Kinematic Age Research Articles

The Galactic kinematics of cataclysmic variables

Kinematical properties of CVs were investigated according to population types and orbital periods, using the space velocities computed from recently updated systemic velocities, proper motions and parallaxes. Reliability of collected space velocity data are refined by removing 34 systems with largest space velocity errors. The 216 CVs in the refined sample were shown to have a dispersion of $53.70 \pm 7.41$ km s$^{-1}$ corresponding to a mean kinematical age of $5.29 \pm 1.35$ Gyr. Population types of CVs were identified using their Galactic orbital parameters. According to the population analysis, seven old thin disc, nine thick disc and one halo CV were found in the sample, indicating that 94% of CVs in the Solar Neighbourhood belong to the thin-disc component of the Galaxy. Mean kinematical ages $3.40 \pm 1.03$ and $3.90 \pm 1.28$ Gyr are for the non-magnetic thin-disc CVs below and above the period gap, respectively. There is not a meaningful difference between the velocity dispersions below and above the gap. Velocity dispersions of the non-magnetic thin-disc systems below and above the gap are $24.95 \pm 3.46$ and $26.60 \pm 4.18$ km s$^{-1}$, respectively. This result is not in agreement with the standard formation and evolution theory of CVs. The mean kinematical ages of the CV groups in various orbital period intervals increase towards shorter orbital periods. This is in agreement with the standard theory for the evolution of CVs. Rate of orbital period change was found to be $dP/dt=-1.62(\pm 0.15)\times 10^{-5}$ sec yr$^{-1}$.

Stellar substructures in the solar neighbourhood

We determine detailed elemental abundances in stars belonging to the so-called Group 1 of the Geneva-Copenhagen survey (GCS) and compare the chemical composition with the Galactic thin- and thick-disc stars, with the GCS Group 2 and Group 3 stars, as well as with several kinematic streams of similar metallicities. The aim is to search for chemical signatures that might give information about the formation history of this kinematic group of stars. High-resolution spectra were obtained with the Fibre-fed Echelle Spectrograph (FIES) spectrograph at the Nordic Optical Telescope, La Palma, and were analysed with a differential model atmosphere method. Comparison stars were observed and analysed with the same method. The average value of [Fe/H] for the 37 stars of Group 1 is -0.20 +- 0.14 dex. Investigated Group 1 stars can be separated into three age subgroups. Along with the main 8- and 12-Gyr-old populations, a subgroup of stars younger than 5 Gyr can be separated as well. Abundances of oxygen, alpha-elements, and r-process dominated elements are higher than in Galactic thin-disc dwarfs. This elemental abundance pattern has similar characteristics to that of the Galactic thick disc and differs slightly from those in Hercules, Arcturus, and AF06 stellar streams. The similar chemical composition of stars in Group 1, as well as in Group 2 and 3, with that in stars of the thick disc might suggest that their formation histories are linked. The chemical composition pattern together with the kinematic properties and ages of stars in the investigated GCS groups provide evidence of their common origin and possible relation to an ancient merging event. A gas-rich satellite merger scenario is proposed as the most likely origin.

KILOPARSEC-SCALE JETS IN THREE RADIO-LOUD NARROW-LINE SEYFERT 1 GALAXIES

We have discovered kiloparsec-scale extended radio emission in three narrow-line Seyfert 1 galaxies (NLS1s) in sub-arcsecond resolution 9 GHz images from the Karl G. Jansky Very Large Array (VLA). We find all sources show two-sided, mildly core-dominated jet structures with diffuse lobes dominated by termination hotspots. These span 20-70 kpc with morphologies reminiscent of FR II radio galaxies, while the extended radio luminosities are intermediate between FR I and FR II sources. In two cases the structure is linear, while a $45^{\circ}$ bend is apparent in the third. Very Long Baseline Array images at 7.6 GHz reveal parsec-scale jet structures, in two cases with extended structure aligned with the inner regions of the kiloparsec-scale jets. Based on this alignment, the ratio of the radio core luminosity to the optical luminosity, the jet/counter-jet intensity and extension length ratios, and moderate core brightness temperatures ($\lesssim10^{10}$ K), we conclude these jets are mildly relativistic ($\beta\lesssim0.3$, $\delta\sim1$-$1.5$) and aligned at moderately small angles to the line of sight (10-15$^{\circ}$). The derived kinematic ages of $\sim10^6$-$10^7$ y are much younger than radio galaxies but comparable to other NLS1s. Our results increase the number of radio-loud NLS1s with known kiloparsec-scale extensions from seven to ten and suggest that such extended emission may be common, at least among the brightest of these sources.

The Very Fast Evolution of Bi-Lobed Planetary Nebulae

We study the kinematical ages of six Galactic bulge planetary nebulae selected purely on their bi-lobed morphology. The ages are derived from pseudo-3D photoionization and kinematical models, using HST images and UVES spectra and assuming axial symmetry. The bi-lobed nebulae show similar kinematical ages (∼1500 yrs), despite of their different sizes, expansion velocities and stellar temperatures.

On the age of the β Pictoris moving group

Jeffries & Binks (2014) and Malo et al. (2014) have recently reported Li depletion boundary (LDB) ages for the {\beta} Pictoris moving group (BPMG) which are twice as old as the oft-cited kinematic age of $\sim$12 Myr. In this study we present (1) a new evaluation of the internal kinematics of the BPMG using the revised Hipparcos astrometry and best available published radial velocities, and assess whether a useful kinematic age can be derived, and (2) derive an isochronal age based on the placement of the A-, F- and G-type stars in the colour-magnitude diagram (CMD). We explore the kinematics of the BPMG looking at velocity trends along Galactic axes, and conducting traceback analyses assuming linear trajectories, epicyclic orbit approximation, and orbit integration using a realistic gravitational potential. None of the methodologies yield a kinematic age with small uncertainties using modern velocity data. Expansion in the Galactic X and Y directions is significant only at the 1.7{\sigma} and 2.7{\sigma} levels, and together yields an overall kinematic age with a wide range (13-58 Myr; 95 per cent CL). The A-type members are all on the zero age-main-sequence, suggestive of an age of $>$20Myr, and the loci of the CMD positions for the late-F- and G-type pre-main-sequence BPMG members have a median isochronal age of 22 Myr ($\pm$ 3 Myr stat., $\pm$ 1 Myr sys.) when considering four sets of modern theoretical isochrones. The results from recent LDB and isochronal age analyses are now in agreement with a median BPMG age of 23 $\pm$ 3 Myr (overall 1{\sigma} uncertainty, including $\pm$2 Myr statistical and $\pm$2 Myr systematic uncertainties).

THE LAUNCH REGION OF THE SVS 13 OUTFLOW AND JET

We present the results of Keck Telescope laser adaptive optics integral field spectroscopy with OSIRIS of the innermost regions of the NGC 1333 SVS 13 outflow that forms the system of Herbig-Haro objects 7-11. We find a bright 0.2 arc sec long microjet traced by the emission of shock-excited [FeII]. Beyond the extent of this jet, we find a series of bubbles and fragments of bubbles that are traced in the lower excitation H_2 1-0 S(1) line. While the most recent outflow activity is directed almost precisely (PA ~145 deg) to the south-east of SVS 13, there is clear indication that prior bubble ejections were pointed in different directions. Within these variations, a clear connection of the newly observed bubble ejection events to the well-known, poorly collimated HH 7-11 system of Herbig-Haro objects is established. Astrometry of the youngest of the expanding shock fronts at 3 epochs covering a time span of over two years gives kinematic ages for two of these. The kinematic age of the youngest bubble is slightly older than the historically observed last photometric outburst of SVS 13 in 1990, consistent with that event launching the bubble and some deceleration of its expansion. A re-evaluation of historic infrared photometry and new data show that SVS 13 has not yet returned to its brightness before that outburst and thus shows a behavior similar to FUor outbursts, albeit with a smaller amplitude. We postulate that the creation of a series of bubbles and the changes in outflow direction are indicative of a precessing disk and accretion events triggered by a repetitive phenomenon possibly linked to the orbit of a close binary companion. We have tried, but failed to detect, the kinematic signature of rotation of the microjet in the [FeII] emission line at 1.644 micrometers.

AN ASYMMETRIC JET-LAUNCHING MODEL FOR THE PROTOPLANETARY NEBULA CRL 618

We propose an asymmetrical jet ejection mechanism in order to model the mirror symmetry observed in the lobe distribution of some protoplanetary nebulae (pPNe), such as the pPN CRL 618. 3D hydrodynamical simulations of a precessing jet launched from an orbiting source were carried out including an alternation in the ejections of the two outflow lobes, depending on which side of the precessing accretion disk is hit by the accretion column from a Roche lobe-filling binary companion. Both synthetic optical emission maps and position-velocity (PV) diagrams were obtained from the numerical results with the purpose of carrying out a direct comparison with observations. Depending on the observer's point of view, multipolar morphologies are obtained which exhibit a mirror symmetry at large distances from the central source. The obtained lobe sizes and their spatial distribution are in good agreement with the observed morphology of the pPN CRL 618. We also obtain that the kinematic ages of the fingers are similar to those obtained in the observations.

From the ashes: JVLA observations of water fountain nebula candidates show the rebirth of IRAS 18455+0448

[abridged] The class of water fountain nebulae is thought to represent the stage of the earliest onset of collimated bipolar outflows during the post-Asymptotic Giant Branch phase. They thus play a crucial role in the study of the formation of bipolar Planetary Nebulae (PNe). To date, 14 water fountain nebulae have been identified. The identification of more sources in this unique stage of stellar evolution will enable us to study the origin of bipolar PNe morphologies in more detail. We present the results of seven sources observed with the JVLA that were identified as water fountain candidates in an Effelsberg 100m telescope survey of 74 AGB and early post-AGB stars. We find that our sample of water fountain candidates displays strong variability in their 22 GHz H2O maser spectra. The JVLA observations show an extended bipolar H2O maser outflow for one source, the OH/IR star IRAS 18455+0448. This source was previously classified as a 'dying' OH/IR star based on the exponential decrease of its 1612 MHz OH maser and the lack of H2O masers. We therefore also re-observed the 1612, 1665, and 1667 MHz OH masers. We confirm that the 1612 MHz masers have not reappeared and find that the 1665/1667 MHz masers have decreased in strength by several orders of magnitude during the last decade. The OH/IR star IRAS 18455+0448 is confirmed to be a new addition to the class of water fountain nebulae. Its kinematic age is approximately 70 yr, but could be lower, depending on the distance and inclination. Previous observations indicate, with significant uncertainty, that IRAS 18455+0448 has a surprisingly low mass compared to available estimates for other water fountain nebulae. The available historical OH maser observations make IRAS 18455+0448 unique for the study of water fountain nebulae and the launch of post-AGB bipolar outflows...

VLT observations of the asymmetric Etched Hourglass Nebula, MyCn 18

Context. The mechanisms that form extreme bipolar planetary nebulae remain unclear. Aims. The physical properties, structure, and dynamics of the bipolar planetary nebula, MyCn 18, are investigated in detail with the aim of understanding the shaping mechanism and evolutionary history of this object. Methods. VLT infrared images, VLT ISAAC infrared spectra, and long-slit optical Echelle spectra are used to investigate MyCn 18. Morpho-kinematic modelling was used to firmly constrain the structure and kinematics of the source. A timescale analysis was used to determine the kinematical age of the nebula and its main components. Results. A spectroscopic study of MyCn 18's central and offset region reveals the detailed make-up of its nebular composition. Molecular hydrogen, atomic helium, and Bracket gamma emission are detected from the central regions of MyCn 18. ISAAC spectra from a slit position along the narrow waist of the nebula demonstrate that the ionised gas resides closer to the centre of the nebula than the molecular emission. A kinematical age of the nebula and its components were obtained by the P-V arrays and timescale analysis. Conclusions. The structure and kinematics of MyCn 18 are better understood using an interactive 3-D modelling tool called shape. A dimensional and timescale analysis of MyCn 18's major components provides a possible mechanism for the nebula's asymmetry. The putative central star is somewhat offset from the geometric centre of the nebula, which is thought to be the result of a binary system. We speculate that the engulfing and destruction of an exoplanet during the AGB phase may have been a key event in shaping MyCn 18 and generating of its hypersonic knotty outflow.

A SPECTROSCOPIC AND PHOTOMETRIC STUDY OF THE PLANETARY NEBULAE Kn 61 AND Pa 5

We present the first morpho-kinematical analysis of the planetary nebulae Kn 61 and Pa 5 and explore the nature of their central stars. Our analysis is based on high resolution and medium resolution spectroscopic observations, deep narrow-band imaging, and integral photometry. This material allows us to identify the morphological components and study their kinematics. The direct images and spectra indicate an absence of the characteristic [N II] and [S II] emission lines in both nebulae. The nebular spectrum of Kn 61 suggests an hydrogen deficient planetary nebula and the stellar spectrum of the central star reveals a hydrogen deficient PG 1159-type star. The [O III] position velocity diagram reveals that Kn 61 is a closed, empty, spherical shell with a thin border and a filamentary surface expanding at 67.6 km s$^{-1}$, and the shell is currently not expanding isotropically. We derived a kinematic age of $\sim$ 1.6$\times$10$^4$ yrs for an assumed distance of 4 kpc. A photometric period of $\sim$ 5.7($\pm$0.4) days has been detected for Kn 61, indicating presence of a possible binary system at its core. A possible link between filamentary, spherical shells and PG 1159-type stars is noted. The morphology of Pa 5 is dominated by an equatorial toroid and faint polar extensions. The equatorial region of this planetary nebula is expanding at 45.2 kms$^{-1}$. The stellar spectrum corresponds to a very hot star and is dominated by a steep blue rising continuum and He II, Balmer and Ca II photospheric lines.

The supergiant shell with triggered star formation in the dwarf irregular galaxy IC 2574: neutral and ionized gas kinematics

We analyse the ionized gas kinematics in the star formation regions of the supergiant shell (SGS) of the IC 2574 galaxy using observations with the Fabry-Perot interferometer at the 6-m telescope of SAO RAS; the data of the THINGS survey are used to analyze the neutral gas kinematics in the area. We perform the 'derotation' of the H-alpha and HI data cubes and show its efficiency in kinematics analysis. We confirm the SGS expansion velocity 25 km/s obtained by Walter & Brinks (1999) and conclude that the SGS is located at the far side of the galactic disc plane. We determine the expansion velocities, kinematic ages, and the required mechanical energy input rates for four star formation complexes in the walls of the SGS; for the remaining ones we give the limiting values of the above parameters. A comparison with the age and energy input of the complexes' stellar population shows that sufficient energy is fed to all HII regions except one. We discuss in detail the possible nature of this region and that of another one, which was believed to be an SNR according to radio observations. We measured the expansion velocity of the latter and confirm its identification as an old SNR. Our observations allowed us to identify a faint diffuse H-alpha emission inside the SGS which was never observed before.

The evolving spectrum of the planetary nebula Hen 2-260

We analysed the planetary nebula Hen 2-260 using optical spectroscopy and photometry. We compared our observations with the data from literature to search for evolutionary changes. The nebular line fluxes were modelled with the Cloudy photoionization code to derive the stellar and nebular parameters. The planetary nebula shows a complex structure and possibly a bipolar outflow. The nebula is relatively dense and young. The central star is just starting $\rm O^+$ ionization ($\rm T_{eff} \approx 30,000 \, K$). Comparison of our observations with literature data indicates a 50% increase of the [OIII] 5007 \AA\ line flux between 2001 and 2012. We interpret it as the result of the progression of the ionization of $\rm O^{+}$. The central star evolves to higher temperatures at a rate of $\rm 45 \pm 7\,K\, yr^{-1}$. The heating rate is consistent with a final mass of $\rm 0.626 ^{+0.003}_{-0.005} \, M_{\odot}$ or $\rm 0.645 ^{+0.008}_{-0.008} \, M_{\odot}$ for two different sets of post-AGB evolutionary tracks from literature. The photometric monitoring of Hen 2-260 revealed variations on a timescale of hours or days. The variability may be caused by pulsations of the star. The temperature evolution of the central star can be traced using spectroscopic observations of the surrounding planetary nebula spanning a timescale of roughly a decade. This allows us to precisely determine the stellar mass, since the pace of the temperature evolution depends critically on the core mass. The kinematical age of the nebula is consistent with the age obtained from the evolutionary track. The final mass of the central star is close to the mass distribution peak for central stars of planetary nebulae found in other studies. The object belongs to a group of young central stars of planetary nebulae showing photometric variability.

A very young, compact bipolar H2O maser outflow in the intermediate-mass star-forming LkHα 234 region

We report multi-epoch Very Long Baseline Interferometry (VLBI) H2O maser observations towards the compact cluster of young stellar objects (YSOs) close to the Herbig Be star LkHα 234. This cluster includes LkHα 234 and at least nine more YSOs that are formed within projected distances of ∼10 arcsec (∼9000 au). We detect H2O maser emission towards four of these YSOs. In particular, our VLBI observations (including proper motion measurements) reveal a remarkable very compact (∼0.2 arcsec = ∼180 au), bipolar H2O maser outflow emerging from the embedded YSO Very Large Array (VLA) 2. We estimate a kinematic age of ∼40 yr for this bipolar outflow, with expanding velocities of ∼20 km s−1 and momentum rate ṀwVw ≃ 10−4–10−3 M⊙ yr−1 km s−1 × (Ω/4π), powered by a YSO of a few solar masses. We propose that the outflow is produced by recurrent episodic jet ejections associated with the formation of this YSO. Short-lived episodic ejection events have previously been found towards high-mass YSOs. We show now that this behaviour is also present in intermediate-mass YSOs. These short-lived episodic ejections are probably related to episodic increases in the accretion rate, as observed in low-mass YSOs. We predict the presence of an accretion disc associated with VLA 2. If detected, this would represent one of the few known examples of intermediate-mass stars with a disc–YSO–jet system at scales of a few hundred astronomical units.

The rapid evolution of the exciting star of the Stingray nebula

SAO244567, the exciting star of the Stingray nebula, is rapidly evolving. Previous analyses suggested that it has heated up from an effective temperature of about 21kK in 1971 to over 50kK in the 1990s. Canonical post-asymptotic giant branch evolution suggests a relatively high mass while previous analyses indicate a low-mass star. Fitting line profiles from static and expanding non-LTE model atmospheres to the observed UV and optical spectra, taken during 1988-2013, allowed us to study the temporal change of effective temperature, surface gravity, mass-loss rate, and terminal wind velocity. In addition, we determined the chemical composition of the atmosphere. We find that the central star has steadily increased its effective temperature from 38kK in 1988 to a peak value of 60kK in 2002. During the same time, the star was contracting, as concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a drop in luminosity. Simultaneously, the mass-loss rate declined from log (dM/dt/Msun/yr)=-9.0 to -11.6 and the terminal wind velocity increased from 1800km/s to 2800km/s. Since around 2002, the star stopped heating and has cooled down again to 55kK by 2006. It has a largely solar surface composition with the exception of slightly subsolar carbon, phosphorus, and sulfur. By comparison with stellar-evolution calculations, we confirm that SAO244567 must be a low-mass star (M < 0.55 Msun). However, the slow evolution of the respective stellar evolutionary models is in strong contrast to the observed fast evolution and the young planetary nebula with a kinematical age of only about 1000 years. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of close-binary evolution. Then SAO244567 would be a low-mass (0.354 Msun) helium prewhite dwarf after the common-envelope phase, during which the planetary nebula was ejected.

Stellar substructures in the solar neighbourhood

From correlations between orbital parameters, several new coherent groups of stars were recently identified in the Galactic disc and suggested to correspond to remnants of disrupted satellites. To reconstruct their origin at least three main observational parameters - kinematics, chemical composition and age - must be known. We determine detailed elemental abundances in stars belonging to the so-called Group 2 of the Geneva-Copenhagen Survey and compare the chemical composition with Galactic thin- and thick-disc stars, as well as with the Arcturus and AF06 streams. The aim is to search for chemical signatures that might give information about the formation history of this kinematic group of stars. High-resolution spectra were obtained with the FIES spectrograph at the Nordic Optical Telescope, La Palma, and were analysed with a differential model atmosphere method. Comparison stars were observed and analysed with the same method. The average value of [Fe/H] for the 32 stars of Group 2 is -0.42 +- 0.10 dex. The investigated group consists mainly of two 8- and 12-Gyr-old stellar populations. Abundances of oxygen, alpha-elements, and r-process-dominated elements are higher than in Galactic thin-disc dwarfs. This elemental abundance pattern has similar characteristics as that of the Galactic thick-disc. The similarity in chemical composition of stars in Group 2 with that in stars of the thick-disc might suggest that their formation histories are linked. The chemical composition together with the kinematic properties and ages of stars in the investigated stars provides evidence of their common origin and possible relation to an ancient merging event. A gas-rich satellite merger scenario is proposed as the most likely origin. Groups 2 and 3 of the Geneva-Copenhagen Survey might have originated in the same merging event.

The origin of the young pulsar PSR J0826+2637 and its possible former companion HIP 13962

We aim to identify the birth place of the young pulsar PSR J0826+2637 in order to determine its kinematic age and give constraints on its radial and spatial (kick) velocity. Since the majority of neutron star (NS) progenitors are in associations or clusters, we search for a possible origin of the NS inside such stellar groups. We trace back the NS and the centres of possible birth associations and clusters to find close encounters. The kinematic age is then given by the time since the encounter. We use Monte Carlo simulations to account for observational uncertainties and the unknown radial velocity of the NS. We evaluate the outcome statistically. In order to find further indication for our findings, we also search for a runaway star that could be the former companion if it exists. We find that PSR J0826+2637 was probably born in the small young cluster Stock 7 ~3 Myr ago. This result is supported by the identification of the former companion candidate HIP 13962 (runaway star with spectral type G0Ia). The scenario predicts a near-zero radial velocity of the pulsar implying an inclination angle of its motion to the line-of-sight of 87+/-11 deg. We also present the chemical abundances of HIP 13962. We do not find enhanced alpha element abundances in the highly evolved star. However, the binary supernova scenario may be supported by the overabundance of r-process elements that could have been ejected during the supernova and accreted by the runaway star. Also, a high rotational velocity of v sin i ~29 km/s of HIP 13962 is consistent with evolution in a pre-SN binary system.

SHORT-TERM RADIO VARIABILITY AND PARSEC-SCALE STRUCTURE IN A GAMMA-RAY NARROW-LINE SEYFERT 1 GALAXY 1H 0323+342

We made simultaneous single-dish and very long baseline interferometer (VLBI) observations of a narrow-line Seyfert 1 galaxy (NLS1) 1H 0323+342, showing gamma-ray activity revealed by Fermi/LAT observations. We found significant variation of the total flux density at 8 GHz on the time scale of one month by the single-dish monitoring. The total flux density varied by 5.5% in 32 days, which is comparable to the gamma-ray variability time scale, corresponding to the variability brightness temperature of $7.0 \times 10^{11}$ K. The source consists of central and southeastern components on the parsec (pc) scale. The flux of only the central component decreased in the same way as the total flux density, indicating that the short-term radio variability, and probably the gamma-ray emitting region, is associated with this component. From the VLBI observations we obtained the brightness temperatures of greater than $(5.2 \pm 0.3) \times 10^{10}$ K, and derived the equipartition Doppler factor of greater than 1.7, the variability Doppler factor of 2.2, and the 8 GHz radio power of $10^{24.6}$ W Hz$^{-1}$. Combining them we conclude that acceleration of radio jets and creation of high-energy particles are ongoing in the central engine, and that the apparent very radio-loud feature of the source is due to the Doppler-boosting effect, resulting in the intrinsic radio loudness to be an order of magnitude smaller than the observed values. We also conclude that the pc-scale jet represents recurrent activity from the the spectral fitting and the estimated kinematic age of pc- and kpc-scale extended components with different position angle.

On the kinematic age of RZ Psc

RZ Psc belongs to the family of young UX Ori stars whose photometric activity is due to strong extinction variations in the circumstellar disks surrounding them. However, in contrast to all the remaining stars of this type, no evidence of youth has been detected for RZ Psc until recently. A rough estimate of the star’s kinematic age was made for the first time in our previous paper (Grinin et al. 2010). It shows that RZ Psc is intermediate in its evolutionary status between young stars in Orion and stars with debris disks. In this paper, we provide a refined estimate of the kinematic age for the star confirming this conclusion. According to this estimate, the age of RZ Psc is approximately 25 ± 5 Myr at M* = 1M⊙.

THE PARSEC-SCALE ENVIRONMENT AND THE EVOLUTIONARY STATUS OF MWC 349A

We report on a study of the ~10' (~5 pc in projection) environs of the peculiar, high-luminosity emission-line star MWC 349A in the IR, radio, and visible domains. Besides the recently discovered X-shaped, arcmin-scale IR nebula centered on MWC 349A (X nebula), with the kinematic age of ~104 yr, we identify several young objects pointing toward an ongoing process of active star formation in this region and estimate some physical parameters of the newly discovered objects. The radiation of the X nebula is due to a geometrically and optically thin dust front heated to Td 60-70 K by the radiation of the central star. The bipolar dust front probably results from the interaction of a powerful stellar wind with the circumstellar disk. One of the related objects is an elongated, cold molecular cloud, ~1 pc in size, adjacent to MWC 349A in projection and having the same radial velocity (V LSR +9 km s–1). The proximity of the molecular cloud may indicate that MWC 349A was born locally rather than being a runaway object ejected from the core of Cyg OB2 several Myr ago. If it is still associated with its natal cloud, MWC 349A may be a rare example of the observable pre-main-sequence stage of a ~30 M ☉ star. If the association with the molecular cloud is an effect of projection, however, MWC 349A may already be an evolved star, even if it was born locally. We discuss future observations that may shed more light on the evolutionary status of this unique object.

BROAD N2H+EMISSION TOWARD THE PROTOSTELLAR SHOCK L1157-B1

We present the first detection of N2H+ towards a low-mass protostellar outflow, namely the L1157-B1 shock, at about 0.1 pc from the protostellar cocoon. The detection was obtained with the IRAM 30-m antenna. We observed emission at 93 GHz due to the J = 1-0 hyperfine lines. The analysis of the emission coupled with the HIFI CHESS multiline CO observations leads to the conclusion that the observed N2H+(1-0) line originates from the dense (> 10^5 cm-3) gas associated with the large (20-25 arcsec) cavities opened by the protostellar wind. We find a N2H+ column density of few 10^12 cm-2 corresponding to an abundance of (2-8) 10^-9. The N2H+ abundance can be matched by a model of quiescent gas evolved for more than 10^4 yr, i.e. for more than the shock kinematical age (about 2000 yr). Modelling of C-shocks confirms that the abundance of N2H+ is not increased by the passage of the shock. In summary, N2H+ is a fossil record of the pre-shock gas, formed when the density of the gas was around 10^4 cm-3, and then further compressed and accelerated by the shock.