bluuebunny/crossref_metadata_2025 huggingface dataset:
Flare Occurrence in the Solar Active Regions with Reversed Helicity Sign
Authors: Bao S. D. , Ai G. X. , Zhang H. Q.
Published: July 2016

Based on the Huairou Solar Observing Station dataset, we computed the current helicity for several hundreds of active regions and found that: (1) Active regions that do not follow the hemispheric helicity sign rule show more flare activity than normal active regions. (2) The relative number of active regions with reversed helicity sign is higher near sunspot maximum. (3) It appears that during solar cycle 22 the southern hemisphere has more the reversed-sign active regions and stronger flare activity than the northern hemisphere.
The Observation and Study of Two Prominence Events in the Infrared
Authors: Cao W. D. , Ye B. X. , Livingston W.
Published: July 2016

This paper reports the first extremely near infrared observation of two prominence events on Jan. 8, 1999 and Feb. 9, 1999 at H Pfund β (46525Å) and H Bracket α (40512Å) using the McMath Telescope. We scanned the spectrograph slit across the prominences parallel to the limb to obtain three-dimensional data cubes (two spatial dimensions and one spectral dimension). By fitting these observed lines, we can directly determine some important physical parameters such as Doppler width (ΔλD), the optical thickness at line center (τΔ0), and the line displacement (Δλ0). Combined with simultaneous observation of Balmer Hα, Hβ, and Ca K lines, the new results of structure and dynamics of limb prominence is presented. These results imply that these infrared lines can penetrate through the prominence and become a potential probe and tool, while Hα and Hβ sense the prominence skin only.
Spectroscopic Features in the EUV Emission of a M8 Flare Observed by SUMER
Authors: Curdt W. , Landi E. , Feldman U. , Innes D. , Dwivedi B. , Wilhelm K.
Published: July 2016

A flare of size M8 occurred while SUMER was recording a spectral scan above the active region NOAA 8537 at the west limb. We recorded spectra during the pre-flare phase, at flare onset, and during the decay phase of this main flare in a series of events. More than 60 flare lines were identified during this observation, which include Fe XVIII - Fe XXIII lines that provide evidence of 107 K plasmas. We also recorded lines from He-like ions, such as Ne IX, Na x, Mg XI, and Si XIII. Accurate wavelength measurements of such lines are of interest in basic atomic physics studies. Using plasma diagnostic techniques, we investigated the temporal evolution of the electron densities and temperatures during the event.
The Formation of G-band Bright Points I: Standard LTE Modelling
Authors: Kiselman D. , Rutten R. J. , Plez B.
Published: July 2016

Assuming LTE, we investigate the formation of the G band in the semiempirical flux-tube model of Briand & Solanki (1995). Preliminary results agree with observations of G-band bright-point contrast, even though the observed contrast depends sensitively on the amount of scattered light. It is the weakening of spectral lines in the flux tube that makes the bright-point contrast greater in the G band than in ordinary continuum. We also present LTE synthetic spectra for the full wavelength range from UV to IR. This serves to point out other promising pass bands for the observations of flux-tube structures.
A Survey of the Size-flux Relation for Pores and Spots
Authors: Li W. , Ai G. , Wang H.
Published: July 2016

Using the vector magnetic field data from the Huairou Solar Observing Station of Beijing Astronomical Observatory, we study their size-flux relations for various magnetic elements identified as pores and spots. More than 725 pores and spots which located near the central meridian are selected. The power-law relationships for the mature sunspots are derived, and, there are different power-law relationships for the different magnetic elements. We also find that there are not evident relations between size and flux of the pores and protosunspots. Such the results may be useful for understanding the physical conditions and dynamical processes of magnetic flux tubes in the solar plasma during the sunspot's formation. In addition, their characteristic size is also given.
Magnetic Configuration and Evolution in a Solar Active Region
Authors: Liu Y. , Zhang H.
Published: July 2016

We present results of the analysis of NOAA 8668, which was observed successively by space satellite (SOHO) and ground-based observatories (BBSO, Huairou). The combined observation offers us a good example of a region observed from low to high solar atmosphere. Several flares and a sigmoid filament were observed in the AR, and we observed the sigmoid filament from its birth to disintegration. The configuration of the magnetic field of the AR changed quickly as well as the loops. From EIT movies, we can even judge the sign of the sigmoid filament's magnetic helicity. The forming and heating of the loops were the result of magnetic reconnection, and the corona seemed heated when the loops became opened.
Velocities in Sunspot Plumes
Authors: Maltby P. , Kjeldseth-Moe O. , Brynildsen N.
Published: July 2016

We have extended our investigation of sunspot regions, with particular attention to the physics of the transition region between the chromosphere and the corona. In sunspot plumes, the most prominent features in the transition region intensity maps, we find downflows exceeding 25 km s−1. The observations show that the flow cannot be maintained by inflow from the corona and prompt a search for the origin of the flow and its possible relation to the origin of the sunspot plume. We find that the downflows in sunspot plumes are maintained by gas at transition region temperatures, streaming in flow channels from locations well outside the sunspot. Hence, the flow pattern in the transition region shows similarities to the inverse Evershed flow in the chromosphere. We present a working hypothesis that is based on the siphon flow mechanism.
The Relationship between CMEs and Prominence Eruptions
Authors: Schmieder B. , van Driel-Gesztelyi L. , Delannée C. , Simnett G. M. , Wiik J. E.
Published: July 2016

We have studied the temporal relationship between three selected prominence eruptions and their associated coronal mass ejections (CME). For these three events it is not possible to predict from the CME time history when the prominence would erupt. However, both are signatures of destabilisation of the global coronal magnetic field. The lack of a precise temporal relationship reflects the fact that several coronal stuctures are destabilised, and in general one will predominate in the CME structure. If this structure happens to be the same as that constraining the prominence, then the latter will erupt following the launch of the CME. If the prominence is constrained by a separate magnetic structure, then it may erupt before or after the main CME. Reconfiguration of the global coronal field probably drives a fast increase of the poloidal field in the prominence flux tube or introduces destabilising shear forces. Events on May 1, 1996, September 25-27, 1996, and May 31 1997 are discussed.
CDS UV Brightenings Explained by Quasi-separatrices and Bald Patches in an S-shape Active Region
Authors: Schmieder B. , Démoulin P. , Fletcher L. , López Fuentes M. C. , Mandrini C. H. , Mason H. E. , Young P. R. , Nitta N.
Published: July 2016

We present multi-instrument observations of AR 8048, made between June 3 and June 5 1997 as part of SoHO JOP033. This active region (AR) has a sigmoid-like global shape and undergoes transient brightenings through which the stored energy is released.

Using a magneto-hydrostatic model, we compute coronal magnetic field. The large-scale magnetic lines confirm the sigmoidal characteristics of the AR. The field lines most closely matching the hotter SoHO/CDS loops extend along the quasi-separatrix-Iayers (QSLs) of the coronal field. Transition region (TR) brightenings observed with SoHO/CDS can be associated with both QSL intersections with the photosphere, and places where separatrices corresponding to bald patches (BPs, sites where field lines are tangent to the photosphere) lie at the photospheric plane. There are suggestions that the element abundances measured in the TR may depend on the type of topological structure present. TR brightenings associated with QSLs have coronal abundances, while those associated with BP separatrices have abundances closer to photospheric values.
The Most Important Origin of Twist and Tilt of Magnetic Fields in Solar Active Regions
Authors: Tian L. , Zhang H. , Bao S.
Published: July 2016

In this paper, we show observed properties of tilt and twist of bipolar active regions magnetic fields, using magnetograph data at Huairou, Beijing Astronomical Observatory. We deduce that origin of the twist of magnetic fields is same as that of tilt of bipolar magnetic fields. In another word, they are primarily produced by Coriolis force acting on a rising flux tube through convection zone.
On the Initial Phase of Solar Radio Burst Event on 23 September 1998
Authors: Yan Y. , Fu Q. , Aurass H. , Lesovoi S. , Altyntsev A. , Liu Y. , Chen Z. , Ji H.
Published: July 2016

On 23 September 1998 there was a 3B/M7.1 flare event starting at 06:40 UT with more than 100 minute duration radio bursts of 1134 sfu peak flux at 2840 MHz and accompanying a CME. In this paper, the event is analyzed with the data observed by Beijing Astronomical Observatory (BAO), OSRA-Tremsdorf (AIP) and SSRT-ISTP. In particular, we compare the fine structures (FS) during the triggering phase revealed in different regimes and discuss their implications.
MHD Seismology of the Solar Corona with SOHO and TRACE
Authors: Nakariakov V. M.
Published: July 2016

Recent discoveries of MHD wave motions in the solar corona done with EUV imaging telescopes onboard SOHO and TRACE provide an observational basis for the MHD seismology of the corona. Measuring the properties of MHD waves and oscillations (periods, wavelengths, amplitudes, temporal and spatial signatures), combined with theoretical modeling of the wave phenomena, allow us to determine values of the mean parameters of the corona (the magnetic field strength, transport coefficients, etc.). As an example, we consider post-flare decaying oscillations of loops, observed with TRACE (14th July 1998 at 12:55 UT). An analysis of the oscillations shows that they are quasi-harmonic, with a period of about 265 s, and quickly decaying with the decay time of about 14.5 min. The period of oscillations allows us to determine the Alfvén speed in the oscillating loop about 770 km/s. This value can be used for deduction of the value of the magnetic field in the loop (giving 10-30 G). The decay time, in the assumption that the decay is caused by viscous (or resistive) dissipation, gives us the Reynolds number of 105.3-6.1 (or the Lundquist number of 105.0-5.8).
The Quiet-Sun Variability as Seen by CDS and SUMER
Authors: Brković A. , Solanki S. K. , Rüedi I.
Published: July 2016

We investigate brightness variations of the quiet Sun using movies obtained with CDS and SUMER time series on SOHO. We compare the variability and relative variability detected with the two instruments and find that SUMER reveals a factor 2-3 higher variability than CDS.
Magnetic Field Structure of the CME Source Region
Authors: Hanaoka Y.
Published: July 2016

The three-dimensional structure of the magnetic field in the source region of CMEs is the key to understand how the stored magnetic energy eventually causes an eruption. A CME accompanied by a filament eruption on 2000 February 26-27 is particularly a good event to study the three-dimensional magnetic field structure. This event was very well observed with the EIT and LASCO of SOHO and the SXT of Yohkoh, and shows the following clues of the magnetic field structure which caused the CME. (1) The filament had a helical structure before the eruption and it was kept throughout the eruption. (2) The coronal loop structure shows that this event was an eruption of a part of the quadrapolar magnetic field structure consisting of two active regions. In this proceeding, we present a brief overview of the event.
CME Properties and Fuzzy Classification
Authors: Tang Y. H. , Han Z. Z.
Published: July 2016

In this paper, the theory and method of fuzzy classification are applied to analyse CME properties, with the use of the data in preliminary report during 1979-1981, the theoretical computation for CME category is performed. According to the original data of average properties for 9 structural classes of CME, the standardized value is obtained by average value and standard deviation. The fuzzy similitude matrix and equivalent matrix are built by use of the correlation coefficient transformation and the method of similitude coefficient, then we can divide 9 different structures into different categories according to different threshold value. Halo was the most energetic, and Single Spike, Diffuse Fan CMEs were the least energetic.
Coronal Heating by MHD Waves: Results from the SECIS Instrument during the 1999 Eclipse
Authors: Phillips K. J. H. , Gallagher P. T. , Williams D. R. , Keenan F. P. , Rudawy P. , Rompolt B. , Berlicki A.
Published: July 2016

We report on results from the Solar Eclipse Coronal Imaging System (SECIS), an instrument using fast CCD cameras designed to search for short-period modulations in visible-light emission from the corona during an eclipse or with a coronagraph. The instrument was successfully used during the total eclipse of 1999 August 11 from a site in Bulgaria. This paper summarizes both the instrument and preliminary results.
Heating the Quiet Corona by Nanoflares: Evidence and Problems
Authors: Benz A. O. , Krucker S.
Published: July 2016

The content of coronal material in the quiet Sun is not constant as soft X-ray and high-temperature EUV line observations have shown. New material, probably heated and evaporated from the chromosphere is occasionally injected even in the faintest parts above the magnetic network cell interiors. Assuming that the smaller events follow the pattern of the well observed larger ones, we estimate the total energy input. Various recent analyses are compared and discussed. The results using similar EUV data from EIT/SOHO and TRACE basically agree on the power-law exponent when the same method is used. The most serious deviations are in the number of nanoflares per energy unit and time unit. It may be explained at least partially by different thresholds for flare detection.
Coronal Heating and the Solar Wind Acceleration
Authors: Hirayama T.
Published: July 2016

We propose a coronal heating theory based on the magnetic twisting, which inevitably produces charge imbalance. The resulting electric field creates supra-thermal electron beams. Beams are then thermalized by classical collisions. The dissipation rate is enough to heat the corona and to accelerate the solar wind.
Long term variations in the Extreme UV corona: the EIT/SoHO perspective
Authors: Hochedez J-F. , Clette F. , Verwichte E. , Berghmans D. , Cugnon P.
Published: July 2016

Since the start of the SoHO mission, EIT -the Extreme ultraviolet Imaging Telescope- offers a global view of the solar corona over the whole rising phase of the current activity cycle. Such a dataset is unprecedented. We give here the current results of an on-going investigation over the entire EIT archive. In the Fe XV images (2 MK), the on-disk and off-disk intensity distributions have been evaluated, and their evolution is described. Additionally, we developed an image processing technique that extracts the smallest detectable features. The cosmic ray hits are statistically disentangled from the solar point-like phenomena, and the trends in both rates are assessed.
Collaborative VLA and LASCO Observations of Nonthermal Energy Release in the Corona
Authors: Willson R. F.
Published: July 2016

We discuss recent collaborative observations of coronal mass ejections and related activity using the Very Large Array (VLA) and the Solar and Heliospheric Observatory (SOHO). VLA observations show the onset of intense 400 cm burst emission during an X1 GOES soft X-ray burst observed by the Yohkoh spacecraft and prior to a CME observed by the Large Angle Spectroscometric C2 Coronagraph (LASCO) on board SOHO. VLA snapshot maps show that the 400 cm burst site varied discontinuously throughout the course of the flare, possibly reflecting abrupt changes in the structure of the coronal magnetic fields along which the energetic particles traveled.
Highlight Results from Ulysses
Authors: Marsden R. G.
Published: July 2016

Launched in October 1990, the ESA-NASA Ulysses mission has conducted the very first survey of the heliosphere within 5 AU of the Sun over the full range of heliolatitudes. The first polar passes took place in 1994 and 1995, enabling Ulysses to characterise the global structure of the heliosphere at solar minimum, when the corona adopts its simplest configuration. The most important findings to date include a confirmation of the uniform nature of the high-speed (~ 750 km s−1) solar wind flow from the polar coronal holes, filling two-thirds of the volume of the inner heliosphere; the sharp boundary, existing from the chromosphere through the corona, between fast and slow solar wind streams; the latitude independence of the radial component of the heliospheric magnetic field; the lower-than-expected latitude gradient of galactic and anomalous cosmic rays; the continued existence of recurrent increases in the flux of low-energy ions and electrons up to the highest latitudes.
Collisionless Reconnection in the Structure and Dynamics of Active Regions
Authors: Somov B. V. , Kosugi T. , Litvinenko Y. E. , Sakao T. , Masuda S. , Bogachev S. A. , Oreshina A. V. , Merenkova E. Y.
Published: July 2016

On the basis of the Yohkoh data on the site and mechanism of magnetic energy transformation into thermal and kinetic energies of superhot plasmas and accelerated particles, a model is developed that explains observed properties of collisionless 3D reconnection in active regions. The model makes intelligible the observed relation between the S-like morphology and eruptive activity.
Propagation of High Energy Electrons in Solar Plasma
Authors: Mel'nik V.
Published: July 2016

It is shown that high energy electrons propagate in solar plasma as beam-plasma structure, a new nonlinear object.
Heliospheric Magnetic Field Configuration and its Coronal Sources
Authors: Zurbuchen T. H.
Published: July 2016

The heliospheric magnetic field configuration is largely determined in the solar atmosphere. The interplanetary magnetic field is therefore intimately linked with the coronal structure and evolution during the solar cycle. We summarize recent experimental results from active satellite experiments on Ulysses and the Advanced Composition Explorer (ACE). These results provide constraints on the sources of the solar wind and also the magnetic structure of the heliosphere and the corona. These results suggest the relevance of reconnection processes and differential rotation effects close to the Sun. This leads to large perturbations from a standard Archimedean spiral configuration which cannot be successfully modeled using coronal models which assume a potential magnetic field.
Components of the Local Interstellar Medium
Authors: Linsky J. L. , Redfield S. , Wood B.
Published: July 2016

HST, EUVE, and Ca II spectra are providing critical velocity and column density data needed to identify individual stuctures (clouds) of warm gas in the local ISM. The Sun is located very close to the edge of and will soon leave the Local Interstellar Cloud (LIC). We will summarize the properties of the LIC and other nearby warm clouds.
Should the Outer Milky Way be Axisymmetric?
Authors: Binney James
Published: July 2016

The shape of the outer Milky Way should be heavily influenced by the shape of our heavy halo. Simulations of the formation of collisionless halos indicate that they should be highly triaxial, with a tendency to pro-lateness. If our halo were as strongly triaxial as these simulations suggest, the outer disk would have ellipticity ϵdisk ≃ 0.06.

Simulations of galaxy formation which include an appropriately small fraction of gaseous matter give rise to halos of much smaller ellipticity, and predict ϵdisk ≃ 0.02. These simulations are not as reliable as purely collisionless simulations, and we should treat their results with some caution. However, it is not incredible that a small fraction of gas could radically reduce the ellipticity of a halo by making the system's potential more centrally concentrated.

The distribution of the apparent ellipticities of spiral galaxies indicates that their disks have slightly eliptical disks, ϵdisk ≃ 0.05. This may merely reflect ephemeral features such as spiral arms, however. The velocity fields of disks provide a more robust probe of the ellipticity ϵΦ of galaxy potentials. The narrowness of the Tully–Fisher relation places an interesting upper limit ϵΦ < 0.1. More detailed studies of individual velocity fields will be required if we are unambiguously to demonstrate ϵΦ > 0 in some cases.
Is there a Bulge Distinct from the Bar?
Authors: Kuijken Konrad
Published: July 2016

It seems by now well-established that in the central kpc the Galaxy is not axisymmetric (Gerhard 1995, Dwek 1995). The kinematics of many different tracers, as well as the distribution of sources on the sky, all point to an elongated distribution of matter and of the potential, with the long axis of the distortion pointing somewhere in the first Galactic quadrant. It is therefore tempting to reconsider our data concerning this part of the Galaxy in terms of ‘the’ Bar. But to what extent should we (or is it useful to) distinguish between the bar and the stellar bulge of old?
Evidence for the Galactic Bar from the Two Color Photometry of the Bulge Red Clump Stars
Authors: Stanek K. Z. , Mateo M. , Udalski A. , Szymański M. , Kałużny J. , Kubiak M. , Krzemiński W.
Published: July 2016

The Optical Gravitational Lensing Experiment (OGLE, Udalski et al. 1994a; Paczynski et al. 1994b – these proceedings; and references therein) is an extensive photometric search for the rare cases of gravitational microlensing of Galactic bulge stars by foreground objects. It provides a huge data base (Szymański & Udalski 1993), from which color-magnitude diagrams have been compiled (Udalski et al. 1993, 1994b). Here we discuss the use a of well-defined population of bulge red clump stars to investigate the presence of the bar in our Galaxy. The results of our earlier studies are described by Stanek et al. (1994).
M Giant Kinematics in Off-Axis Fields Between 150 and 300 PC from the Galactic Center
Authors: Blum R. D. , Carr J. S. , Sellgren K. , Terndrup D. M.
Published: July 2016

We present radial velocities for approximately 40 stars in each of four optically obscured, off-axis fields toward the Galactic bulge. The mean heliocentric radial velocity and velocity dispersion are −75 ± 24 km s–1 and 127 ± 16 km s–1 2 ± 23 km s–1 and 127 ± 14 km s–1, −14 ± 22 km s–1 and 126 ± 14 km s–1, and −31 ± 28 km s–1 and 153 ± 17 km s–1 for fields located at 299, 288, 171, and 160 pc projected radius, respectively. The dispersions generally match Kent's (1992) axisymmetric mass model but may be higher than the model's predictions at small projected radius.
Position-Velocity Diagrams as a Probe of the Bar in Edge-On Galaxies
Authors: Wada Keiichi , Hasegawa Tetsuo , Sofue Yoshiaki , Taniguchi Yoshiaki , Habe Asao
Published: July 2016

Analyses of the distribution of far infrared point sources in the Galactic bulge have suggested that from a face-on perspective the bulge has a bar like shape. Here, we investigate how a rotating bar-like bulge affects the global gas dynamics in a disk and compare the longitude-velocity (LV) maps from selfgravitating hydrodynamical simulations with observed maps of neutral hydrogen and carbon monoxide in the Galaxy. We found that the features on the numerical LV maps depend strongly on four factors: the pattern speed of the bar, the position angle of the Sun, the strength of the bar potential and the ratio of the gas mass to total dynamical mass. We conclude that our Galaxy has a rotating, weak, bar-like bulge (a/b ∼ 0.8) observed from nearly end on (θp < 20°). The allowed range of pattern speed of the bar is surprisingly narrow (∼ 20 km s–1 kpc–1) and is consistent with recent observations of bulge stars. Selfgravity of the interstellar matter is needed to account for some of the observations.
Is There a Compact Central Mass Concentration in Our Galaxy?
Authors: Townes Charles H.
Published: July 2016

Since the center of our own galaxy is the closest region of this type, it should provide a proving ground for our understanding of such regions. And fortunately, recent developments in radio, infrared, and x-ray astronomy allow us to look at the center of our galaxy in considerable detail – a privilege not possible in the visible region because of the rather dense dust clouds surrounding it. We find there many complex and interesting phenomena, including evidence for a compact central mass of a few million solar masses from the dynamics of stars and gases. However, its existence has been doubted because the radiant energy coming from the very center is much less than normal theoretical expectations for a black hole of such a mass. This discrepancy has led to speculation whether the velocity measurements could be misleading, whether the concentrated mass is in some form other than the expected black hole, whether the radiation might be taking place in an exotic form, being directed away from us or happening to be low at the moment, or whether our theories of such radiation are as yet incomplete. The general characteristics of the very central region of our galaxy will be reviewed along with the dynamic evidence for a large concentrated mass and comparison of the radiation it emits with that of various theoretical models.
Is Sgr A∗ Underfed, Underefficient, or Underdone?
Authors: Ozernoy Leonid M. , Genzel Reinhard
Published: July 2016

We argue that the wind from IRS 16 and He I stars in the central 1 pc of the Galaxy is responsible for the peculiar features of accretion onto a putative black hole at the Galactic center. What makes Sgr A∗ unique is not that it is just underfed but, in addition, it has a much lower efficiency of accretion and possibly a lower mass, compared to the AGN case.
What are the Radio Filaments Near the Galactic Center?
Authors: Morris Mark
Published: July 2016

A population of nonthermally-emitting radio filaments tens of parsecs in length has been observed within a projected distance of ∼130 pc of the Galactic center. More or less perpendicular to the Galactic plane, they appear to define the flux lines of a milligauss magnetic field. The characteristics of the known filaments are summarized. Three fundamental questions raised by these structures are discussed: 1) Do they represent magnetic flux tubes embedded within an ubiquitous, dipole magnetic field permeating the inner Galaxy, but which have been illuminated by some local source of relativistic particles, or are they instead isolated, self-sustaining current paths with an approximately force-free magnetic configuration in pressure equilibrium with the interstellar medium? 2) What is the source of either the magnetic field or the current? and 3) What is the source of the relativistic particles which provide the illuminating synchrotron radiation? We are nearer an answer to the the last of these questions than to the others, although several interesting models have been proposed.
The Nature of the Galactic Center Arc
Authors: Serabyn E.
Published: July 2016

Ever since the Galactic Center Arc was resolved into its component filaments a decade ago, it has been clear that its linear structure arises from the influence of a strong magnetic field. However, the origin and nature of the contributory phenomena have remained elusive. Since what is seen is synchrotron emission from relativistic particles, of prime interest is a knowledge of the acceleration mechanism involved. Interferometric imaging of the molecular gas in the vicinity of the Arc has now provided a tantalizing clue to the Arc's origin: molecular clumps coinciding with the endpoints of a number of the Arc's filaments point to these clumps as the source of the relativistic particles. This suggests that as dense molecular clumps course through the ambient magnetic field at the Galactic Center, magnetic energy is liberated in their leading layers via field reconnection, precipitating rapid acceleration of free charges to high energy.
Two New Views of the Galactic Center
Authors: Jaffe D.T. , Plume R. , Pak S.
Published: July 2016

The inner few hundred parsecs of the Galactic Center contains ∼10% of the molecular ISM in the Galaxy. The conditions in this gas are significantly different from those in molecular clouds elsewhere in the Galaxy. Typical temperatures, densities, and internal velocity dispersions are higher (Güesten 1989). There is also evidence for a large amount of molecular gas which is not bound to distinct clouds (Stark et al. 1989). High velocity bulk gas motions and velocity discontinuities open up the possibility of a role for powerful large-scale shocks in ISM excitation. The very different nature of the dense ISM in the inner Galaxy make it useful as a laboratory for physical effects in the interstellar medium and a proving ground for ideas about the interaction of gas and stars in the nuclei of other galaxies.
A Survey of OH in the Galactic Centre Region
Authors: Boyce P.J. , Cohen R. J.
Published: July 2016

The galactic centre contains the largest concentration of molecular clouds in the Galaxy. The clouds in the central region are unusual in having large linewidths and masses, and large non-circular motions. Previous surveys of their distribution in the central region have been carried out in OH (Robinson & McGee 1970; Cohen & Few 1976), H2CO (Whiteoak & Gardner 1979; Cohen & Few 1981), CO (Bania 1977; Dame et al. 1987; Bally et al. 1987, 1988) and CS (Bally et al. 1987, 1988). The OH groundstate lines at 18cm wavelength have certain advantages for such a survey. The OH lines appear in absorption against the galactic centre continuum sources, and against the continuum emission from the disk of the Galaxy. The absorption spectra are sensitive to relatively small molecular column densities. In addition they can give information on the relative positions of the molecular gas and the radio continuum sources. This paper describes results from an absorption line survey of the galactic centre region in the OH main lines at 1667.359 MHz and 1665.402 MHz (Boyce & Cohen 1994).
Observational Properties of SiO Masers in the Galactic Bulge IRAS Sources
Authors: Izumiura H.
Published: July 2016

We surveyed 313 color-selected IRAS sources in the Bulge in the SiO J=1−0 (v=1, 2) lines with the Nobeyama 45m telescope, which resulted in 222 detections (Nakada et al. 1992; Izumiura et al. 1994ab). The detection rates exceeded 80% for the brighter samples (Table 1). We discuss the observed SiO maser characteristics and the implications of the obtained 1-v diagram.
Report on a Survey of H2O Masers Towards the Galactic Center
Authors: Levine Deborah A. , Morris Mark
Published: July 2016

Galactic H2O masers are associated either with the circumstellar shells of late-type stars or with star-forming regions (SFRs). Previous surveys for H2O masers had revealed relatively few near the Galactic Center (GC). We report on the continuation of a survey using the VLA to make 22.2 GHz observations of IRAS point sources which lie within 2° of the GC and which have either 12 μm or 25 μm flux > 8 Jy. Taylor, Morris and Schulman (TMS, 1992, AJ, 106, 1978) previously observed 97 of 342 candidate sources. We have observed 160 more and plan to complete the survey in early 1995.
Is the Dynamical History at Odds with the Chemical History?
Authors: Matteucci F.
Published: July 2016

The paper of Eggen, Lynden-Bell and Sandage (1962)(hereafter ELS) titled “Evidence from the motion of old stars that the Galaxy collapsed” was the first attempt to understand the formation and evolution of our Galaxy. From a study of a kinematically selected sample of high velocity stars, ELS had found a remarkable correlation between chemical abundance and orbital eccentricity, in the sense that stars with the largest ultraviolet excess (a measure of stellar metallicity, in particular Fe), i.e. the lowest metallicity, are invariably moving in highly elliptical orbits. As the average < [Fe/H] > (in the usual notation [Fe/H] = log(Fe/H)∗ – log(Fe/H)⊙) is expected to increase with time, as a consequence of the progressive chemical enrichment of the gas, stars with the lowest [Fe/H] are, on average, the oldest. ELS also found a correlation between abundance and motion of stars perpendicular to the Galactic plane. This correlation suggests a continuous decrease of the perpendicular velocity with decreasing [Fe/H]. To explain these relations ELS proposed that the Galaxy collapsed from a protocloud to a thin disk on a timescale of a few times 108 years, with progressive chemical enrichment as the collapse proceeded. This model was subsequently criticized mainly because of selection effects in their data, i.e. given the data available to ELS one would not expect the sample to contain low abundance, low orbital eccentricity objects even if they existed. They would be absent from the high velocity catalog they used. In addition, the ELS simple model did not account for the fact that almost half of the halo stars have retrograde orbits. This fact led Larson (1969) to consider models of clumpy and turbulent protogalaxies with collapse times that sometimes exceeded 1 Gyr.
Globular Clusters and the Formation of the Galactic Halo
Authors: van den Bergh Sidney
Published: July 2016

The radii of globular clusters are used to study the early evolution of the Galactic halo. It is found that such cluster radii increase with Galactocentric distance and depend on orbit shape. It is suggested that metal enrichment in the Galactic halo might have been mainly a local phenomenon, whereas cluster radii may have been determined by more global properties of the protogalaxy.
Did the Bulge Form All at Once?
Authors: Rich R.M.
Published: July 2016

It is reasonable to say that if Jan Oort were alive today, he would no doubt find recent developments in the study of the Galactic bulge to be fascinating. Oort considered the Galactic bulge in two contexts. First, he was interested in the use of the RR Lyrae stars as probes to determine the distance to the Galactic Center. No doubt, Oort would have been excited about the growing evidence of the bulge's triaxiality, as well as by the debate over the age of the bulge. His second interest was in the nature of activity at the center, an issue that I will not discuss in this review. The latter also remains an unsolved problem of the Milky Way, and (based on his work) one that might have been nearer to his heart than this one. Yet the question of when the bulge formed is ultimately a question about the formation history of the Galaxy. The oldest stars (those whose ages we are certain of) are found in Galactic globular clusters, the sum total of which are ≈ 5 × 107M⊙. The field population of the bulge is ≈ 2–3 × 1010M⊙, an order of magnitude more massive than the field population of the metal poor spheroid. So if the bulge formed all at once, and early, then the Milky Way had a luminous, even cataclysmic youth. But if the bulge formed later in the history of our galaxy, as a starburst or dynamical instability of the central disk, then the young Milky Way may have been inconspicuous and primeval galaxies may be hard to find indeed. If our bulge formed very early, its stellar population might have much in common with the giant ellipticals, while a late bulge might teach us much about processes that affect galaxy evolution.
Metallicity Structures of the Milky Way
Authors: Buser Roland , Rong Jianxiang
Published: July 2016

The metallicity-sensitive (U – G) colors from the new homogeneous catalog of photographic RGU data in seven high-latitude fields have been used to determine the larger-scale metallicity distributions of the Galactic population components. For the thick disk, preliminary analysis based on our best structural models provides a mean metallicity 〈[M/H]〉 = −0.6 ± 0.3dex and a marginal vertical metallicity gradient ≈ −0.1dex/kpc. The observed color distributions are further consistent with the (old) thin disk having mean abundance 〈[M/H]〉 = −0.3 ± 0.2dex and abundance gradient ∂[M/H]/∂z = −0.6dex/kpc.
Different Laws of Increasing of Stellar Dispersion: Observational Flaws or Natural Property of Stellar Population?
Authors: Fridman A.M. , Khoruzhii O.V. , Piskunov A.E.
Published: July 2016

Observations show that in the solar neighborhood the velocity dispersions of disk stars increase with their age. In this work we present the results of a critical analysis of the existing interpretations of the data, as well as of previous theoretical explanations of the heating phenomenon. It is shown that different relaxation mechanisms based on star-cloud collisions can result in a wide set of age–velocity dispersion relations (AVDR). Thus the observed differing power laws of the heating of the stellar component can be a consequence of the different relaxation mechanisms.
UBVI CCD Photometry of an old Open Cluster AM-2
Authors: Lee Myung Gyoon
Published: July 2016

AM-2 is a sparse cluster located at low galactic latitude. It has been suspected to be a globular cluster. We present a study of AM-2 based on the deep UBVI CCD photometry obtained using the Las Campanas duPont 2.5m telescope. The color-magnitude diagrams of AM-2 show (a) a main-sequence extending up to V ≈ 19 mag at (B – V) ≈ 1.1 mag, (b) a small number of red giant clump giants, (c) the brightest red giant at V ≈ 16.1 mag and (B – V) ≈ 1.9 mag, and (d) a small group of mysterious blue stars at V ≈ 16.6 mag and (B – V) ≈ 0.9 mag. We have estimated the reddening using the color-color diagram, E(B – V) = 0.56 ± 0.04. The metallicity of the main-sequence stars has been estimated from the ultraviolet excess, δ(U – B)0.6 = 0.09 ± 0.04, to be [Fe/H] = −0.4 ± 0.2 dex. The distance to the cluster has been measured using the Zero-Age-Main-Sequence fitting method, (m – M)0 = 14.8 ± 0.3 (d = 9.1 ± 1.4 kpc). Finally we have estimated the age of the cluster using the Revised Yale isochrones and the Morphological Age Ratio (MAR) method, obtaining a value of 5 ± 1 Gyrs (Fig. 1). This shows that AM-2 is not a globular cluster, but an old open cluster.
What do Gamma Rays tell us about the ISM?: New Insight from the Compton Observatory
Authors: Bloemen Hans
Published: July 2016

Gamma-ray astronomy has become a rich field of research and matured significantly since the launch of NASA's Compton Gamma Ray Observatory in April 1991. Studies of the diffuse γ-ray emission of the Galaxy can now be performed in far more detail and extended into the MeV regime, including both continuum and line emission. These studies provide unique insight into various aspects of the interstellar medium, in particular of the cosmic-ray component. This paper gives a brief review on the diffuse Galactic γ-ray emission and summarizes early results and prospects from the Compton Observatory.
Why is the HI Scale Height Constant with Galactocentric Radius?
Authors: Ferrara A.
Published: July 2016

The vertical distribution of neutral hydrogen has been studied over the last three decades (Schmidt 1957; Baker & Burton 1979; Lockman 1984; Savage & Massa 1987). The best fit curve to the HI z-distribution in the solar neighborhood has a FWHM of about 230 pc, and it extends appreciably beyond 500 pc (Dickey & Lockman 1990).
Dynamics of Thick Spirals
Authors: Patsis P.A. , Grosbøl P.
Published: July 2016

Most studies of spiral structure in galaxies use 2D models, since the motion of the stars is supposed to be confined very close to the plane of symmetry. However, galactic disks have a finite thickness and the vertical variation of spiral perturbations should be considered. The effect of 3D orbits in the dynamics of normal spiral galaxies is examined in this paper. We present the basic orbital behavior of the families of periodic orbits, which support a thick spiral pattern.
The Influence of Velocity Dispersion on Dynamical Friction in Stellar Discs
Authors: Wahde Mattias , Johan Donner Karl , Sundelius Björn
Published: July 2016

We have investigated the force of dynamical friction on a softened point mass orbiting inside a disc galaxy on a planar circular orbit both analytically and by means of numerical simulations. Including a velocity dispersion large enough to stabilise the disc markedly changes the physical picture of the mechanisms producing friction, whereas the numerical value of the friction is almost unaffected. Self-gravity of the disc enhances the friction only by a moderate amount.
On the Spiral-Induced Nonlinear Dissipative Process to the Evolution of the Galactic Disk
Authors: Zhang Xiaolei
Published: July 2016

It is well known that in dynamical systems governed by long-range forces, the secular evolution of the system is driven predominantly by collective effects. In the current paper, we report the discovery of such a collective dissipation process induced by a self-sustained spiral density wave, which leads to important secular evolution effects in spiral galaxies.
Stability of a Collisional Galactic Disk
Authors: Griv E.
Published: July 2016

Until recently, only collisionless models have been investigated in the kinetic treatment of stellar disk stability (e.g., Fridman and Polyachenko [1984]). This is due to the fact that the frequency of ordinary binary stellar gravitational (elastic) encounters in the Galaxy is much smaller than the variation of the gravitational field for the process being studied. However, in the pioneering paper Spitzer and Schwarzschild (1951) proposed a different kind of encounter: interaction of stars with gas clouds of the interstellar medium having a mass of rougly 106M⊙. In recent years this hypothesis was partially confirmed by observations: it was discovered in the Galaxy a few thousand giant molecular clouds of mass Mc ≥ 105M⊙. Other evidence of dynamical relaxation of the star–cloud disk in the solar neighborhood was found by Grivnev and Fridman (1990); the time of relaxation was estimated equal to τ = (2 – 4) × 109 years. Hence the study of collisional star–cloud system is not only of academic interest – on the time span t ≥ 109 years an actual galaxy may be a collisional ensemble of stars and clouds.
Local Variations of the Velocity Ellipsoid
Authors: Moreno M. , Torra J. , Oblak E.
Published: July 2016

We have analyzed the distribution of residual velocities of nearby stars (within 200 pc of the Sun) looking for space variations on the velocity ellipsoid. We used a sample of 1071 main sequence stars of spectral types B, A and F selected from the Hipparcos Input Catalogue [7] with uvbyHβ photometric data. Ages have been calculated following [1]. Six subsamples with 8.07 ≤ log(age) ≤ 9.45 have been considered.
Hierarchical and Non-Hierarchical Triples
Authors: Anosova J. , Kiseleva L. , Eggleton P. , Orlov V.
Published: July 2016

Triple stars and galaxies are referred to as having hierarchical or non-hierarchical structure. Thus, the dynamically stable and unstable triple systems are usually distinguished observationally by the ratio of the larger and smaller angular separation. However, (i) projection effects may hide the true configuration; and (ii) apparent hierarchical forms may occur in both types (Anosova 1986).
Criteria of Dynamical Isolation of Binaries and Multiples
Authors: Kiseleva Ludmila , Anosova Joanna
Published: July 2016

In order to obtain an objective criterion for dynamical isolation of binaries within systems of large multiplicity we study numerically the dynamical behavior and average parameters of stable hierarchical triple systems containing a close binary. Using the new criterion for hierarchical stability of triple systems with different mass ratios of components (Kiseleva, Eggleton, Anosova 1994; Kiseleva, Eggleton, Orlov 1994) the perturbing force from the outer body on the close inner binary is estimated. On this basis, the critical separations are obtained when both inner and outer orbits are practically not perturbed. Because the dispersion of masses has a very strong influence on dynamical processes in N-body systems, mass ratios of subsystems, and sometimes within subsystems, should always be taken into account.
The Unsolved Problem of Stellar Origins
Authors: Lada Charles J.
Published: July 2016

We now stand at the threshold of the 21st century having witnessed perhaps the greatest era of astronomical discovery in the history of mankind. During the twentieth century the subject of astronomy was revolutionized and completely transformed by technology and physics. Advances in technology that produced radio astronomy, infrared astronomy, UV, X and γ ray astronomy, large telescopes on the ground, in balloons, aircraft and space coupled with advances in nuclear, atomic and high energy physics forever changed the way in which the universe is viewed. Indeed, it is altogether likely that future historians of science will consider the twentieth century as the Golden Age of observational astronomy. As a measure of how far we have come in the last 100 years, recall that at the turn of this century the nature of spiral nebulae and of the Milky Way itself as an island universe were not yet revealed. The expansion of the universe and the microwave background were not yet discovered and exotic objects such as quasars, pulsars, gamma-ray bursters and black holes were not even envisioned by the most imaginative authors of science fiction. The interstellar medium, with its giant molecular clouds, magnetic fields and obscuring dust was unknown. Not even the nature of stars, these most fundamental objects of the astronomical universe, was understood.
Far-Outer Galaxy Molecular Clouds
Authors: Brand J. , Wouterloot J.G.A.
Published: July 2016

The outer Galaxy (defined as those parts of the Galaxy with galactocentric distances R>R0=8.5 kpc) in general and particularly the outermost regions (R ≳ 16 kpc; the far-outer Galaxy or FOG) has not received the attention that has been dedicated to the inner Galaxy and the solar neighbourhood.
The Structure of the Neutral Interstellar Medium: A Theory of Interstellar Turbulence
Authors: Whitworth Anthony
Published: July 2016

We present a model for the development of density structure in the neutral interstellar medium. In this model, bulk kinetic energy is injected mainly at small scales, by jets and expanding nebulae generated by young and/or massive stars. Subsequently, this bulk kinetic energy propagates to, and is dissipated on, larger scales.

This is in contrast to standard (incompressible) Kolmogorov turbulence, where kinetic energy is injected at larger scales, then propagates to, and is dissipated on, smaller scales. The sense in which the Second Law of Thermodynamics drives the propagation of turbulent energy is reversed in the interstellar medium because virialized self-gravitating gas clumps and ensembles of clumps (clouds) have negative effective specific heat.

The model is able to explain Larson's relations (between the mass, size, and velocity-dispersion of molecular clouds and clumps; Larson 1981), the maximum masses of giant molecular cloud complexes, the velocity dispersions observed (at low resolution) in face-on spirals like the Milky Way, and the apparent scaling of the interstellar magnetic field with density.