Berkeley-Illinois-Maryland Array Research Articles

The chromosphere above sunspots at millimeter wavelengths

Aims: The aim of this paper is to demonstrate that millimeter wave data can be used to distinguish between various atmospheric models of sunspots, whose temperature structure in the upper photosphere and chromosphere has been the source of some controversy. Methods: We use observations of the temperature contrast (relative to the quiet Sun) above a sunspot umbra at 3.5 mm obtained with the Berkeley-Illinois-Maryland Array (BIMA), complemented by submm observations from Lindsey & Kopp (1995) and 2 cm observations with the Very Large Array. These are compared with the umbral contrast calculated from various atmospheric models of sunspots. Results: Current mm and submm observational data suggest that the brightness observed at these wavelengths is low compared to the most widely used sunspot models. These data impose strong constraints on the temperature and density stratifications of the sunspot umbral atmosphere, in particular on the location and depth of the temperature minimum and the location of the transition region. Conclusions: A successful model that is in agreement with millimeter umbral brightness should have an extended and deep temperature minimum (below 3000 K). Better spatial resolution as well as better wavelength coverage are needed for a more complete determination of the chromospheric temperature stratification above sunspot umbrae.

The relationship between chromospheric emissions and magnetic field strength

Aims. We analyze observational data from 4 instruments to study the correlations between chromospheric emission, spanning the heights from the temperature minimum region to the middle chromosphere, and photospheric magnetic field. Methods: The data consist of radio images at 3.5 mm from the Berkeley-Illinois-Maryland Array (BIMA), UV images at 1600 A from TRACE, Ca II K-line filtergrams from BBSO, and MDI/SOHO longitudinal photospheric magnetograms. For the first time interferometric millimeter data with the highest currently available resolution are included in such an analysis. We determine various parameters of the intensity maps and correlate the intensities with each other and with the magnetic field. Results: The chromospheric diagnostics studied here show a pronounced similarity in their brightness structures and map out the underlying photospheric magnetic field relatively well. We find a power law to be a good representation of the relationship between photospheric magnetic field and emission from chromospheric diagnostics at all wavelengths. The dependence of chromospheric brightness on magnetic field is found to be different for network and internetwork regions.

High-resolution millimeter-interferometer observations of the solar chromosphere

The use of millimeter-interferometer data for the study of chromospheric structure and dynamics is tested using 85 GHz observations with the 10-element Berkeley-Illinois-Maryland Array (BIMA). Interferometer data have the advantage over single-dish data that they allow both high spatial resolution and dense temporal sampling simultaneously. However, snapshot imaging of the quiet solar atmosphere with a small number of dishes is challenging. We demonstrate that techniques are available to carry out this task successfully using maximum entropy deconvolution from a default image constructed from the entire observation: one of our results is that the solar chromosphere at millimeter wavelengths exhibits features that are long-lasting and the map of the entire observation is significant provided that atmospheric phase errors do not prevent deconvolution. We compare observations of quiet Sun, active region and coronal hole targets. The interferometer is not sensitive to the disk emission and the positivity constraint of the maximum entropy algorithm used forces the zero level in the images to be at the temperature of the coolest feature in each field. The brightest features in the images are typically 1000–1500 K above the zero level, with a snapshot noise level of order 100 K. We use extensive tests to determine whether oscillation power can be recovered from sequences of snapshot images and show that individual sources can be down to quite weak levels at locations in the image where significant flux is present; oscillation power located in cool regions of the image is not well recovered due to the deconvolution method used and may be redistributed to brighter regions of the millimeter image. We then investigate whether the data do show oscillation power using uninterrupted 30-min scans of the target regions. Intensity oscillations with significant power in the frequency range 1.5–8.0 mHz are found in the quiet-Sun and active region targets. For the quiet-Sun region we compare the oscillation properties of network boundaries and cell interiors (internetwork) in the spatially-resolved time series. In agreement with investigations at other wavelengths, in the millimeter data the power in the network tends to be at periods of 5 min and longer while power in the internetwork is present also at shorter (3-min) periods.

Simple 1 MM Receivers with a Fixed Tuned Double Sideband SIS Mixer and a Wideband INP MMIC Amplifier

We report on techniques to broaden the intermediate frequency (IF) bandwidth of the Berkeley‐Illinois‐Maryland Array (BIMA) 1mm Superconductor‐Insulator‐Superconductor (SIS) heterodyne receivers by combining fixed tuned Double Side Band (DSB) SIS mixers and wideband Monolithic Microwave Integrated Circuit (MMIC) IF amplifiers. To obtain the flattest receiver gain across the IF band we tested three schemes for keeping the mixer and amplifier as electrically close as possible. In Receiver I, we connected separate mixer and MMIC modules by a 1 ″ stainless steel SMA elbow. In Receiver II, we integrated mixer and MMIC into a modified BIMA mixer module. In Receiver III, we devised a thermally split block in which mixer and MMIC can be maintained at different temperatures–in this receiver module the mixer at 4 K sees very little of the 10–20 mW heat load of the biased MMIC at 10 K. The best average receiver noise we achieved by combining SIS mixer and MMIC amplifier is 45 ‐50 K DSB for νLO = 215–240 GHz and below 80 K DSB for νLO = 205 ‐ 270 GHz. Over an IF frequency band of 1 – 4 GHz we have demonstrated receiver DSB noise temperatures of 40 – 60 K. Of the three receiver schemes, we feel Receiver III shows the most promise for continued development.

The solar chromosphere as seen in high-resolution millimeter observations

We report on chromospheric oscillations studied with 15 sec cadence observations obtained at a wavelength of 3.5 mm with the Berkeley-Illinois-Maryland Array (BIMA). Dif- ferent solar structures, including active and quiet-Sun regions, are analyzed. We compare the high-resolution millimeter 2-D images with Ca II K line images from Big Bear Solar Observa- tory. Using Fourier and wavelet time series analysis techniques we find signatures of intensity oscillations with periods in the range of 2-7 minutes. We discuss the results in the context of the dynamical model of the solar atmosphere by Carlsson & Stein (1995, 1997).

High‐Resolution Imaging of CO Outflows in OMC‐2 and OMC‐3

A large scale, high resolution map of CO(1-0) emission toward the OMC-2 and OMC-3 star forming regions is presented. The map is a mosaic of 46 fields using the Berkeley-Illinois-Maryland Array (BIMA) and covers $\sim 10'\times 15'$ at $\sim 10" $ resolution. These data are combined with singledish FCRAO observations and analyzed to identify and determine the properties of nine protostellar outflows. The BIMA data alone almost completely resolve out the cloud emission at central velocities and only recover 1/20 of the flux in the high velocity gas showing that outflows are generally broadly dispersed over $\sim 1'$ angular scales. All nine identified outflows emanate from known Class 0 or borderline Class 0/I sources, are associated with knots of shocked \h2\ emission, and have short dynamical times. It is suggested that only the youngest, most spatially compact, and energetic outflows have been found and that more distributed high velocity gas undetected by BIMA is due to older outflows continuing through the Class I phase of protostellar evolution. The mechanical energy injection rate into the cloud is estimated to be $\sim 1.5 L_\odot$ which is comparable to the turbulent energy dissipation rate. Outf`lows appear capable, therefore, of sustaining cloud turbulence but a high starformation rate is required implying a short cloud lifetime $\leq 5$ `Myr.

The Simultaneous Spectrum of Sagittarius A* from 20 Centimeters to 1 Millimeter and the Nature of the Millimeter Excess

We report results of a multiwavelength campaign to measure the simultaneous spectrum of the supermassive black hole candidate Sgr A* in the Galactic center from centimeter to millimeter wavelengths using the Very Large Array, the Berkeley-Illinois-Maryland Array (BIMA), the Nobeyama 45 m, and the Institut de Radioastronomie Millimetrique (IRAM) 30 m telescopes. The observations confirm that the previously detected millimeter excess is an intrinsic feature of the spectrum of Sgr A*. The excess can be interpreted as and effect of the presence of an ultracompact component of relativistic plasma with a size of a few Schwarzschild radii near the black hole. If so, Sgr A* might offer a unique possibility to image the putative black hole against the background of this component with future millimeter VLBI experiments.

A High Spatial Resolution Study of the ON2 Massive Star‐forming Region

We present 5''-10'' resolution images of the ON2 cloud core in the molecular lines 12CO(J = 1-0), SiO(v = 0, J = 2-1), H13CO+(J = 1-0), and SO2(83,5-92,8) and 3 millimeter continuum obtained with the Berkeley-Illinois-Maryland Array (BIMA) millimeter-wave interferometer. For the 12CO line, additional observations were obtained with the NRAO 12 m telescope to recover extended emission missed by the interferometer. We find evidence for at least four massive, energetic molecular outflows from young stellar objects within a 1.5 pc region. Three of these are identified by bipolar high-velocity 12CO emission, and a fourth is suggested by SiO and SO2 emission. Continuum emission is detected from the ultracompact (UC) H II region, G75.78+0.34, and also from two deeply embedded YSOs, one of which (source 3) may be the driving engine of the dominant central outflow G75 C. Approximately half of the SiO emission observed is not obviously linked to the proposed molecular outflows and perhaps traces additional shock activity within ON2. The high-resolution observations support the conclusion reached previously that a near-simultaneous massive star formation event occurred in the ON2 complex ~104 yr ago. The presence of molecular outflows in the cloud core shows that this event was not isolated to the cloud edges.

NGC 6946: Molecular Spiral Arms Masquerading as a Bar?

NGC 6946 is an example of a galaxy with a barlike molecular gas distribution from which the presence of a stellar bar has been inferred. We present high dynamic range Berkeley-Illinois-Maryland Array (BIMA) observations of CO 1-0 emission at a resolution of 4'' and a deep IR mosaic of K'-band emission. The CO distribution shows two strong offset ridges of emission curving out from a central CO peak; however, the CO distribution is complex, with evidence for an m = 4 structure. A central oval distortion in the stellar distribution has radius of 35, too small to have caused the barlike molecular distribution. There is evidence in the K'-band image for a weak bar with a diameter of 120'', although the barlike structure can also be attributed to blending of the spiral arms. The main CO ridges lie on the trailing side of the IR ridges both north and south of the nucleus. The IR ridges appear to be an enhancement in the old stellar population. If this is the case, the offset implies that the CO and IR trace spiral arms since the compressed gas in a bar would be located on the leading side of the bar. On the other hand, if the IR ridge enhancements are due entirely to K supergiants (which trace recent star formation), the CO must trace the dust lanes of a bar. If NGC 6946 has a bar, it is larger but weaker than previously inferred, and it has characteristics atypical of known bars. We conclude that it can be difficult to determine whether a galaxy is weakly barred from the CO and IR morphology.

The distribution of HCN, H(13)CN, and CN in IRC + 10216

We present aperture synthesis images of millimeter-wavelength molecular emission lines of HCN (J = 1-0 at 88.6 GHz), H(13)CN )J = 1-0 at 86.3 GHz), and CN (N = 1-0, J = 3/2-1/2 at 113.5 GHZ) toward the carbon star IRC + 10216. The HCN and H(13)CN images were made with the Berkeley-Illinois-Maryland Array (BIMA) interferometer, while the CN images were from a combination of interferometer and single-dish data. The HCN and H(13)CN images have an angular resolution of 8 arcseconds and a velocity resolution of approximately 1.3 km/s; the CN images have an angular resolution of 11 arcsecs and a velocity resolution of approximately 0.9 km/s. The images show that the HCN and H(13)CN are found concentrated toward the center of the envelope, while CN is found in the shell surrounding the central star. The data for HCN and H(13)CN are compared with the output of a statistical equilibrium code, which takes into account the excitation by IR photons and collisions with H2, and calculates the radial brightness profiles and spectra. The models are used to determine the abundance distribution of H(13)CN. HCN is too optically thick for accurate modeling; we infer the HCN abundance by scaling H(13)CN up by a factor of 40. For CN, a simple Local Thermodynamic Equilibrium (LTE) calculation is used to derive the abundance of the molecule as a function of distance form the star. The distributions of HCN and CN are qualitatively consistent with chemical models for asymptotic giant branch (AGB) star envelopes. Our best estimate of the peak CN/HCN abundance ratio is about 0.12, and the radius of the peak CN abundance is 19 arcsecs from the star. Both of these values are significantly less than recent photochemical model predictions. We suggest that CN phtodissociates or reacts with other radicals or ions more rapidly than these models assume.

The molecular envelope of NGC 7027

We present maps of the J=1-0 emission line of CO toward the planetary nebula NGC 7027. The images were made by combining interferometer observations from the Berkeley-Illinois-Maryland Array (BIMA) with single-dish observations from the NRAO 12 m telescope. The maps have an angular resolution of 5″ and contain all the flux in the CO J=1-0 line. The integrated intensity map shows that the circumstellar molecular envelope has a diameter to zero intensity of ∼70″, corresponding to a linear size of 0.30 pc at distance of 900 pc.

High spatial resolution observations of solar flares at 3.3 mm wavelength

We present the first high-spatial-resolution interferometric observations of solar flares at millimeter wavelengths, carried out with the Berkeley-Illinois-Maryland Array (BIMA). The observations represent an improvement of an order of magnitude in both sensitivity and spatial resolution compared with previous solar observations at these wavelengths. Most of the flares occurring within the field of view during the observations were detected by BIMA, including both very impulsive and longer-duration events. It appears that millimeter burst sources are not much smaller than microwave sources. If the emission in the flash phase is predominantly due to gyrosynchrotron emission, we can rule out thermal gyrosynchrotron models for the radio emission because the flux at millimeter wavelengths is too high. During the Flares 22 campaign we will collect both imaging data as well as dedicated patrol observations of flare time profiles at millimeter wavelengths.

40. Radio Astronomy (Radio Astronomie)

The following commission members have contributed to this report:W.J. Altenhoff, H. Andernach, J. Baars, R. Chini, R.J. Cohen, J.J. Condon, Y. Fukui, R. Güsten, J.M. van der Hulst, C. Henkel, W. Huchtmeier, M. Ishiguro, T. Krichbaum, M, Kundu, A. Lazenby, W. Reich, P. Roelfsema, S.E. Thorsett, R. Tuffs, CM. Walmsley, H. Wendker, R. Wielebinski, T.L. Wilson (organizer), A. WitzelAperture Synthesis Telescopes - Commissioning tests and observations have been made using three antennas in the Australia Telescope (AT), spaced up to 2km apart, and the first image of a radio source observed at 6cm was produced in 1989. The IRAM millimeter array in France comprizing three 15-m antennas has been commissioned at 3mm. A fourth antenna is planned as well as operation at 1mm. The three element Berkeley-Illinois-Maryland Array (BIMA) at Hat Creek is being expanded to six 6-m antennas and a new correlator with 1024 channel up to 830MHz bandwidth. Plans to expand to 9 antennas are final. The Owens Valley three-element millimeter array has been operated at 1mm, the shortest wavelength for the radio interferometry; a digital correlator with 500MHz bandwidth is under construction and the array will be extended to 6 antennas. In Japan at NRO the Nobeyama Millimeter Array equipped with SIS receivers and 320MHz bandwidth "FX" correlator is now operational at 3mm and 7mm wavelengths. New SIS receivers for 1mm and 2mm are under construction and a sixth 10-m antenna is planned. The Cambridge 5-km telescope, now the Ryle Telescope, has been substantially upgraded. The overall sensitivity of the instrument is being increased by a factor of ~20. The Giant Metrewave Radio Telescope, being built at Khodad near Pune in India, has made considerable progress and is expected to be operational by 1992-93.

Millimeter and Microwave Activity of the Sun

The emission of solar flares at millimeter wavelengths is of great interest both in its own right and because it is generated by the energetic electrons which also emit gamma rays. Since high-resolution imaging at gamma-ray energies is not presently possible, millimeter observations can act as a substitute. Except for that class of flares known as gamma-ray flares the millimetric emission is optically thin. It can be used as a powerful diagnostic of the energy distribution of electrons in solar flares and its evolution, and of the magnetic field. We have carried out high-spatial-resolution millimeter observations of solar flares this year using the Berkeley-Illinois-Maryland Array (BIMA), and report on the preliminary results in this paper (Kundu et al 1990; White et al 1990). We also report some recent results obtained from multifrequency observations using the VLA (White et al 1990).