A. Alberdi

Hydrodynamical Models of Superluminal Sources

We present numerical simulations of the generation, evolution, and radio emission of superluminal components in relativistic jets. We perform the fluid dynamical calculations using a relativistic time-dependent code based on a high-resolution shock-capturing scheme, and then we calculate the radio emission by integrating the transfer equations for synchrotron radiation. These simulations show that a temporary increase in the flow velocity at the base of the jet produces a moving perturbation that contains both a forward and a reverse shock and is trailed by a rarefaction. The perturbation appears in the simulated maps as a region of enhanced emission moving downstream at a superluminal apparent velocity. Interactions of the perturbation with the underlying steady jet result in changes in the internal brightness distribution of the superluminal component, which are manifested as low-level fluctuations about the long-term evolution of both the apparent velocity and the exponential decay of the light curves.

On the simultaneous optical and near-infrared variability of pre-main sequence stars

For a complete understanding of the physical processes causing the photometric variability of pre-main sequence systems, simultaneous optical and near-IR observations are required to disentangle the emission from the stars and that from their associated circumstellar disks. Data of this sort are extremely rare and little systematic work has been reported to date. The work presented in this paper is a systematic attempt in this direction. It presents an analysis of the simultaneous optical and near-IR photometric variability of 18 Herbig Ae/Be and T Tauri stars which were observed in October 98 by the EXPORT collaboration. The time dierence between the UBVRI and JHK measurements is less than 1 hour in50% of the data and the largest dierence is around 2 hours in only10% of the data. Twelve stars appear to show a correlation between the optical and near-IR variability trends, which suggests a common physical origin such as spots and/or variable extinction. The optical and near-IR variability is uncorrelated in the rest of the objects, which suggests it originates in distinctly dierent regions. In general, the optical variability qualitatively follows the predictions of starspots or variable extinction. As far as the near-IR is concerned, the simultaneity of the observations demonstrates that for most objects the flux is largely produced by their circumstellar disks and, consequently, in many cases the near-IR fluctuations must be attributed to structural variations of such disks producing variations of their thermal emission and/or scattered light. The observed near-IR changes of up to around 1 mag on timescales of 1{2 days provide interesting challenges for understanding the mechanisms generating such remarkable variabilities, an issue insuciently investigated until now but one which deserves further theoretical and modeling eorts.

Molecules as tracers of galaxy evolution: an EMIR survey - I. Presentation of the data and first results

Aims. We investigate the molecular gas properties of a sample of 23 galaxies in order to find and test chemical signatures of galaxy evolution and to compare them to IR evolutionary tracers. Methods. Observation at 3 mm wavelengths were obtained with the EMIR broadband receiver, mounted on the IRAM 30 m telescope on Pico Veleta, Spain. We compare the emission of the main molecular species with existing models of chemical evolution by means of line intensity ratios diagrams and principal component analysis. Results. We detect molecular emission in 19 galaxies in two 8 GHz-wide bands centred at 88 and 112 GHz. The main detected molecules are CO, (CO)-C-13, HCN, HNC, HCO+, CN, and C2H. We also detect HC3N J = 10-9 in the galaxies IRAS 17208, IC 860, NGC 4418, NGC 7771, and NGC 1068. The only HC3N detections are in objects with HCO+/HCN 0.8). The brightest HC3N emission is found in IC 860, where we also detect the molecule in its vibrationally excited state. We find low HNC/HCN line ratios (<0.5), that cannot be explained by existing PDR or XDR chemical models. The intensities of HCO+ and HNC appear anti-correlated. No correlation is found between the HNC/HCN line ratio and dust temperature. All HNC-bright objects are either luminous IR galaxies (LIRG) or Seyferts. Galaxies with bright polycyclic aromatic hydrocarbons (PAH) emission show low HNC/HCO+ ratios. The CO/(CO)-C-13 ratio is positively correlated with the dust temperature and is generally higher than in our galaxy. The emission of CN and (CO)-O-18 is correlated. Conclusions. Bright HC3N emission in HCO+-faint objects may imply that these are not dominated by X-ray chemistry. Thus the HCN/HCO+ line ratio is not, by itself, a reliable tracer of XDRs. Bright HC3N and faint HCO+ could be signatures of embedded star-formation, instead of AGN activity. Mechanical heating caused by supernova explosions may be responsible for the low HNC/HCN and high HCO+/HCN ratios in some starbursts. We cannot exclude, however, that the discussed trends are largely caused by optical depth effects or excitation. Chemical models alone cannot explain all properties of the observed molecular emission. Better constraints to the gas spacial distribution and excitation are needed to distinguish abundance and excitation effects.

Evidence for Evolution of the Outflow Collimation in Very Young Stellar Objects

We present Very Long Baseline Array proper-motion measurements of water masers toward two young stellar objects (YSOs) of the W75 N star-forming region. We find that these two objects are remarkable for having a similar spectral type, being separated by 07 (corresponding to 1400 AU), and sharing the same environment, but with a strikingly different outflow ejection geometry. One source has a collimated, jetlike outflow at a 2000 AU scale, while the other has a shell outflow at a 160 AU scale expanding in multiple directions with respect to a central compact radio continuum source. This result reveals that outflow collimation is not only a consequence of ambient conditions but is something intrinsic to the individual evolution of stars and brings to light the possibility of noncollimated outflows in the earliest stages of YSOs.

Constraints on the Progenitor System and the Environs of SN 2014J from Deep Radio Observations

We report deep EVN and eMERLIN observations of the Type Ia SN 2014J in the nearby galaxy M82. Our observations represent, together with JVLA observations of SNe 2011fe and 2014J, the most sensitive radio studies of Type Ia SNe ever. By combining data and a proper modeling of the radio emission, we constrain the mass-loss rate from the progenitor system of SN 2014J to (M) over dot less than or similar to 7.0 x 10(-10) M yr(-1) (for a wind speed of 100 km s(-1)). If the medium around the supernova is uniform, then n(ISM) less than or similar to 1.3 cm(-3), which is the most stringent limit for the (uniform) density around a Type Ia SN. Our deep upper limits favor a double-degenerate (DD) scenario-involving two WD stars-for the progenitor system of SN 2014J, as such systems have less circumstellar gas than our upper limits. By contrast, most single-degenerate (SD) scenarios, i.e., the wide family of progenitor systems where a red giant, main-sequence, or sub-giant star donates mass to an exploding WD, are ruled out by our observations. (While completing our work, we noticed that a paper by Margutti et al. was submitted to The Astrophysical Journal. From a non-detection of X-ray emission from SN 2014J, the authors obtain limits of (M) over dot less than or similar to 1.2 x 10(-9) M-circle dot yr(-1) (for a wind speed of 100 km s(-1)) and n(ISM) less than or similar to 3.5 cm(-3), for the rho proportional to r(-2) wind and constant density cases, respectively. As these limits are less constraining than ours, the findings by Margutti et al. do not alter our conclusions. The X-ray results are, however, important to rule out free-free and synchrotron self-absorption as a reason for the radio non-detections.) Our estimates on the limits on the gas density surrounding SN2011fe, using the flux density limits from Chomiuk et al., agree well with their results. Although we discuss the possibilities of an SD scenario passing observational tests, as well as uncertainties in the modeling of the radio emission, the evidence from SNe 2011fe and 2014J points in the direction of a DD scenario for both.

An extremely prolific supernova factory in the buried nucleus of the starburst galaxy IC 694

Context. The central kiloparsec of many local uminous infrared galaxies are known to host intense bursts of massive star formation, leading to numerous explosions of core-collapse supernovae (CCSNe). However, the dust-enshrouded regions where those supernovae explode hamper their detection at optical and near-infrared wavelengths. Aims. We investigate the nuclear region of the starburst galaxy IC 694 (=Arp 299-A) at radio wavelengths, aimed at discovering recently exploded CCSNe, as well as determining their rate of explosion, which carries crucial information about star formation rates, the initial mass function, and the starburst processes in action. Methods. We use the electronic European VLBI Network to image with milliarcsecond resolution the 5.0 GHz compact radio emission of the innermost nuclear region of IC 694. Results. Our observations detect a rich cluster of 26 compact radio emitting sources in the central 150 pc of the nuclear starburst in IC 694. The high brightness temperatures observed for the compact sources are indicative of a non-thermal origin for the observed radio emission, implying that most, if not all, of those sources are young radio supernovae (RSNe) and supernova remnants (SNRs). We find evidence of at least three relatively young, slowly evolving, long-lasting RSNe (A0, A12, and A15) that appear to have unusual properties, suggesting that the conditions in the local circumstellar medium (CSM) play a significant role in determining the radio behaviour of expanding SNe. Their radio luminosities are typical of normal RSNe, which result from the explosion of type IIP/b and type IIL SNe. All of these results provide support for a recent (less than 10-15 Myr) instantaneous starburst in the innermost regions of IC 694.

Nuclear Star-forming Structures and the Starburst-Active Galactic Nucleus Connection in Barred Spirals: NGC 3351 and NGC 4303

A high-resolution Hubble Space Telescope WFPC2 F218W UV image of the barred spiral NGC 4303 (classified as a LINER-type active galactic nucleus [AGN]) reveals for the first time the existence of a nuclear spiral structure of massive star-forming regions all the way down to the UV-bright unresolved core (size ≤ 8 pc) of an active galaxy. The spiral structure, as traced by the UV-bright star-forming regions, has an outer radius of 225 pc and widens as the distance from the core increases. The UV luminosity of NGC 4303 is dominated by the massive star-forming regions, and the unresolved LINER-type core contributes only 16% of the integrated UV luminosity. The nature of the UV-bright LINER-type core—stellar cluster or pure AGN—is still unknown. In contrast to NGC 4303, the UV F218W image of the non-AGN barred galaxy NGC 3351 shows a nuclear star-forming ring of 315 pc (semimajor axis) with a faint core. In the ring, the star formation is arranged in clumps of about 60-85 pc in diameter. Each clump consists of a few compact UV-bright clusters embedded in a more diffuse component. The integrated IUE spectrum of NGC 3351 shows the presence of Si IV 1400 A and C IV 1550 A absorption lines, typical features of young, 4-5 Myr old, massive star clusters. The presence of ring and spiral star-forming structures in the nuclear regions of these two barred spirals supports the bar-induced gas-fueling scenario by which bars accumulate gas in the nuclear regions of galaxies, produce nuclear star-forming rings (NGC 3351), and might eventually generate or feed an AGN (NGC 4303).

A Decade of SN 1993J: Discovery of Radio Wavelength Effects in the Expansion Rate

We studied the growth of the shell-like radio structure of supernova SN 1993J in M 81 from September 1993 to October 2003 with very-long-baseline interferometry (VLBI) observations at the wavelengths of 3.6, 6, and 18 cm. We developed a method to accurately determine the outer radius (R) of any circularly symmetric compact radio structure such as SN 1993J. The source structure of SN 1993J remains circularly symmetric (with deviations from circularity under 2%) over almost 4000 days. We characterize the decelerated expansion of SN 1993J until approximately day 1500 after explosion with an expansion parameter m = 0.845 ± 0.005 (R ∝ t m ). However, from that day onwards the expansion differs when observed at 6 and 18 cm. Indeed, at 18 cm, the expansion can be well characterized by the same m as before day 1500, while at 6 cm the expansion appears more decelerated, and is characterized by another expansion parameter, m6 = 0.788 ± 0.015. Therefore, since about day 1500 onwards, the radio source size has been progressively smaller at 6 cm than at 18 cm. These findings differ significantly from those of other authors in the details of the expansion. In our interpretation, the supernova expands with a single expansion parameter, m = 0.845 ± 0.005, and the 6 cm results beyond day 1500 are caused by physical effects, perhaps also coupled to instrumental limitations. Two physical effects may be involved: (a) a changing opacity of the ejecta to the 6 cm radiation; and (b) a radial decrease of the magnetic field in the emitting region. We also found that at 6 cm about 80% of the radio emission from the backside of the shell behind the ejecta is absorbed (our average estimate, since we cannot determine any possible evolution of the opacity), and the width of the radio shell is (31 ± 2)% of the outer radius. The shell width at 18 cm depends on the degree of assumed absorption. For 80% absorption, the width is (33.5 ± 1.7)%, and for 100% absorption, it is (37.8 ± 1.3)%. A comparison of our VLBI results with optical spectral line velocities shows that the deceleration is more pronounced in the radio than in the optical. This difference might be due to a progressive penetration of ejecta instabilities into the shocked circumstellar medium, as also suggested by other authors.

AGN and starbursts at high redshift: High resolution EVN radio observations of the Hubble Deep Field

We present deep, wide-field European VLBI Network (EVN) 1.6 GHz observations of the Hubble Deep Field (HDF) region with a resolution of 0.025 arcseconds. Above the 210 μ Jy/beam (

Detection of jet precession in the active nucleus of M 81

5\sigma