyer-Zhao, Zheng; Montenegro-Montes, Francisco; Montgomerie, William; Muders, Dirk; Nishioka, Hiroaki; Norton, Timothy J.; Olivares, Rodrigo; Perez-Beaupuits, Juan Pablo; Parra, Rodrigo; Poirier, Michael; Pradel, Nicolas; Raffin, Philippe A.; Ramirez, Jorge; Reynolds, Mark; Saez-Madain, Alejandro F.; Santana, Jorge; Silva, Kevin M.; Sousa, Don; Stahm, William; Torstensson, Karl; Venegas, Paulina; Walther, Craig; Wieching, Gundolf; Wijnands, Rudy; Wouterloot, Jan G. A.; Event Horizon Telescope Collaboration., We present Event Horizon Telescope (EHT) 1.3 mm measurements of the radio source located at the position of the supermassive black hole Sagittarius A* (Sgr A*), collected during the 2017 April 5–11 campaign. The observations were carried out with eight facilities at six locations across the globe. Novel calibration methods are employed to account for Sgr A*'s flux variability. The majority of the 1.3 mm emission arises from horizon scales, where intrinsic structural source variability is detected on timescales of minutes to hours. The effects of interstellar scattering on the image and its variability are found to be subdominant to intrinsic source structure. The calibrated visibility amplitudes, particularly the locations of the visibility minima, are broadly consistent with a blurred ring with a diameter of ∼50 μas, as determined in later works in this series. Contemporaneous multiwavelength monitoring of Sgr A* was performed at 22, 43, and 86 GHz and at near-infrared and X-ray wavelengths. Several X-ray flares from Sgr A* are detected by Chandra, one at low significance jointly with Swift on 2017 April 7 and the other at higher significance jointly with NuSTAR on 2017 April 11. The brighter April 11 flare is not observed simultaneously by the EHT but is followed by a significant increase in millimeter flux variability immediately after the X-ray outburst, indicating a likely connection in the emission physics near the event horizon. We compare Sgr A*'s broadband flux during the EHT campaign to its historical spectral energy distribution and find that both the quiescent emission and flare emission are consistent with its long-term behavior. © 2022. The Author(s). Published by the American Astronomical Society., The Event Horizon Telescope Collaboration thanks the following organizations and programs: the Academia Sinica; the Academy of Finland (projects 274477, 284495, 312496, 315721); the Agencia Nacional de Investigación y Desarrollo (ANID), Chile via NCN19_058 (TITANs) and Fondecyt 1221421, the Alexander von Humboldt Stiftung; an Alfred P. Sloan Research Fellowship; Allegro, the European ALMA Regional Centre node in the Netherlands, the NL astronomy research network NOVA and the astronomy institutes of the University of Amsterdam, Leiden University and Radboud University; the ALMA North America Development Fund; the Black Hole Initiative, which is funded by grants from the John Templeton Foundation and the Gordon and Betty Moore Foundation (although the opinions expressed in this work are those of the author(s) and do not necessarily reflect the views of these Foundations); Chandra DD7-18089X and TM6-17006X; the China Scholarship Council; China Postdoctoral Science Foundation fellowship (2020M671266); Consejo Nacional de Ciencia y Tecnología (CONACYT, Mexico, projects U0004-246083, U0004-259839, F0003-272050, M0037-279006, F0003-281692, 104497, 275201, 263356); the Consejería de Economía, Conocimiento, Empresas y Universidad of the Junta de Andalucía (grant P18-FR-1769), the Consejo Superior de Investigaciones Científicas (grant 2019AEP112); the Delaney Family via the Delaney Family John A. Wheeler Chair at Perimeter Institute; Dirección General de Asuntos del Personal Académico-Universidad Nacional Autónoma de México (DGAPA-UNAM, projects IN112417 and IN112820); the Dutch Organization for Scientific Research (NWO) VICI award (grant 639.043.513) and grant OCENW.KLEIN.113; the Dutch National Supercomputers, Cartesius and Snellius (NWO Grant 2021.013); the EACOA Fellowship awarded by the East Asia Core Observatories Association, which consists of the Academia Sinica Institute of Astronomy and Astrophysics, the National Astronomical Observatory of Japan, Center for Astronomical Mega-Science, Chinese Academy of Sciences, and the Korea Astronomy and Space Science Institute; the European Research Council (ERC) Synergy Grant "BlackHoleCam: Imaging the Event Horizon of Black Holes" (grant 610058); the European Union Horizon 2020 research and innovation programme under grant agreements RadioNet (No 730562) and M2FINDERS (No 101018682); the Generalitat Valenciana postdoctoral grant APOSTD/2018/177 and GenT Program (project CIDEGENT/2018/021); MICINN Research Project PID2019-108995GB-C22; the European Research Council for advanced grant "JETSET: Launching, propagation and emission of relativistic jets from binary mergers and across mass scales" (Grant No. 884631); the Institute for Advanced Study; the Istituto Nazionale di Fisica Nucleare (INFN) sezione di Napoli, iniziative specifiche TEONGRAV; the International Max Planck Research School for Astronomy and Astrophysics at the Universities of Bonn and Cologne; DFG research grant "Jet physics on horizon scales and beyond" (Grant No. FR 4069/2-1); Joint Princeton/Flatiron and Joint Columbia/Flatiron Postdoctoral Fellowships, research at the Flatiron Institute is supported by the Simons Foundation; the Japan Ministry of Education, Culture, Sports, Science and Technology (MEXT; grant JPMXP1020200109); the Japanese Government (Monbukagakusho: MEXT) Scholarship; the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for JSPS Research Fellowship (JP17J08829); the Joint Institute for Computational Fundamental Science, Japan; the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (CAS, grants QYZDJ-SSW-SLH057, QYZDJSSW-SYS008, ZDBS-LY-SLH011); the Leverhulme Trust Early Career Research Fellowship; the Max-Planck-Gesellschaft (MPG); the Max Planck Partner Group of the MPG and the CAS; the MEXT/JSPS KAKENHI (grants 18KK0090, JP21H01137, JP18H03721, JP18K13594, 18K03709, JP19K14761, 18H01245, 25120007); the Malaysian Fundamental Research Grant Scheme (FRGS) FRGS/1/2019/STG02/UM/02/6; the MIT International Science and Technology Initiatives (MISTI) Funds; the Ministry of Science and Technology (MOST) of Taiwan (103-2119-M-001-010-MY2, 105-2112-M-001-025-MY3, 105-2119-M-001-042, 106-2112-M-001-011, 106-2119-M-001-013, 106-2119-M-001-027, 106-2923-M-001-005, 107-2119-M-001-017, 107-2119-M-001-020, 107-2119-M-001-041, 107-2119-M-110-005, 107-2923-M-001-009, 108-2112-M-001-048, 108-2112-M-001-051, 108-2923-M-001-002, 109-2112-M-001-025, 109-2124-M-001-005, 109-2923-M-001-001, 110-2112-M-003-007-MY2, 110-2112-M-001-033, 110-2124-M-001-007, and 110-2923-M-001-001); the Ministry of Education (MoE) of Taiwan Yushan Young Scholar Program; the Physics Division, National Center for Theoretical Sciences of Taiwan; the National Aeronautics and Space Administration (NASA, Fermi Guest Investigator grant 80NSSC20K1567, NASA Astrophysics Theory Program grant 80NSSC20K0527, NASA NuSTAR award 80NSSC20K0645); NASA Hubble Fellowship grant HST-HF2-51431.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555; the National Institute of Natural Sciences (NINS) of Japan; the National Key Research and Development Program of China (grant 2016YFA0400704, 2017YFA0402703, 2016YFA0400702); the National Science Foundation (NSF, grants AST-0096454, AST-0352953, AST-0521233, AST-0705062, AST-0905844, AST-0922984, AST-1126433, AST-1140030, DGE-1144085, AST-1207704, AST-1207730, AST-1207752, MRI-1228509, OPP-1248097, AST-1310896, AST-1440254, AST-1555365, AST-1614868, AST-1615796, AST-1715061, AST-1716327, AST-1716536, OISE-1743747, AST-1816420, AST-1935980, AST-2034306); NSF Astronomy and Astrophysics Postdoctoral Fellowship (AST-1903847); the Natural Science Foundation of China (grants 11650110427, 10625314, 11721303, 11725312, 11873028, 11933007, 11991052, 11991053, 12192220, 12192223); the Natural Sciences and Engineering Research Council of Canada (NSERC, including a Discovery Grant and the NSERC Alexander Graham Bell Canada Graduate Scholarships-Doctoral Program); the National Youth Thousand Talents Program of China; the National Research Foundation of Korea (the Global PhD Fellowship Grant: grants NRF-2015H1A2A1033752, the Korea Research Fellowship Program: NRF-2015H1D3A1066561, Brain Pool Program: 2019H1D3A1A01102564, Basic Research Support Grant 2019R1F1A1059721, 2021R1A6A3A01086420, 2022R1C1C1005255); Netherlands Research School for Astronomy (NOVA) Virtual Institute of Accretion (VIA) postdoctoral fellowships; Onsala Space Observatory (OSO) national infrastructure, for the provisioning of its facilities/observational support (OSO receives funding through the Swedish Research Council under grant 2017-00648); the Perimeter Institute for Theoretical Physics (research at Perimeter Institute is supported by the Government of Canada through the Department of Innovation, Science and Economic Development and by the Province of Ontario through the Ministry of Research, Innovation and Science); the Spanish Ministerio de Ciencia e Innovación (grants PGC2018-098915-B-C21, AYA2016-80889-P, PID2019-108995GB-C21, PID2020-117404GB-C21); the University of Pretoria for financial aid in the provision of the new Cluster Server nodes and SuperMicro (USA) for a SEEDING GRANT approved towards these nodes in 2020; the Shanghai Pilot Program for Basic Research, Chinese Academy of Science, Shanghai Branch (JCYJ-SHFY-2021-013); the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofísica de Andalucía (SEV-2017- 0709); the Spinoza Prize SPI 78-409; the South African Research Chairs Initiative, through the South African Radio Astronomy Observatory (SARAO, grant ID 77948), which is a facility of the National Research Foundation (NRF), an agency of the Department of Science and Innovation (DSI) of South Africa; the Toray Science Foundation; Swedish Research Council (VR); the US Department of Energy (USDOE) through the Los Alamos National Laboratory (operated by Triad National Security, LLC, for the National Nuclear Security Administration of the USDOE (Contract 89233218CNA000001); and the YCAA Prize Postdoctoral Fellowship. APEX is a collaboration between the Max-Planck-Institut für Radioastronomie (Germany), ESO, and the Onsala Space Observatory (Sweden). The SMA is a joint project between the SAO and ASIAA and is funded by the Smithsonian Institution and the Academia Sinica. The JCMT is operated by the East Asian Observatory on behalf of the NAOJ, ASIAA, and KASI, as well as the Ministry of Finance of China, Chinese Academy of Sciences, and the National Key Research and Development Program (No. 2017YFA0402700) of China and Natural Science Foundation of China grant 11873028. Additional funding support for the JCMT is provided by the Science and Technologies Facility Council (UK) and participating universities in the UK and Canada. The LMT is a project operated by the Instituto Nacional de Astrófisica, Óptica, y Electrónica (Mexico) and the University of Massachusetts at Amherst (USA). The IRAM 30-m telescope on Pico Veleta, Spain is operated by IRAM and supported by CNRS (Centre National de la Recherche Scientifique, France), MPG (Max-Planck-Gesellschaft, Germany) and IGN (Instituto Geográfico Nacional, Spain). The SMT is operated by the Arizona Radio Observatory, a part of the Steward Observatory of the University of Arizona, with financial support of operations from the State of Arizona and financial support for instrumentation development from the NSF. Support for SPT participation in the EHT is provided by the National Science Foundation through award OPP-1852617 to the University of Chicago. Partial support is also provided by the Kavli Institute of Cosmological Physics at the University of Chicago. The SPT hydrogen maser was provided on loan from the GLT, courtesy of ASIAA. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), supported by NSF grant ACI-1548562, and CyVerse, supported by NSF grants DBI-0735191, DBI-1265383, and DBI-1743442. XSEDE Stampede2 resource at TACC was allocated through TG-AST170024 and TG-AST080026N. XSEDE JetStream resource at PTI and TACC was allocated through AST170028. This research is part of the Frontera computing project at the Texas Advanced Computing Center through the Frontera Large-Scale Community Partnerships allocation AST20023. Frontera is made possible by National Science Foundation award OAC-1818253. This research was carried out using resources provided by the Open Science Grid, which is supported by the National Science Foundation and the U.S. Department of Energy Office of Science. Additional work used ABACUS2.0, which is part of the eScience center at Southern Denmark University. Simulations were also performed on the SuperMUC cluster at the LRZ in Garching, on the LOEWE cluster in CSC in Frankfurt, on the HazelHen cluster at the HLRS in Stuttgart, and on the Pi2.0 and Siyuan Mark-I at Shanghai Jiao Tong University. The computer resources of the Finnish IT Center for Science (CSC) and the Finnish Computing Competence Infrastructure (FCCI) project are acknowledged. This research was enabled in part by support provided by Compute Ontario (http://computeontario.ca), Calcul Quebec (http://www.calculquebec.ca) and Compute Canada (http://www.computecanada.ca).

X-ray observations have been made of a sample of 20 classical Cepheids, including two new observations (Polaris and {\it l} Car) reported here. The occurrence of X-ray flux around the pulsation cycle is discussed. Three Cepheids are detected ($\delta$ Cep, $\beta$ Dor, and Polaris). X-rays have also been detected from the low--mass F, G, and K companions of 4 Cepheids (V473 Lyr, R Cru, V659 Cen, and W Sgr), and one hot companion (S Mus). Upper limits on the X-ray flux of the remaining Cepheids provide an estimate that 28\% have low mass companions. This fraction of low--mass companions in intermediate mass Cepheids is significantly lower than expected from random pairing with the field IMF. Combining the companion fraction from X-rays with that from ultraviolet observations results in a binary/multiple fraction of 57\% $\pm$12\% for Cepheids with the ratios q $>$ 0.1 and separations a $>$ 1 au. This is a lower limit since M stars are not included. X-ray observations detect less massive companions than other existing studies of intermediate mass stars. Our measured occurrence rate of unresolved, low-mass companions to Cepheids suggests that intermediate-period binaries derive from a combination of disk and core fragmentation and accretion. This yields a hybrid mass-ratio distribution that is skewed toward small values compared to a uniform distribution but is still top-heavy compared to random pairings drawn from the IMF., Comment: Accepted by ApJ