Mass Of Central Star Research Articles

Planetary nebula candidates in extragalactic young star clusters

Context. During an analysis of optical spectra of 80 young star clusters in several nearby spiral galaxies, [Oiii] and [Nii] emission lines were noted in some cases. Three of these emission line sources are associated with clusters older than 30 Myr, and are identified as likely planetary nebula (PN) candidates. Aims. These objects may represent a rare opportunity to study PNe whose progenitor stars are known to be of intermediate masses, although detailed analysis is challenging because of the underlying strong continuum from the cluster stars. This paper presents and discusses basic properties of the PN candidates and their host clusters. Methods. Based on the observed emission line fluxes, the excitation parameters and luminosities of the nebulae are derived. This allows a crude placement of the central stars in two of the objects on the H-R diagram. Cluster ages and masses are estimated from broad-band colours and by fitting model SSP spectra to the observed spectra. Results. The two PN candidates where central star luminosities and temperatures can be estimated are found to be consistent with post-AGB model tracks for a central star mass of about 0.60 M . One of the host clusters has an age of 32–65 Myr, corresponding to a main sequence turn-off mass of MTO = 6.6–9.0 M . For the other cluster the age is 282–407 Myr, corresponding to MTO = 3.2–3.6 M . By estimating the number of stars evolving off the main sequence per year, a total of 6 PNe are expected in the full sample of 80 clusters for a PN lifetime of 104 years. The factor of two disagreement with the actual observed number may be due, among other things, to uncertainties in PN lifetimes. It is interesting to note that all three PN candidates are associated with clusters which are more diffuse than average. Conclusions. While PNe have previously been found in some old globular clusters, the candidates identified here are among the first identified in young star clusters.

Dense molecular clouds and associated star formation in the Magellanic Clouds

AbstractMultiwavelengths studies of massive star formation regions in the LMC and SMC reveal that a second generation of stars is being formed in dense molecular clouds located in the surroundings of the massive clusters. These dense molecular clouds have survived the action of massive star UV radiation fields and winds and they appear as compact dense H2 knots in regions of weak CO emission. Alternatively, we have found that large molecular clouds, probably remnants of the parental giant molecular clouds where the first generation of stars were formed, are suffering the interaction of the winds and UV radiation field in their surfaces in the direction of the central massive cluster or massive stars. These molecular regions show 1.2 mm continuum emission form cold dust and they show embedded IR sources as determined from deep ground base JHKs imaging. The distribution of young IR sources as determined from their Mid IR colors obtained by SPITZER concentrate in the maxima of CO and dust emission. IR spectroscopy of the embedded sources with high IR excess confirm their nature as massive young stellar objects (MYSO's). Our results are suggestive of contagious star formation where triggering and induced star formation could be taking place.

On near Chandrasekhar mass central stars of planetary nebulae

A recent spectroscopic analysis of central stars of planetary nebulae (CSPNe) claims that the sample includes five CSPNe with masses very close to the Chandrasekhar limit of white dwarfs. This claim should be verified or discarded from the available kinematical and chemical abundance information. Kinematical parameters are extracted from Galactic orbits and compared with parameters expected for populations of different ages. The chemistry of the nebulae is compared with average values for different types. The reported high masses are not supported by our investigation. The claimed high central star masses are in contradiction with all other evidence. A more consistent picture emerges if CSPN masses close to the peak of the white dwarf mass distribution are assumed.

AChandraACIS Study of 30 Doradus. I. Superbubbles and Supernova Remnants

We present an X-ray tour of diffuse emission in the 30 Doradus star-forming complex in the Large Magellanic Cloud using high spatial resolution X-ray images and spatially resolved spectra obtained with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. The dominant X-ray feature of the 30 Doradus nebula is the intricate network of diffuse emission generated by interacting stellar winds and supernovae working together to create vast superbubbles filled with hot plasma. We construct maps of the region showing variations in plasma temperature (T = 3-9 million degrees), absorption [NH = (1-6) × 1021 cm-2], and absorption-corrected X-ray surface brightness [SX = (3-126) × 1031 ergs s-1 pc-2]. Enhanced images reveal the pulsar wind nebula in the composite supernova remnant N157B, and the Chandra data show spectral evolution from nonthermal synchrotron emission in the N157B core to a thermal plasma in its outer regions. In a companion paper we show that R136, the central massive star cluster, is resolved at the arcsecond level into almost 100 X-ray sources. Through X-ray studies of 30 Doradus the complete life cycle of such a massive stellar cluster can be revealed.

The disc-jet connection

A large body of theoretical and computational work shows that jets - modelled as magnetized disk winds - exert an external torque on their underlying disks that can efficiently remove angular momentum and act as major drivers of disk accretion. These predictions have recently been confirmed in direct HST measurements of the jet rotation and angular momentum transport in low mass protostellar systems. We review the theory of disc winds and show that their physics is universal and scales to jets from both low and high mass star forming regions. This explains the observed properties of outflows in massive star forming regions, before the central massive star generates an ultracompact HII region. We also discuss the recent numerical studies on the formation of massive accretion disks and outflows through gravitational collapse, including our own work on 3D Adaptive Mesh simulations (using the FLASH code) of the hydromagnetic collapse of an initial rotating, and cooling Bonner-Ebert sphere. Magnetized collapse gives rise to outflows. Our own simulations show that both a jet-like disk wind on sub AU scales, and a larger scale molecular outflow occur (Banerjee and Pudritz 2005).

Photoevaporated Disks around Massive Young Stars

We present a parametric model of an isothermal photoevaporating wind from a Keplerian disk around a young massive star. The wind is produced by high-energy photons (hν > 13.6 eV) from the massive central star that ionize and heat the material of the circumstellar disk, as discussed several years ago by Hollenbach and coworkers. The model gives the velocity and density structure of the ionized winds. Because of its simplicity, it is very useful to explore the physical conditions of this type of wind. The model can also describe the photoevaporation of disks around low-mass young stars. We obtain the free-free continuum emission of the photoevaporated flows and compare with the spectral energy distribution of the known bipolar sources MWC 349 A and NGC 7538 IRS 1.

The Low‐ and Intermediate‐Mass Stellar Population in the Small Magellanic Cloud: The Central Stars of Planetary Nebulae

We present a study on the central stars (CSs) of planetary nebulae (PNs) observed in the Small Magellanic Cloud (SMC) with the Space Telescope Imaging Spectrograph (STIS) instrument on board the Hubble Space Telescope (HST). The stellar magnitudes have been measured using broadband photometry, and Zanstra analysis of the nebulae provided the stellar temperatures. From the location of the CSs on the H-R diagram, and by comparing the observed CSs with current models of stellar evolution, we infer the CS masses. We examine closely the possibility of light contamination in the bandpass from an unrecognized stellar companion, and we establish strong constraints on the existence and nature of any binary companion. We find an average mass of 0.63 M☉, which is similar to the mass obtained for a sample of CSs in the LMC (0.65 M☉). However, the SMC and LMC CS mass distributions differ slightly, with the SMC sample lacking an intermediate-mass stellar population (0.65-0.75 M☉). We discuss the significance and possible reasons for the difference between the two mass distributions. In particular, we consider the differences in the star formation history between the clouds and the mass-loss rate dependence on metallicity.

Blue Blob in the Red Rectangle

ASTROPHYSICS The Red Rectangle nebula is an unusual protoplanetary nebula. Its central star, HD44179, is dying, and substantial mass is being blown off the star in a bipolar outflow, creating the rectangular shape and a large amount of dust. It is the brightest source of unidentified infrared band emission (UIR), and much of this comes from carbon compounds. Given that 20 to 30% of the Milky Way's infrared radiation is in the UIR and that carbon is an important component of stars, planets, and terrestrial life, it is desirable to identify the carbon species that account for the emissions. At shorter wavelengths, in the visible to ultraviolet, the Red Rectangle looks like a blue blob, produced by dust-scattered light from the central star. Vijh et al. obtained spectra at these wavelengths, and the peaks correspond to those in fluorescence spectra of electronic transitions in neutral polycyclic aromatic hydrocarbons (PAHs) with three to four rings, such as anthracene and pyrene. Thus, not only is the Red Rectangle producing a lot of PAH dust, but blue luminescence may be used to identify many carbon and silicon species that fluoresce at specific wavelengths. — LR Astrophys. J. , in press (astro-ph/0403522).

The evolution of planetary nebulae

We follow hydrodynamically the evolution of spherical model planetary nebulae subject to different initial conditions and with various central stars, investigating how combinations of central-star mass and asymptotic giant branch mass-loss rate determine the shape and kinematics of a planetary nebula. With this approach we aim at constituting a framework useful for the interpretation of the evolutionary status and previous mass-loss history of observed individual nebulae, making use of their kinematical properties and surface brightness characteristics. In particular, the models are compared with the observed morphologies and kinematics of double shell nebulae. The dynamical structure of all the models is characterized by a more or less complicated shock wave pattern set up by ionization and wind interaction whose combined action results in general in a typical double-shell structure. We have found that models with simple initial structures based on a constant AGB mass-loss rate fail to comply with observed shell morphologies and surface-brightness distributions. A reasonable agreement with the observations is only found for a model where the mass-loss rate is strongly increasing towards the end of the asymptotic giant-branch evolution. Depending on the central star's evolutionary speed and the density of the cool wind expelled along the asymptotic giant-branch, planetary nebulae may never get optically thin. This is primarily the case for the more massive central stars, and this fact offers a rather natural explanation for the long standing problem of the very existence of molecular hydrogen in the immediate vicinity of hot central stars. We also show that distances to planetary nebulae based on expansion parallaxes are systematically too small by a significant amount.

The correlation between planetary nebula morphology and radio brightness temperatures

We have used model central star evolutionary tracks to evaluate the expected variation of 5 GHz brightness tempera- tures TB in planetary nebulae. Using these results, and previously derived central star mass functions, we find that there should be a correlation between TB and the mean central star massMCS� , and also between TB and nebular morphology. An analysis of the available observational results suggests that such a correlation between TB and nebular morphology does indeed exist. Bipolar nebulae appear to be concentrated towards high brightness temperatures, whilst circular sources tend to possess lower brightness temperature shells. Elliptical sources are present (and common) at all values of TB .T here are differences however between the observed and deduced morphological variations, and these enable us to place constraints upon the progenitor masses of the nebulae. In particular, it is clear that higher mass progenitors are likely to give rise to both bipolar and elliptical outflows; a result which is consistent with analyses of nebular abundances, Zanstra temperatures, and line excitations.

Post–Asymptotic Giant Branch Evolution in the Large Magellanic Cloud: A Study of the Central Stars of Planetary Nebulae

We present medium- and broadband Hubble Space Telescope (HST) photometry of a sample of 35 central stars (CSs) of planetary nebulae (PNs) in the Large Magellanic Cloud (LMC). The observations were made with the Wide Field Planetary Camera 2 (WFPC2) and Space Telescope Imaging Spectrograph instruments on board the HST. By observing LMC objects, our sample is free of the distance uncertainty that is the dominant source of error in the determination of CS luminosities in Galactic PNs. By observing with the HST we resolve the nebula, and therefore we often detect the CSs unambiguously. We obtain core masses of 16 of the objects by comparing their positions on the H-R diagram with theoretical evolutionary tracks, once we determine the stellar effective temperature through Zanstra analysis. This sample of CS masses is the largest and most reliable set obtained in an extragalactic environment. We find an average mass of 0.65 M?, although a few of the objects have very high mass. This average value is consistent with the average mass of the white dwarf population in the Galaxy. As the immediate precursors of white dwarfs, the study of the mass distribution of PN CSs should help to constrain the initial-to-final mass relation within environments of differing metallicity. Finally, by exploring the physical connections between the star and the nebula, we establish the importance of the study of PNs in the LMC to constrain the energy input from the wind during the post-asymptotic giant branch phase.

Observed Evolution of the Nebulae and Central Stars

Several points involved in observational studies of evolution of planetary nebulae and their central stars are discussed. It concerns observed masses of nebulae and central stars, nature of H-deficient central stars, role of time-dependent ionization in the nebular haloes, and a recent discovery of strong enrichements in some planetary nebulae.

Self‐similar Champagne Flows in HiiRegions

We consider the idealized expansion of an initially self-gravitating, static, singular, isothermal cloud core. For t ≥ 0, the gas is ionized and heated to a higher uniform temperature by the formation of a luminous but massless star in its center. The approximation that the mass and gravity of the central star are negligible for the subsequent motion of the H II region holds for distances r much greater than ~100 AU and for the massive cloud cores that give rise to high-mass stars. If the initial ionization and heating are approximated to occur instantaneously at t = 0, then the subsequent flow (for r ≫ 100 AU) caused by the resulting imbalance between self-gravity and thermal pressure is self-similar. Because of the steep density profile (ρ ∝ r-2), pressure gradients produce a shock front that travels into the cloud, accelerating the gas to supersonic velocities in what has been called the "champagne phase." The expansion of the inner region at t > 0 is connected to the outer envelope of the now ionized cloud core through this shock, whose strength depends on the temperature of the H II gas. In particular, we find a modified Larson-Penston (L-P) type of solution as part of the linear sequence of self-similar champagne outflows. The modification involves the proper insertion of a shock and produces the right behavior at infinity (v → 0) for an outflow of finite duration, reconciling the long-standing conflict on the correct (inflow or outflow) interpretation for the original L-P solution. For realistic heating due to a massive young central star that ionizes and heats the gas to ~104 K, we show that even the self-gravity of the ionized gas of the massive molecular cloud core can be neglected. We then study the self-similar solutions of the expansion of H II regions embedded in molecular clouds characterized by more general power-law density distributions: ρ ∝ r-n with 3/2 < n < 3. In these cases, the shock velocity is an increasing function of the exponent n and diverges as n → 3. We show that this happens because the model includes an origin where the pressure driving the shock diverges because the enclosed heated mass is infinite. Our results imply that the continued photoevaporation of massive reservoirs of neutral gas (e.g., surrounding disks and/or globules) near the embedded ionizing source is required in order to maintain over a significant timescale the emission measure observed in champagne flows.

The entropic index of the planets of the solar-system

By application of the Tsallis-2 entropy and additional constraints the entropic index q of the set of solar-system planets is calculated as q=0.602. Confirmation of this value comes from the equation 1/(1− q)=1/ α min−1/ α max, where α min, α max are the end-points of the f( α) curve of the system. This produces the approximation q=0.608. For an entire planetary system, i.e. the central star-mass and the planet-masses, one can state that normally the entropic index is practically q=1.

The Correlations between Planetary Nebula Morphology and Central Star Evolution: Analysis of the Northern Galactic Sample

Northern Galactic planetary nebulae (PNs) are studied to disclose possible correlations between the morphology of the nebulae and the evolution of the central stars (CSs). To this end, we have built the best database available to date, accounting for homogeneity and completeness. We use updated statistical distances and an updated morphological classification scheme, and we calculate Zanstra temperatures for a large sample of PNs. With our study we confirm that round, elliptical, and bipolar PNs have different spatial distributions within the Galaxy, with average absolute distances to the Galactic plane of 0.73, 0.38, and 0.21 kpc, respectively. We also find evidence that the distributions of the CS masses are different across these morphological groups, although we do not find that CSs hosted by bipolar PNs are hotter, on average, than CSs within round and elliptical PNs. Our results are in broad agreement with previous analyses, indicating that round, elliptical, and bipolar PNs evolve from progenitors in different mass ranges and might belong to different stellar populations, as also indicated by the helium and nitrogen abundances of PNs of different morphology.

Are There Unstable Planetary Systems around White Dwarfs?

The presence of planets around solar-type stars suggests that many white dwarfs should have relic planetary systems. While planets closer than $\sim$ 5~AU will most likely not survive the post-main sequence lifetime of its parent star, any planet with semimajor axis $>$ 5~AU will survive, and its semimajor axis will increase as the central star loses mass. Since the stability of adjacent orbits to mutual planet-planet perturbations depends on the ratio of the planet mass to the central star's mass, some planets in previously stable orbits around a star undergoing mass loss will become unstable. We show that when mass loss is slow, systems of two planets that are marginally stable can become unstable to close encounters, while for three planets the timescale for close approaches decreases significantly with increasing mass ratio. These processes could explain the presence of anomalous IR excesses around white dwarfs that cannot be explained by close companions, such as G29-38, and may also be an important factor in explaining the existence of DAZ white dwarfs. The onset of instability through changing mass-ratios will also be a significant effect for planetary embryos gaining mass in protoplanetary disks.

Molecular tracers of photo-evaporating disks around young stars

Disks around massive young stellar objects, and disks around low-mass stars irradiated by nearby OB associations, are eroded by photo-evaporation. In the latter case, this erosion may be an important factor in planet formation. As Johnstone et al. (1998) have shown, photo-evaporating material is gravitationally retained within a critical radius from the star, and constitutes an envelope similar to a Photon-Dominated Region (PDR) that normally arises at the edge of a molecular cloud irradiated by a massive star. We explore the chemistry of such a PDR/disk system to examine the contribution that it may make to the molecular species that may be observed. The model is in two phases; firstly, a collapse from low density to a high density appropriate for a disk; and, secondly, a 2D calculation of the irradiation of disk material by the radiation field of the central massive star or nearby OB association. The model follows the chemistry self-consistently through both phases. We compute the column densities of species through the PDR/disk system, averaged over the disk. We validate our model by comparing predicted averaged molecular column densities with those of several species detected in the disk around the 10 solar mass star GL 2591, currently the sole example known of this kind of object. Results are in good agreement for a model in which the outer part of the PDR is hot while the inner part is cool, and in which the local ionization rate is comparable with that caused by cosmic rays in the local interstellar medium. We show that in addition to the four detected species, there should be many others also detectable in this system, including HCN, NH3 and CS. Similar conclusions should apply to other disks around massive stars. Disks around low-mass stars are much more common; our models show that when irradiated by a nearby OB association such disks with their attendant PDRs also generate a rich chemistry. No detections of molecules in such objects have yet been reported. However, the models suggest that averaged molecular column densities should be comparable to those detected in disks around massive stars (see references listed in Table 4 for molecules in GL 2591). Potential tracers of irradiated disks around low-mass stars include OH, CH3 and C2H. We note that the detection in a disk of PDR-type chemistry is a clear signature that the disk is undergoing erosion. Its duration is therefore limited, with consequences for planet formation.

The determination of progenitor masses for planetary nebulae

We have evaluated the likely progenitor masses MPG of nebulae having elliptical, circular and bipolar morphologies, using observed ratios between the populations of these sources, and deduced central star mass functions. We find that most bipolar nebulae (BPNe) are likely to arise from progenitors having mass MPG>2.3 M⊙ and spectral types earlier than A3.2, whilst circular sources are associated with progenitors of mass 1.0 M⊙<MPG<1.2 M⊙ and spectral range G1.9–F7.8. Elliptical sources arise from intermediate-mass progenitors. The procedures employed to determine these values are relatively insensitive to uncertainties in scaleheights and population ratios, and completely insensitive to uncertainties in the distance scale. They are, however, dependent upon the precise forms adopted for the initial–final and central star mass functions, and we discuss the sensitivity of MPG to uncertainties in these functions.

Chemical Abundances of the Planetary Nebula IC 4634 and Its Central Star

We have measured the spectral line intensities of the metal poor planetary nebula IC 4634. Using a photo-ionization model calculation, we try to fit the the optical and UV region spectra, i.e., Hamilton Echelle and IUE observations. From direct images, one expects complicated density variations, but the model predicts a range in densities that may be smaller than actually exist. We find N(sub epsilon) approximates 5000 /cubic meter. In spite of the geometrical complexity of the S shaped double-lobed structure, the simple photoionization model with a spherical symmetry can fit most emission lines, fairly well. The derived chemical composition has been compared with previous estimates and also with the Sun - The metallicity in IC 4634 appears to be lower than in the Sun or the average planetary nebula. The most likely temperature of the central ionizing source of IC 4634 appears to be about 55,000 K. We find a central star mass of about 0.55 Solar Mass from comparison with theoretical evolutionary tracks.

Chemical Abundances of Planetary Nebulae in the Bulge and Disk of M31

We derive abundances and central star parameters for 15 planetary nebulae (PNe) in M31: 12 in the bulge and 3 in a disk field 14 kpc from the nucleus. No single abundance value characterizes the bulge stars: although the median abundances of the sample are similar to those seen for PNe in the LMC, the distribution of abundances is several times broader, spanning over 1 decade. None of the PNe in our sample approach the super metal-rich ([Fe/H] ~ +0.25) expectations for the bulge of M31, although a few PNe in the sample of Stasinska, Richer, & Mc~Call (1998) come close. This [O/H] vs [Fe/H] discrepancy is likely due to a combination of factors, including an inability of metal-rich stars to produce bright PNe, a luminosity selection effect, and an abundance gradient in the bulge of M31. We show that PNe that are near the bright limit of the [OIII] 5007 planetary nebula luminosity function (PNLF) span nearly a decade in oxygen abundance, and thus, support the use of the PNLF for deriving distances to galaxies (Jacoby 1996) with differing metallicities. We also identify a correlation between central star mass and PN dust formation that partially alleviates any dependence of the PNLF maximum magnitude on population age. Additionally, we identify a spatially compact group of 5 PNe having unusually high O/H; this subgroup may arise from a recent merger, but velocity information is needed to assess the true nature of the objects.