Negative Spectral Indices Research Articles

High-resolution radio emission from RCW 49/Westerlund 2

RCW49 and its ionizing cluster form an extensive, complex region, widely studied at IR and optical wavelengths. Molonglo 843 MHz and ATCA data at 1.4 and 2.4 GHz showed two shells. Recent high-resolution IR imaging revealed a complex dust structure and ongoing star formation. New high-bandwidth and high-resolution data of the RCW49 field were obtained to survey the radio emission at arcsec scale and investigate the small-scale features and nature of the HII region.- Data were collected with the new 2-GHz bandwidth receivers and the CABB correlator of the ATCA, at 5.5 and 9.0 GHz. In addition, archival observations at 1.4 and 2.4 GHz have been re-reduced and re-analyzed, together with optical, IR, X-ray, and gamma-ray observations.- The CABB data result in the most detailed radio continuum images of RCW49 to date. The radio emission closely mimics the near-IR emission observed by Spitzer, showing pillars and filaments. The brightest continuum emission comes from the region known as the bridge. The overall flattish spectral index is consistent with a free-free emission mechanism. However, hints of nonthermal components are also present in the bridge. A jet-like structure surrounded by a bubble feature whose nature is still unclear has been discovered close to the Westerlund~2 core. Two apparent bow shocks and a number of discrete sources have been detected as well in the surroundings of RCW49. We also report on and discuss the possible detection of a H RRL.- The radio results support an association between the cm continuum and molecular emission. The detection of the RRL kinematically favors a RCW49 distance of 6--7 kpc. If the negative spectral indices measured at the bridge are caused by synchrotron emission, we propose a scenario where high-energy emission could be produced. Finally, the newly discovered jet-like structure appears to deserve a detailed study by itself.

THE GOULD's BELT VERY LARGE ARRAY SURVEY. I. THE OPHIUCHUS COMPLEX

We present large-scale ($\sim$ 2000 square arcminutes), deep ($\sim$ 20 $\mu$Jy), high-resolution ($\sim$ 1$''$) radio observations of the Ophiuchus star-forming complex obtained with the Karl G.\ Jansky Very Large Array at $\lambda$ = 4 and 6 cm. In total, 189 sources were detected, 56 of them associated with known young stellar sources, and 4 with known extragalactic objects; the other 129 remain unclassified, but most of them are most probably background quasars. The vast majority of the young stars detected at radio wavelengths have spectral types K or M, although we also detect 4 objects of A/F/B types and 2 brown dwarf candidates. At least half of these young stars are non-thermal (gyrosynchrotron) sources, with active coronas characterized by high levels of variability, negative spectral indices, and (in some cases) significant circular polarization. As expected, there is a clear tendency for the fraction of non-thermal sources to increase from the younger (Class 0/I or flat spectrum) to the more evolved (Class III or weak line T Tauri) stars. The young stars detected both in X-rays and at radio wavelengths broadly follow a G\"udel-Benz relation, but with a different normalization than the most radio-active types of stars. Finally, we detect a $\sim$ 70 mJy compact extragalactic source near the center of the Ophiuchus core, which should be used as gain calibrator for any future radio observations of this region.

The Galactic centre mini-spiral in the mm-regime

The mini-spiral is a feature of the interstellar medium in the central ~2 pc of the Galactic center. It is composed of several streamers of dust and ionised and atomic gas with temperatures between a few 100 K to 10^4 K. There is evidence that these streamers are related to the so-called circumnuclear disk of molecular gas and are ionized by photons from massive, hot stars in the central parsec. We attempt to constrain the emission mechanisms and physical properties of the ionized gas and dust of the mini-spiral region with the help of our multiwavelength data sets. Our observations were carried out at 1.3 mm and 3 mm with the mm interferometric array CARMA in California in March and April 2009, with the MIR instrument VISIR at ESO's VLT in June 2006, and the NIR Br-gamma with VLT NACO in August 2009. We present high resolution maps of the mini-spiral, and obtain a spectral index of 0.5 for Sgr A*, indicating an inverted synchrotron spectrum. We find electron densities within the range 0.8-1.5x10^4 cm-3 for the mini-spiral from the radio continuum maps, along with a dust mass contribution of ~0.25 solar masses from the MIR dust continuum, and extinctions ranging from 1.8-3 at 2.16 micron in the Br-gamma line. We observe a mixture of negative and positive spectral indices in our 1.3 mm and 3 mm observations of the extended emission of the mini-spiral, which we interpret as evidence that there are a range of contributions to the thermal free-free emission by the ionized gas emission and by dust at 1.3 mm.

Australia Telescope Compact Array Radio Imaging of the Proplyd‐like Objects in the Giant HiiRegion NGC 3603

Three cometary-shaped objects in the giant H II region NGC 3603, originally found and identified as proto-planetary disks (proplyds) by Brandner and coworkers using the Hubble Space Telescope and the Very Large Telescope in the optical and near-infrared, have been detected with the Australia Telescope Compact Array in the radio continuum at 3 and 6 cm. All three proplyd-like objects are clearly resolved with an extent of a few arcseconds. The integrated 6 cm fluxes are up to 1.3 times higher than the 3 cm fluxes with spectral indices averaged over the whole clump between α = -0.1 and -0.5 (Sν ∝ να), indicating the likely presence of nonthermal emission in at least some of the sources. We present spectral-index maps and show that the sites of negative radio spectral indices are predominantly concentrated in the direction of the tails in at least two of the three proplyd-like nebulae, while positive spectral indices are found in the region facing the ionizing star cluster. We propose that thermal bremsstrahlung and nonthermal synchrotron radiation are at work in all three proplyd-like sources. In at least one of the three objects, optically thin nonthermal synchrotron emission appears to dominate when averaged over its whole spatial extent, while the spectrum of a second source shows a marginal indication of a nonthermal spectrum. The average spectrum of the third source is in agreement with thermal bremsstrahlung. All measured fluxes are at least one order of magnitude higher than those predicted by Brandner and coworkers. Upper limits for mass-loss rates due to photoevaporation are calculated to be ~10-5 M☉ yr-1 and for electron densities to be ~104 cm-3. Due to the unexpectedly large radio luminosities of the proplyd-like features and because the radio emission is extended, a (proto-) stellar origin of the nonthermal emission from a dust enshrouded star appears unlikely. Instead, we propose that magnetized regions within the envelope of the proplyd-like nebulae exist.

Mass loss rate determination of southern OB stars

A sample of OB stars (eleven Of, one O and one B supergiant) has been surveyed with the Australia Telescope Compact Array at 4.8 and 8.64 GHz with a resolution of ~2´´-4´´. Five stars were detected; three of them have negative spectral indices, consistent with non-thermal emission, and two have positive indices. The thermal radiation from HD 150135 and HD 163181 can be explained as coming from an optically thick ionized stellar wind. The non-thermal radiation from CD-47° 4551, HD 124314 and HD 150136 possibly comes from strong shocks in the wind itself and/or in the wind colliding region if the stars have a massive early-type companion. The percentage of non-thermal emitters among detected O stars has increased up to ~50% . The Of star HD 124314 clearly shows flux density variations. Mass loss rates (or upper limits) were derived for all the observed stars and the results compared with non-radio measurements and theoretical predictions.

Radio Spectral Indices of the Powering Sources of Outflows

Eight regions containing molecular or HH out—ows (Mon R2E, S287, L1654, L483, PV Cephei, L1203, L1251A, and CB 247) have been observed with the VLA at 6 cm, and total of 55 sources have been detected above 5 p level. By combining our observations at 6 cm with previous data obtained at 3.6 cm, we estimate the spectral indices of the sources in the —elds. For 32 of the sources we discriminate the sign of the spectral index and thus, we obtain information about the nature of the emission. For seven out—ows (Mon R2E, S287-B, L1654, L483, PV Cephei, L1203, and L1251A) we found at least central source with an spectral index consistent with thermal free-free emission from thermal radio jets. Multiple sources, separated D10A, are found near the center of the Mon R2E, S287-B, L1654, and L1251A out—ows. In the case of S287-B we found two sources that are valid candidates to drive the S287-B molecular out—ow and could be related to the out—ow excitation. Under the assumption that the observed emission arises from thermal radio jets, we discuss the mechanism responsible for the ionization and the constraints that introduces on their physical parameters, and we estimate that the —ow of ionized material has to originate at few AU from the exciting star. Most of the remaining, noncentral, sources are characterized by negative spectral indices ( a [0.1) and are probably nonthermal background objects unrelated to the studied star-forming regions.

Non-linear evolution of cosmological power spectra

Hamilton et al. have suggested an invaluable scaling formula which describes how the power spectra of density fluctuations evolve into the nonlinear regime of hierarchical clustering. This paper presents an extension of their method to low-density universes and universes with nonzero cosmological constant. We pay particular attention to models with large negative spectral indices, and give a spectrum-dependent fitting formula which is of significantly improved accuracy by comparison with an earlier version of this work. The tendency of nonlinear effects to increase power on small scales is stronger for spectra with more negative spectral indices, and for lower densities. However, for low-density models with a cosmological constant, the nonlinear effects are less strong than for an open universe of the same $\Omega$.

The Nature of the Radio Sources within the Cepheus A Star-forming Region

view Abstract Citations (124) References (58) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Nature of the Radio Sources within the Cepheus A Star-forming Region Garay, Guido ; Ramirez, Solange ; Rodriguez, Luis F. ; Curiel, Salvador ; Torrelles, Jose M. Abstract We present multifrequency, matching-beam, VLA radio continuum observations of the Cep A East radio source, known to consist of 16 compact ( 1") components clustered within a 25" radius region, most of which are aligned in stringlike structures. We find that the spectral indices of the emission from these compact objects, in the frequency interval from 1.5 to 15 GHz, cover a wide range, from -0.6 to 0.7. Positive spectral indices are exhibited by sources 2, 3b, 3c, and 3d. The first and last of these objects, the brightest sources within Cep A East, exhibit in addition elongated morphologies and angular-size and flux-density dependences with frequency that suggest they correspond to confined jets of ionized gas. Most of the objects that appear in string structures exhibit a mixture of flat and negative spectral indices across their faces, which indicates the presence of both thermal and nonthermal emission. The spectral indices of the integrated emission from sources 1b, 4, 6, and 7a are in the range between - 0.3 and -0.1 while those of sources 1 a, 5, 7b, and 7c are even more negative (-0.6 < alpha < -0.4). We suggest that the radio emission from the string sources arises in shocks resulting from the interaction of confined stellar winds with the surrounding medium. The duality in emission mechanisms is expected in shock waves where a small fraction of the electrons are accelerated to relativistic velocities, giving rise to nonthermal emission, while most of the electrons produce thermal free-free emission. We find that the nonthermal emission dominates the thermal emission when the density of the thermal electrons is below a critical density of 5 x 103 cm -3. We also observed the Cep A West radio source, which consists of two compact components and an elongated, diffuse, champagne-like structure. We find that the spectral index between 1.5 and 5 GHz of the peak and of the integrated emission from the latter component are -0.1 and -0.4, respectively. We suggest that the radio emission from the diffuse source arises from shocked gas at the edge of a cavity driven by a wind that originates from the brightest compact radio object within this region, which shows a spectral index at the peak of the emission of 0.6. Publication: The Astrophysical Journal Pub Date: March 1996 DOI: 10.1086/176882 Bibcode: 1996ApJ...459..193G Keywords: ISM: H II REGIONS; ISM: INDIVIDUAL NAME: CEPHEUS A; ISM: JETS AND OUTFLOWS; RADIO CONTINUUM: STARS; STARS: FORMATION; STARS: PRE-MAIN-SEQUENCE full text sources ADS | data products SIMBAD (38)

On the radio variability of V410 Tauri

The radio flux density of the star V410 Tauri has been monitored at 6 and 2 cm wavelengths at monthly intervals over a one-year period with the VLA. In contrast to earlier observations, the radio emission was typically at the level of roughly 1 mJy with flat or negative spectral indices. No large radio 'flare' events were detected. However, a periodicity analysis of the quiescent data shows statistically significant evidence for a modulation of the radio emission, with a period of 0.933 days, or one-half the optical period reported by Vrba, et al. (1988). If the modulation is real, it would imply that (1) the radio emission is produced either in localized zones on two opposite hemispheres of the star, or in a single, presumably high-latitude, structure that remains visible for an entire rotation period; (2) this zone or zones must be comparable to the stellar radius in size; (3) the energy spectrum of the radiating electrons is stable over long times (greater than 1 yr) but the absolute intensity level in the quiescent state may vary by factors of a few; and (4) the emission solid angle varies with viewing angle of the emission zone. It is suggested that gyrosynchrotron radiation from closed-loop magnetic structures may be responsible for the quasisteady low-level radio emission reported here. A geometric model for the radio emission structure may explain qualitatively the modulation of the radio emission. By analogy with the premain-sequence star DoAr 21, this same physical mechanism may produce both the 'quiescent' and 'flaring' radio emission. The monitoring results suggest that strong radio outbursts, such as were previously observed for V410 Tau, are relatively rare.

Spectral evolution of young stellar objects

An evolutionary sequence, from protostars to pre-main sequence stars, for the classification of young stellar objects is derived by comparing the predictions of the theoretical protostar models of Adams and Shu (AS, 1986) with the morphological classification scheme of Lada and Wilking (1984). It is shown that the AS models adequately explain the emergent spectral energy distributions of unidentified objects with negative spectral indices in the mid-IR and near-IR in both Taurus and Ophiuchus. If the infalling dust envelope is then completely removed, the spectra of the underlying stars and nebular disks used by AS provide a natural explanation for the near-IR and mid-IR excesses and the positive spectral indices of embedded T Tauri stars. It is found that the addition of a simple physical model for residual dust envelopes can reproduce the far-IR excesses found in some of these T Tauri stars.