Hα Sources Research Articles

A Nascent Tidal Dwarf Galaxy Forming within the Northern H i Streamer of M82

Abstract We identify a ∼600 pc wide region of active star formation located within a tidal streamer of M82 via Hα emission (F Hα ∼ 6.5 × 10−14 erg s−1 cm−2), using a pathfinder instrument based on the Dragonfly Telephoto Array. The object is kinematically decoupled from the disk of M82 as confirmed via Keck/LRIS spectroscopy and is spatially and kinematically coincident with an overdensity of H i and molecular hydrogen within the “northern H i streamer” induced by the passage of M81 several hundred Myr ago. From H i data, we estimate that ∼5 × 107 M ⊙ of gas is present in the specific overdensity coincident with the Hα source. The object’s derived metallicity (12+ log ( O / H ) ≃ 8.6 ), position within a gas-rich tidal feature, and morphology (600 pc diameter with multiple star-forming clumps), indicate that it is likely a tidal dwarf galaxy in the earliest stages of formation.

New luminous blue variable candidates in the NGC 247 galaxy

ABSTRACT We search for luminous blue variable (LBV) stars in galaxies outside the Local Group. Here we present a study of two bright Hα sources in the NGC 247 galaxy. Object j004703.27–204708.4 (MV = −9.08 ± 0.15 mag) shows the spectral lines typical for well-studied LBV stars: broad and bright emission lines of hydrogen and helium He i with P Cyg profiles, emission lines of iron Fe ii, silicon Si ii, nitrogen N ii and carbon C ii, forbidden iron [Fe ii] and nitrogen [N ii] lines. The variability of the object is ΔB = 0.74 ± 0.09 mag and ΔV = 0.88 ± 0.09 mag, which makes it a reliable LBV candidate. The star j004702.18–204739.93 (MV = −9.66 ± 0.23 mag) shows many emission lines of iron Fe ii, forbidden iron lines [Fe ii], bright hydrogen lines with broad wings, and also forbidden lines of oxygen [O i] and calcium [Ca ii] formed in the circumstellar matter. The study of the light curve of this star did not reveal significant variations in brightness (ΔV = 0.29 ± 0.09 mag). We obtained estimates of interstellar absorption, the photosphere temperature, as well as bolometric magnitudes $M_\text{bol}=-10.5^{+0.5}_{-0.4}$ and $M_\text{bol}=-10.8^{+0.5}_{-0.6}$, which correspond to bolometric luminosities $\log (L_\text{bol}/{\rm L}_{\odot })=6.11^{+0.20}_{-0.16}$ and $6.24^{+0.20}_{-0.25}$ for j004703.27–204708.4 and j004702.18–204739.93, respectively. Thus, the object j004703.27–204708.4 remains a reliable LBV candidate, while the object j004702.18–204739.93 can be classified as a B[e]-supergiant.

Point and Compact Hα Sources in the Interior of M33

Abstract A variety of interesting objects such as Wolf–Rayet stars, tight OB associations, planetary nebulae, X-ray binaries, etc., can be discovered as point or compact sources in Hα surveys. How these objects distribute through a galaxy sheds light on the galaxy star formation rate and history, mass distribution, and dynamics. The nearby galaxy M33 is an excellent place to study the distribution of Hα-bright point sources in a flocculant spiral galaxy. We have reprocessed an archived WIYN continuum-subtracted Hα image of the inner 6.′5 × 6.′5 of M33 and, employing both eye and machine searches, have tabulated sources with a flux greater than approximately 10−15 erg cm−2s−1. We have effectively recovered previously mapped H ii regions and have identified 152 unresolved point sources and 122 marginally resolved compact sources, of which 39 have not been previously identified in any archive. An additional 99 Hα sources were found to have sufficient archival flux values to generate a Spectral Energy Distribution. Using the SED, flux values, Hα flux value, and compactness, we classified 67 of these sources.

Study of an Extended EUV Filament Using SoHO/SUMER Observations of the Hydrogen Lyman Lines

A filament and its channel close to the solar disk were observed in the complete hydrogen Lyman spectrum, and in several EUV lines by the SUMER (Solar Ultraviolet Measurement of Emitted Radiation) and CDS (Coronal Diagnostic Spectrometer) spectrographs on the SoHO satellite, and in Hα by ground-based telescopes during a multi-instrument campaign in May 2005. It was a good opportunity to get an overview of the volume and the density of the cold plasma in the filament channel; these are essential parameters for coronal mass ejections. We found that the width of the filament depends on the wavelength in which the filament is observed (around 15 arcsec in Hα, 30 arcsec in Lα, and 60 arcsec in EUV). In Lα the filament is wider than in Hα because cool plasma, not visible in Hα, is optically thick at the Lα line center, and its presence blocks the coronal emission. We have derived physical plasma properties of this filament fitting the Lyman spectra and Hα profiles by using a 1D isobaric NLTE model. The vertical temperature profile of the filament slab is flat (T≈7000 K) with an increase to ≈ 20 000 K at the top and the bottom of the slab. From an analysis of the Lα and Hα source functions we have concluded that these lines are formed over the whole filament slab. We have estimated the geometrical filling factor in the filament channel. Its low value indicates the presence of multi-threads.

A WARM MOLECULAR HYDROGEN TAIL DUE TO RAM-PRESSURE STRIPPING OF A CLUSTER GALAXY

We have discovered a remarkable warm (130–160 K) molecular hydrogen tail with a H2 mass of approximately 4 × 107 M☉ extending 20 kpc from a cluster spiral galaxy, ESO 137-001, in Abell 3627. At least half of this gas is lost permanently to the intracluster medium, as the tail extends beyond the tidal radius of the galaxy. We also detect a hot (400–550 K) component in the tail that is approximately 1% of the mass. The large H2 line to IR continuum luminosity ratio in the tail indicates that star formation is not a major excitation source and that the gas is possibly shock-heated. This discovery confirms that the galaxy is currently undergoing ram-pressure stripping, as also indicated by its previously discovered X-ray and Hα tails. We estimate that the galaxy is losing its warm H2 gas at a rate of ∼2–3 M☉ yr−1. The true mass-loss rate is likely higher if we account for cold molecular gas and atomic gas. We predict that the galaxy will lose most of its gas in a single pass through the core and place a strong upper limit on the ram-pressure timescale of 1 Gyr. We also study the star-forming properties of the galaxy and its tail. We identify most of the previously discovered external Hα sources within the tail in our 8 μm data but not in our 3.6 μm data; IRS spectroscopy of the region containing these Hα sources also reveals aromatic features typically associated with star formation. From the positions of these H ii regions, it appears that star formation is not occurring throughout the molecular hydrogen tail but only immediately downstream of the galaxy. Some of these H ii regions lie outside the tidal radius of the galaxy, indicating that ram-pressure stripping can be a source of intracluster stars.

On the Extended Knotted Disks of Galaxies

The stellar disks of many spiral galaxies are twice as large as generally thought. We use archival data from the Galaxy Evolution Explorer mission to quantify the statistical properties of young stellar clusters in the outer, extended disks of a sample of 11 nearby galaxies. We find an excess of sources between 1.25 and 2 optical radii, R25, for five of the galaxies, which statistically implies that at least a quarter of such galaxies have this cluster population (90% confidence level), and no significant statistical excess in the sample as a whole beyond 2R25, even though one galaxy (M83) individually shows such an excess. Although the excess is typically most pronounced for blue (FUV - NUV < 1, NUV < 25) sources, there is also an excess of sources with redder colors. Although from galaxy to galaxy the number of sources varies significantly, on average the galaxies with such sources have 75 ± 10 blue sources at radii between 1.25R25 and 2R25. In addition, the radial distribution is consistent with the extended dust emission observed in the far-IR and with the properties of Hα sources, assuming a constant cluster formation rate over the last few hundred megayears. All of these results suggest that the phenomenon of low-level star formation well outside the apparent optical edges of disks (R ~ R25) is common and long lasting.

A Study of Compact Radio Sources in Nearby Face-on Spiral Galaxies. II. Multiwavelength Analyses of Sources in M51

We report the analysis of deep radio observations of the interacting galaxy system M51 from the Very Large Array, with the goal of understanding the nature of the population of compact radio sources in nearby spiral galaxies. We detect 107 compact radio sources, 64% of which have optical counterparts in a deep Hα Hubble Space Telescope image. Thirteen of the radio sources have X-ray counterparts from a Chandra observation of M51. We find that six of the associated Hα sources are young supernova remnants (SNRs) with resolved shells. Most of the SNRs exhibit steep radio continuum spectral indices consistent with synchrotron emission. We detect emission from the Type Ic SN 1994I nearly a decade after explosion: the emission (160 ± 22 μJy beam-1 at 20 cm, 46 ± 11 μJy beam-1 at 6 cm, α = -1.02 ± 0.28) is consistent with light-curve models for Type Ib/Ic supernovae. We detect X-ray emission from the supernova; however, no optical counterpart is present. We report on the analysis of the Seyfert 2 nucleus in this galaxy, including the evidence for bipolar outflows from the central black hole.

Energetics and Dynamics of an Impulsive Flare on March 10, 2001

We present Hα observations from ARIES (Nainital) of a compact and impulsive solar flare that occurred on March 10, 2001 and which was associated with a CME. We have also analyzed HXT, SXT/Yohkoh observations as well as radio observations from the Nobeyama Radio Observatory to derive the energetics and dynamics of this impulsive flare. We coalign the Hα, SXR, HXR, MW, and magnetogram images within the instrumental spatial-resolution limit. We detect a single HXR source in this flare, which is found spatially associated with one of the Hα bright kernels. The unusual feature of HXR and Hα sources, observed for the first time, is the rotation during the impulsive phase in a clockwise direction. We propose that the rotation may be due to asymmetric progress of the magnetic reconnection site or may be due to the change of the peak point of the electric field. In MW emission we found two sources. The main source is at the main flare site and another is in the southwest direction. It appears that the remote source is formed by the impact of accelerated energetic electrons from the main flare site. From the spatial correlation of multiwavelength images of the different sources, we conclude that this flare has a three-legged structure.

Spitzer Observations of the Supergiant Shell Region in IC 2574

We present spatially resolved Spitzer Space Telescope imaging of the supergiant shell region of the M81 group dwarf galaxy IC 2574 obtained as part of the Spitzer Infrared Nearby Galaxies Survey. This region harbors one of the best nearby examples of a kinematically distinct H I shell, with an associated remnant stellar cluster; the shell is initiating sequential star formation as it interacts with the surrounding interstellar medium. This region dominates the infrared luminosity of IC 2574 and is spatially resolved in all Spitzer imaging bands. We study the differences in dust temperature as a function of local environment and compare local star formation rates as inferred from Hα and total infrared luminosities. We find that the strong Hα sources are associated with regions of warm dust; however, the most luminous infrared and Hα sources are not necessarily cospatial. The coolest dust is found in the regions farthest from the rim of the shell; these regions show the best agreement between star formation rates derived from Hα and from total infrared luminosities (although discrepancies at the factor of 3-4 level still exist). There is considerable variation in the radio-far-infrared correlation in different regions surrounding the shell. The low dust content of the region may influence the scatter seen in these relations; these data demonstrate that the expanding shell is dramatically affecting its surroundings by triggering star formation and altering the dust temperature.

Relativistic MHD simulations of pulsar bow-shock nebulae

Pulsar bow-shock nebulae are a class of pulsar wind nebulae (PWNe) that form when the pulsar wind is confined by the ram pressure of the ambient medium, and are usually associated with old pulsars, that have already emerged from the progenitor Supernova Remnant (SNR). Until a few years ago these nebulae were mainly observed as Hα sources; recently, also non-thermal emission has been detected. This is the signature of accelerated particles gyrating in a magnetic field. In the same way as Hα radiation is a tool for studying the layer of shocked Interstellar Medium (ISM), the non-thermal radiation might be used to infer the properties of the shocked pulsar wind. However theoretical and numerical models have been presented so far only in the hydrodynamical (HD) regime, while in order to properly model the internal flow structure and the emission properties of these nebulae a magnetohydrodynamical (MHD) treatment is required. We present here relativistic MHD (RMHD) axisymmetric simulations of pulsar wind bow-shock nebulae. The structure and fluid dynamics of such objects is investigated for various values of the pulsar wind magnetization. Simulated synchrotron maps are computed and comparison of the emission pattern with observations is discussed.

Discovery of a Huge Young Stellar Object Interaction Region in Camelopardalis

During the course of a wide-field VI survey of galaxies in the IC 342/Maffei Group, a large nebula, which looks like an inclined disk with a jetlike plume emerging from it, was discovered in Camelopardalis. The object is most prominent in I. The predominating disk component is 68 across, which corresponds to 4.0 ± 1.6 pc at the estimated distance of 2.0 ± 0.8 kpc (the Perseus Arm). The plume extends 38 (2.2 pc) outward from the core along a direction that is about 20° from the minor axis of the disk. The disk lies along the edge of a filament of dust and molecular gas in the Milky Way. The plume points toward the core of the filament. No large-scale emission is seen at Hα, and the nebula is invisible in Two Micron All Sky Survey (2MASS) images. About 30'' from the center of the disk is IRAS 04261+6339, which is a pair of unresolved Hα sources whose IRAS colors and spectra reveal them to be young stellar objects (YSOs). The northern of the two exhibits a near-infrared tail, which is 15'' (0.15 pc) long in H and directed 66° away from the plume. Although the stars are exposed, as in Class II YSOs, the spectral energy distribution of the pair rises beyond 2 μm, typical of Class I systems. It appears that they are transitional YSOs, with characteristics similar to those of Holoea (IRAS 05327+3404). The total brightness of the plume plus disk exceeds that of the stars by 1.6 mag in I, yet the V-I color is bluer by only 0.50 mag. Thus, the nebula cannot be a consequence of reflection, even allowing for differential extinction. It is tentatively identified as a remnant of an outflow from a binary YSO, glowing from the photoluminescence of silicon nanoparticles.

Star Formation Across the Taffy Bridge: UGC 12914/15

We present Berkeley-Illinois-Maryland Association (BIMA) two-field mosaic CO (1–0) images of the Taffy galaxies (UGC 12914/15), which show the distinct taffy-like radio continuum emission bridging the two spiral disks. Large amounts of molecular gas (1.4 × 1010 M⊙, using the standard Galactic CO-to-H2 conversion applicable to Galactic disk giant molecular clouds) were clearly detected throughout the taffy bridge between the two galaxies, which, as in the more extreme case of H I, presumably results from a head-on collision between the two galaxies. The highest CO concentration between the two galaxies corresponds to the Hα source in the taffy bridge near the intruder galaxy UGC 12915. This H II region is also associated with the strongest source of radio continuum in the bridge and shows both morphological and kinematic connections to UGC 12915. The overall CO distribution of the entire system agrees well with that of the radio continuum emission, particularly in the taffy bridge. This argues for the star formation origin of a significant portion of the radio continuum emission. Compared with the H I morphology and kinematics, which are strongly distorted owing to the high-speed collision, CO better defines the orbital geometry and impact parameter of the interaction, as well as the disk properties (e.g., rotation, orientation) of the progenitor galaxies. Based on the 20 cm–to–CO ratio maps, we conclude that the starburst sites are primarily located in UGC 12915 and the Hα source in the bridge and show that the molecular gas in the taffy bridge is forming into stars with star formation efficiency comparable to that of the target galaxy UGC 12914 and similar to that in the Galactic disk.

A Neighboring Dwarf Irregular Galaxy Hidden by the Milky Way

We have obtained VLA and optical follow-up observations of the low-velocity H I source HIZSS 3 discovered by Henning et al. and Rivers in a survey for nearby galaxies hidden by the disk of the Milky Way. Its radio characteristics are consistent with this being a nearby (~1.8 Mpc) low-mass dwarf irregular galaxy (dIm). Our optical imaging failed to reveal a resolved stellar population but did detect an extended Hα emission region. The location of the Hα source is coincident with a partially resolved H I cloud in the 21 cm map. Spectroscopy confirms that the Hα source has a similar radial velocity to that of the H I emission at this location, and thus we have identified an optical counterpart. The Hα emission (100 pc in diameter and with a luminosity of 1.4 × 1038 ergs s-1) is characteristic of a single H II region containing a modest population of OB stars. The galaxy's radial velocity and distance from the solar apex suggests that it is not a Local Group member, although a more accurate distance is needed to be certain. The properties of HIZSS 3 are comparable to those of GR 8, a nearby dIm with a modest amount of current star formation. Further observations are needed to characterize its stellar population, determine the chemical abundances, and obtain a more reliable distance estimate.

Fabry-PÉérot Hα Observations of the Barred Spiral NGC 3367

We report the gross properties of the velocity field of the barred spiral galaxy NGC 3367. We obtained the following values: the inclination with respect to the plane of the sky, i = 30; the position angle (P.A.) of the receding semimajor axis, P.A. = 51°; and the systemic velocity, Vsys = 3032 km s-1. Large velocity dispersions are observed of up to 120 km s-1 in the nuclear region and of up to 70 km s-1 near the eastern bright sources just beyond the edge of the stellar bar, where three spiral arms seem to start, and in the western bright sources at about 10 kpc. Deviations from normal circular velocities are observed from the whole disk but mainly from the semicircle formed by the string of southwestern Hα sources. An estimate of the dynamical mass is Mdyn = 2 × 1011 M⊙.

Space Telescope Imaging Spectrograph Parallel Observations of the Planetary Nebula M94‐2011Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under the NASA contract NAS 5‐26555.

ABSTRACTThe planetary nebula M94‐20 in the Large Magellanic Cloud was serendipitously observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope as part of the Hubble Space Telescope Archival Pure Parallel Program. We present spatially resolved imaging and spectral data of the nebula and compare them with ground‐based data, including detection of several emission lines from the nebula and the detection of the central star. We find that the total Hα + [N ii] flux is 7.3 × 10- 15 ergs s−1 cm−2, and we estimate the magnitude of the central star to be mV = 26.0 ± 0.2. Many other Hα sources have been found in M31, M33, and NGC 205 as well. We discuss the use of the parallel observations as a versatile tool for planetary nebula surveys and for other fields of astronomical research.

Discovery of Candidate Luminous Blue Variables in M31

Luminous blue variables (LBVs) constitute a short-lived, eruptive phase in the evolution of some of the most massive stars. Only a handful have yet been identified in the Galaxy and in each of the nearby galaxies; there are four known LBVs in M31. We have found an efficient method to identify candidate LBV stars in nearby galaxies. The candidates are identified in a pair of deep, continuum-subtracted narrowband Hα and [S II] images as objects with extremely low [S II] to Hα ratios, and with coincident stellar objects in continuum images. Five of the most promising new candidates identified by these criteria in the northeastern half of M31 were subsequently confirmed by optical spectroscopy to show spectra similar to the previously identified M31 LBV, HS var 15. These five candidates also have much in common with B[e] stars, of which none were known to exist in M31. They are bright Hα sources, (120 L☉ < LHα < 1300 L☉) with no detectable [S II] emission, large Hα equivalent widths (-60 to -400 Å), and broad wings on the Hα profiles (FWZI = 1200-2000 km s-1). Most candidates have Fe II emission. We discuss the environments of the candidates and find that many objects are likely older than several million years because they tend not to be located inside bright H II regions. We predict, based on the current results, that at least 20-25 LBV/B[e] candidates may be present in M31.

A Late-Time Optical Detection of SN 1985L in NGC 5033

An apparent late-time optical recovery of SN 1985L (Type II-L) in the Sb galaxy NGC 5033 is presented and discussed. An Hα emission point source is found within 1'' of both SN 1985L's historically reported position 70'' west and 55'' north of NGC 5033's center and its late-time, radio-measured position. This Hα source is not visible on V-band or narrow red continuum (7000 ± 125 A) images, but is detected using a 6510 A filter (FWHM = 30 A). Its detection at 6510 A can be attributed to blueshifted Hα emission because of a broad (±5000 km s-1) Hα line profile like that seen in late-time Type II-L spectra. The SN 1985L source has an estimated total Hα flux of (5 ± 1) × 10-16 ergs cm-2 s-1, making it, at a distance of 13 Mpc, about as luminous as the late-time Hα emission from SN 1970G. The presence of Hα emission from SN 1985L suggests that it has a spectrum similar to that of other comparably aged Type II-L supernovae, and further helps establish a late-time radio-optical link for this subclass of Type II events.

On the Origin of the Southwest H alpha Regions of the Spiral Galaxy NGC 3367

view Abstract Citations (8) References (54) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Origin of the Southwest H alpha Regions of the Spiral Galaxy NGC 3367 Garcia-Barreto, J. A. ; Franco, J. ; Carrillo, R. Abstract New narrow-band, continuum-free, Hα, and broadband R and I imaging data from the barred galaxy NGC 3367 are presented. Our Hα observations indicate that there is Hα almost all over the disk of the galaxy, and there are no particular signs of either nuclear or outer rings. Strong Hα emission is detected from the compact nucleus and from different sources in the disk. In particular, there are H II regions at the ends of the stellar bar and in the inner spiral arms. Also, and very strikingly, a string of Hα emission sources spanning almost 180^deg^, from southeast to northwest, is observed in the southwest region of the galaxy, at an almost constant radius from the center. The string is located outside the stellar bar and does not belong to any particular spiral arm. There are several possible interpretations regarding the origin of these Hα knots (i.e., due to an outer Lindblad resonance ring, to the interaction of NGC 3367 with a hot intergalactic medium, or to a direct impact with a neighbor), but we favor the possibility that the string is due to an off-center impact by an intruder. This scenario can also explain the emission from the compact nucleus and is consistent with the expected lifetimes of the Hα knots and the kinematic timescales for an induced ring by an intruder. The string of Hα sources is probably evidence of having cloud overlap, which has triggered new massive star formation. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177766 Bibcode: 1996ApJ...469..138G Keywords: GALAXIES: PHOTOMETRY; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 3367; GALAXIES: SPIRAL; GALAXIES: STRUCTURE full text sources ADS | data products NED (25) SIMBAD (18)

Star formation in NGC 4449: MAMA-detector UV imagery and Fabry-Perot Balmer-line imagery

view Abstract Citations (31) References (50) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Star Formation in NGC 4449: MAMA-Detector UV Imagery and Fabry-Perot Balmer-Line Imagery Hill, Robert S. ; Home, Allen T. ; Smith, Andrew M. ; Bruhweiler, Fred C. ; Cheng, K. P. ; Hintzen, Paul M. N. ; Oliversen, Ronald J. Abstract Using far-ultraviolet (FUV) and Balmer-line imagery, we investigate the star formation history of 22 large OB complexes in the Magellanic irregular galaxy NGC 4449. The FUV luminosity of NGC 4449 is comparable to those of late-type spirals and is greater than that of the LMC by 2.4 mag, indicating substantial star formation in the last 10^8^ yr. FUV data were taken using a sounding-rocket telescope with a Multianode Microchannel Array (MAMA) detector, and Balmer-line data were taken using the Goddard Fabry-Perot Imager. The resulting imagery shows bright, roughly coincident FUV and Hα sources throughout the extent of the visible galaxy. We model these sources using cluster-evolution codes. Although all sources are a few Myr old, clear age differences are found. In particular, several of the most recently active star formation regions are located together in the galaxy's northern periphery, which is apparently coincident with a large H I reservoir. The brightest and most massive OB complexes are found along the northeast-southwest surface brightness ridgeline (the "bar"). Over the entire galaxy, star formation rates are consistent on timescales of 10^6^, 10^8^, and 10^9^ yr. A history of recent star formation is suggested with two main episodes, one predominantly in the bar ending ~5 Myr ago, and an ongoing one associated with an observed H I cloud. Publication: The Astrophysical Journal Pub Date: August 1994 DOI: 10.1086/174431 Bibcode: 1994ApJ...430..568H Keywords: Balmer Series; Galactic Clusters; Galactic Evolution; Irregular Galaxies; Luminosity; Star Formation; Ultraviolet Spectroscopy; Astronomical Models; Cosmology; Fabry-Perot Interferometers; Multi-Anode Microchannel Arrays; Spectrum Analysis; Ultraviolet Astronomy; Ultraviolet Photometry; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 4449; GALAXIES: STELLAR CONTENT; STARS: EVOLUTION; STARS: FORMATION; TECHNIQUES: INTERFEROMETRIC; ULTRAVIOLET: STARS full text sources ADS | data products SIMBAD (29) NED (7)

The roots of coronal structure in the Sun's surface

We have compared the structures seen on X-ray images obtained by a flight of the NIXT sounding rocket payload on July 11, 1991 with near-simultaneous photospheric and chromospheric structures and magnetic fields observed at Big Bear. The X-ray images reflect emission of both Mgx and Fexvi, formed at 1 × 106 K and 3 × 106 K, respectively. The brightest Hα sources correspond to a dying sub-flare and other active region components, all of which reveal coronal enhancements situated spatially well above the Hα emission. The largest set of X-ray arches connected plages of opposite polarity in a large bipolar active region. The arches appear to lie in a small range of angle in the meridian plane connecting their footpoints. Sunspots are dark on the surface and in the corona. For the first time we see an emerging flux region in X-rays and find the emission extends twice as high as the Hα arches. Many features which we believe to correspond to ‘X-ray bright points’ (XBPs) were observed. Whether by resolution or spectral band, the number detected greatly exceeds that from previous work. All of the brighter XBPs correspond to bipolar Hα features, while unipolar Hα bright points are the base of more diffuse comet-like coronal arches, generally vertical. These diverge from individual features by less than 30°, and give a good measure of what the ‘canopies’ must do. The Hα data shows that all the Hα features were present the entire day, so they are not clearly disappearing or reappearing. We find a new class of XBPs which we call ‘satellite points’, elements of opposite polarity linked to nearby umbrae by invisible field lines. The satellite points change rapidly in X-ray brightness during the flight. An M1.9 flare occurred four hours after the flight; examination of the pre-flare structures reveals nothing unusual.