Star Formation Mechanisms Research Articles

Discovery of high-velocity EHB stars in the globular cluster ω Centauri (NGC 5139)

We report the discovery of 45 high-velocity extreme horizontal branch (EHB) stars in the globular cluster Omega Centauri (NGC 5139). The tangential velocities of these EHB stars are determined to be in the range 93∼313 km s−1, with an average uncertainty of ∼27 km s−1. The central escape velocity of the cluster is determined to be in the range 60∼105 km s−1. These EHB stars are significantly more concentrated toward the cluster core compared with other cluster members. The formation mechanisms of these EHB stars are discussed. Our conclusions can be summarized as follows: (1) A comparison of the tangential velocities of these EHB stars to the central escape velocity of the cluster shows that most if not all of these EHB stars are unbound to the cluster; (2) These EHB stars obtained high velocities in the central cluster region no longer than ∼1 Myr ago and may be subsequently ejected from the cluster in the next ∼1 Myr; (3) If the progenitors of these EHB stars were single stars, then they may have experienced a fast mass-loss process. If the progenitors were in close binaries, then they may have formed through disruptions by the intermediate-mass black hole in the cluster center.

The abundance of26Al-rich planetary systems in the Galaxy

One of the most puzzling properties of the solar system is the high abundance at its birth of 26Al, a short-lived radionuclide with a mean life of 1 Myr. Now decayed, it has left its imprint in primitive meteoritic solids. The origin of 26Al in the early solar system has been debated for decades and strongly constrains the astrophysical context of the Sun and planets formation. We show that, according to the present understanding of star-formation mechanisms, it is very unlikely that a nearby supernova has delivered 26Al into the nascent solar system. A more promising model is the one whereby the Sun formed in a wind-enriched, 26Al-rich dense shell surrounding a massive star (M > 32 M⊙). We calculate that the probability of any given star in the Galaxy being born in such a setting, corresponding to a well-known mode of star formation, is of the order of 1%. It means that our solar system, though not the rule, is relatively common and that many exo-planetary systems in the Galaxy might exhibit comparable enrichments in 26Al. Such enrichments played an important role in the early evolution of planets because 26Al is the main heat source for planetary embryos.

The VISTA Orion mini-survey: star formation in the Lynds 1630 North cloud

The Orion cloud complex presents a variety of star formation mechanisms and properties and it is still one of the most intriguing targets for star formation studies. We present VISTA/VIRCAM near-infrared observations of the L1630N star forming region, including the stellar clusters NGC 2068 and NGC 2071, in the Orion molecular cloud B and discuss them in combination with Spitzer data. We select 186 young stellar object (YSO) candidates in the region on the basis of multi-colour criteria, confirm the YSO nature of the majority of them using published spectroscopy from the literature, and use this sample to investigate the overall star formation properties in L1630N. The K-band luminosity function of L1630N is remarkably similar to that of the Trapezium cluster, i.e., it presents a broad peak in the range 0.3-0.7 M$_\odot$ and a fraction of sub-stellar objects of $\sim$20%. The fraction of YSOs still surrounded by disk/envelopes is very high ($\sim$85%) compared to other star forming regions of similar age (1-2 Myr), but includes some uncertain corrections for diskless YSOs. Yet, a possibly high disk fraction together with the fact that 1/3 of the cloud mass has a gas surface density above the threshold for star formation ($\sim$129 M$_\odot$ pc$^{-2}$), points towards a still on-going star formation activity in L1630N. The star formation efficiency (SFE), star formation rate (SFR) and density of star formation of L1630N are within the ranges estimated for galactic star forming regions by the Spitzer "core to disk" and "Gould's Belt" surveys. However, the SFE and SFR are lower than the average value measured in the Orion A cloud and, in particular, lower than that in the southern regions of L1630. This might suggest different star formation mechanisms within the L1630 cloud complex.

The contribution of major mergers to the creation of spheroidal galaxies and the build up of stellar mass at z≃2

AbstractWe investigate the contribution of major mergers to star formation in spheroidal galaxies at z ~ 2. Galaxies are visually classified from a sample of massive galaxies in CANDELS. At the redshifts used, the observed morphological disturbances are due to recent major mergers as minor mergers are too faint. The percentage of blue spheroids showing morphological disturbances is 21 ± 4%, indicating that major mergers are not the dominant star formation mechanism in these galaxies. Thus, minor mergers or cold accretion are likely to be the main drivers of star formation. We investigate the U-band luminosity emission of the sample and find that only a small fraction of the cosmic L(U) is from galaxies involved in a major merger, ~30%. Using the ratio of specific star formation rate for LTGs to mergers and combining this with the results for the luminosity budget shows that only ~6% of the total L(U) emitted at z ~ 2 is due to the major merger process.

Comparing simulations of ionization triggered star formation and observations in RCW 120

Massive clumps within the swept-up shells of bubbles, like that surrounding the galactic HII region RCW 120, have been interpreted in terms of the Collect and Collapse (C&C) mechanism for triggered star formation. The cold, dusty clumps surrounding RCW 120 are arranged in an almost spherical shell and harbour many young stellar objects. By performing high-resolution, three-dimensional SPH simulations of HII regions expanding into fractal molecular clouds, we investigate whether the formation of massive clumps in dense, swept-up shells necessarily requires the C&C mechanism. In a second step, we use RADMC-3D to compute the synthetic dust continuum emission from our simulations, in order to compare them with observations of RCW 120 made with APEX-LABOCA at 870 micron. We show that a distribution of clumps similar to the one seen in RCW 120 can readily be explained by a non-uniform initial molecular cloud structure. Hence, a shell-like configuration of massive clumps does not imply that the C&C mechanism is at work. Rather, we find a hybrid form of triggering, which combines elements of C&C and Radiatively Driven Implosion (RDI). In addition, we investigate the reliability of deriving clump masses from their 870 micron emission. We find that for clumps with more than 100 M_sun the observational estimates are accurate to within a factor of two and that, even at these long wavelengths, it is important to account for the radiative heating from triggered, embedded protostars.

CHARACTERIZING THE BROWN DWARF FORMATION CHANNELS FROM THE INITIAL MASS FUNCTION AND BINARY-STAR DYNAMICS

The stellar initial mass function (IMF) is a key property of stellar populations. There is growing evidence that the classical star-formation mechanism by the direct cloud fragmentation process has difficulties reproducing the observed abundance and binary properties of brown dwarfs and very-low-mass stars. In particular, recent analytical derivations of the stellar IMF exhibit a deficit of brown dwarfs compared to observational data. Here we derive the residual mass function of brown dwarfs as an empirical measure of the brown dwarf deficiency in recent star-formation models with respect to observations and show that it is compatible with the substellar part of the Thies-Kroupa IMF and the mass function obtained by numerical simulations. We conclude that the existing models may be further improved by including a substellar correction term that accounts for additional formation channels like disk or filament fragmentation. The term "peripheral fragmentation" is introduced here for such additional formation channels. In addition, we present an updated analytical model of stellar and substellar binarity. The resulting binary fraction and the dynamically evolved companion mass-ratio distribution are in good agreement with observational data on stellar and very-low-mass binaries in the Galactic field, in clusters, and in dynamically unprocessed groups of stars if all stars form as binaries with stellar companions. Cautionary notes are given on the proper analysis of mass functions and the companion mass-ratio distribution and the interpretation of the results. The existence of accretion disks around young brown dwarfs does not imply that these form just like stars in direct fragmentation.

Observational perspective of the youngest phases of intermediate-mass stars

Intermediate-mass protostars are the long-forgotten actors in the star-formation scenario. Historically, intermediate-mass stars have been associated to the low-mass regime and not too much attention has been drawn to them as a separate group. However, although their pre-main sequence evolution should be similar to that of low-mass protostars, intermediate-mass young stellar objects share many characteristics with their higher mass counterparts. This suggests that the star-formation mechanism could be similar for all luminosities.

On the Origin of the Gamma-Ray Burst Redshift Distribution in the Early Universe

Gamma-ray bursts (GRBs) are the most powerful events in the universe, and are promising standard candles for cosmological studies. The data from NASA's Swift satellite reveal a distribution for the GRB number density that peaks at a redshift between 1 and 3. In this paper, we classify GRBs based on their duration and discuss the origin of their progenitors. We shed light on the formation mechanism of supermassive black holes and massive stars at the early universe, and show how this process and other related events can lead to a relatively high number of GRBs that peak at high redshift.

The H i-dominated low-surface-brightness galaxy KKR 17

We present new narrow-band (H$\alpha$ and [OIII]) imagings and optical spectrophotometry of HII regions for a gas-rich low surface brightness irregular galaxy, KKR 17. The central surface brightness of the galaxy is $\mu_0(B)$ = 24.15 $\pm$0.03 mag~sec$^{-2}$. The galaxy was detected by \emph{Arecibo Legacy Fast ALFA survey} (ALFALFA), and its mass is dominated by neutral hydrogen (HI) gas. In contrast, both the stellar masses of the bright HII and diffuse stellar regions are small. In addition, the fit to the spectral energy distribution to each region shows the stellar populations of HII and diffuse regions are different. The bright HII region contains a large fraction of O-type stars, revealing the recent strong star formation, whereas the diffuse region is dominated by median age stars, which has a typical age of $\sim$ 600 Myrs. Using the McGaugh's abundance model, we found that the average metallicity of KKR 17 is 12 + (O/H) = 8.0 $\pm$ 0.1. The star formation rate of KKR 17 is 0.21$\pm$0.04 M$_{\odot}$/yr, which is $\sim$1/5 of our Milky Way's. Based on the analysis results to young stellar clusters in HII region, it is found that the bright HII region showed two sub-components with different velocities and metallicities. This may be caused by the outflow of massive stars or merging events. However, the mechanism of triggering star formation in the HII region is still uncertain.

A CLASSIFICATION SCHEME FOR YOUNG STELLAR OBJECTS USING THEWIDE-FIELD INFRARED SURVEY EXPLORERAllWISE CATALOG: REVEALING LOW-DENSITY STAR FORMATION IN THE OUTER GALAXY

We present an assessment of the performance of WISE and the AllWISE data release in a section of the Galactic Plane. We lay out an approach to increasing the reliability of point source photometry extracted from the AllWISE catalog in Galactic Plane regions using parameters provided in the catalog. We use the resulting catalog to construct a new, revised young star detection and classification scheme combining WISE and 2MASS near and mid-infrared colors and magnitudes and test it in a section of the Outer Milky Way. The clustering properties of the candidate Class I and II stars using a nearest neighbor density calculation and the two-point correlation function suggest that the majority of stars do form in massive star forming regions, and any isolated mode of star formation is at most a small fraction of the total star forming output of the Galaxy. We also show that the isolated component may be very small and could represent the tail end of a single mechanism of star formation in line with models of molecular cloud collapse with supersonic turbulence and not a separate mode all to itself.

An Element in a Paradigm Shift? – Could Population II Stars Actually be Younger than Population I stars?

Recent astronomical observations have indicated the existence of supermassive black holes at the centres of most, perhaps all, galaxies, and also that immensely powerful relativistic jets of protons and electrons may occasionally emanate from some such supermassive black holes. Together these observations suggest a possible mechanism for alternative star formation, which may require a revision of the traditional classification of stars into a more recent Population I and a more ancient Population II. Rather than assumedly being members of an older generation of stars preceding Population I, maybe many Population II stars could perhaps actually be younger than Population I stars, with their lower metallicity being an effect of their much more recent creation from the pristine hydrogen left by such relativistic jets.

Binary pulsar J0737−3039 – evidence for a new core collapse and neutron star formation mechanism

The binary pulsar J0737-3039 is the only known system having two observable pulsars, thus offering a unique laboratory to test general relativity and explore pulsar physics. Based on the low eccentricity and the position within the galactic plane, Piran & Shaviv (2004, 2005) argued that pulsar B had a non-standard formation scenario with little or no mass ejection. They have also predicted that the system would have a very slow proper motion. Pulsar timing measurements (Kramer et al. 2006; Deller et al. 2009) confirmed this prediction. The recent observations of the alignment between the spin of pulsar A and the binary orbit is also in agreement with this scenario. Detailed simulations of the formation process of pulsar B enable us to show that its progenitor, just before the collapse, was a massive O-Ne-Mg white dwarf surrounded by a tenuous, 0.1-0.16 M_sun, envelope. This envelope was ejected when the white dwarf collapsed to form a neutron star. Pulsar B was born as a slow rotator (spin period ~ 1 s) and a kick received when the pulsar formed changed its spin direction to the current one. This realization sheds light on the angular momentum evolution of the progenitor star, a process which is strongly affected by interaction with the binary companion. The slow proper motion of the system also implies that the system must have undergone a phase of mass transfer in which Star A shed a significant fraction of its mass onto B.

Stellar distribution in the star-forming region Gamma Velorum

Different star formation scenarios predict a variety of forms for young star clusters. The recent discovery of two dynamically distinct populations in the young cluster Gamma Velorum allows us to investigate the star formation mechanism.

Numerical simulations of bubble-induced star formation in dwarf irregular galaxies with a novel stellar feedback scheme

To study the star formation and feedback mechanism, we simulate the evolution of an isolated dwarf irregular galaxy (dIrr) in a fixed dark matter halo, similar in size to WLM, using a new stellar feedback scheme. We use the new version of our original N-body/smoothed particle chemodynamics code, GCD+, which adopts improved hydrodynamics, metal diffusion between the gas particles and new modelling of star formation and stellar wind and supernovae (SNe) feedback. Comparing the simulations with and without stellar feedback effects, we demonstrate that the collisions of bubbles produced by strong feedback can induce star formation in a more widely spread area. We also demonstrate that the metallicity in star forming regions is kept low due to the mixing of the metal-rich bubbles and the metal-poor inter-stellar medium. Our simulations also suggest that the bubble-induced star formation leads to many counter-rotating stars. The bubble-induced star formation could be a dominant mechanism to maintain star formation in dIrrs, which is different from larger spiral galaxies where the non-axisymmetric structures, such as spiral arms, are a main driver of star formation.

A statistical study of gaseous environment of Spitzer interstellar bubbles

The expansion of interstellar bubbles is suggested to be an important mechanism of triggering material accumulation and star formation. In this work, we investigate the gaseous environment of a large sample of interstellar bubbles identified by the Spitzer space telescope, aiming to explore the possible evidence of triggered gas accumulation and star formation in a statistical sense. By cross-matching 6124 Spitzer interstellar bubbles from the Milky Way Project (MWP) and more than 2500 Galactic H ii regions collected by us, we obtain the velocity information for 818 MWP bubbles. To study the gaseous environment of the interstellar bubbles and get rid of the projection effect as much as possible, we constrain the velocity difference between the bubbles and the 13CO(1−0) emission extracted from the Galactic Ring Survey (GRS). Three methods: the mean azimuthally averaged radial profile of 13CO emission, the surface number density of molecular clumps and the angular cross-correlation function of MWP bubbles and the GRS molecular clumps are adopted. Significant over density of molecular gas is found to be close to the bubble rims. 60 per cent of the studied bubbles were found to have associated molecular clumps. By comparing the clump-associated and the clump-unassociated MWP interstellar bubbles, we reveal that the bubbles in associations tend to be larger and thicker in physical sizes. From the different properties shown by the bubble-associated and bubble-unassociated clumps, we speculate that some of the bubble-associated clumps result from the expansion of bubbles. The fraction of the molecular clumps associated with the MWP bubbles is estimated to be about 20 per cent after considering the projection effect. For the bubble-clump complexes, we found that the bubbles in the complexes with associated massive young stellar object(s) (MYSO(s)) have larger physical sizes, hence the complexes tend to be older. We propose that an evolutionary sequence might exist between the relatively younger MYSO-unassociated bubble-clump complexes and the MYSO-associated complexes.

Planet packing in circumbinary systems

The recent discovery of planets orbiting main sequence binaries will provide crucial constraints for theories of binary and planet formation. The formation pathway for these planets is complicated by uncertainties in the formation mechanism of the host stars. In this paper, we compare the dynamical states of single and binary star planetary systems. Specifically, we pose two questions: (1) What does it mean for a circumbinary system to be dynamically packed? (2) How many systems are required to differentiate between a population of packed or sparse planets? We determine when circumbinary systems become dynamically unstable as a function of the separation between the host-stars and the inner planet, and the first and second planets. We show that these represent unique stability constraints compared to single-star systems. We find that although the existing Kepler data is insufficient to distinguish between a population of packed or sparse circumbinary systems, a more thorough study of circumbinary TTVs combined with an order of magnitude increase in the number of systems may prove conclusive. Future space missions such as TESS provide the best opportunity for increasing the sample size.

INTRINSICALLY POLARIZED STARS AND IMPLICATION FOR STAR FORMATION IN THE CENTRAL PARSEC OF OUR GALAXY

We have carried out adaptive-optics assisted observations at the Subaru telescope, and have found 11 intrinsically polarized sources in the central parsec of our Galaxy. They are selected from 318 point sources with Ks<15.5, and their interstellar polarizations are corrected using a Stokes Q/I - U/I diagram. Considering brightness, near-infrared color excess, and the amount of intrinsic polarization, two of them are good young stellar object (YSO) candidates with an age of ~10^5 yr. If they are genuine YSOs, their existence provides strong constraints on star formation mechanisms in this region. In the remaining sources, two are known as bow-shock sources in the Northern arm. One other is also located in the Northern arm and shows very similar properties, and thus likely to be a so far unknown bow-shock source. The origin of the intrinsic polarization of the other sources is as yet uncertain.

Direct detection of the tertiary component in the massive multiple HD 150136 with VLTI

Massive stars are of fundamental importance for almost all aspects of astrophysics, but there still exist large gaps in our understanding of their properties and formation because they are rare and therefore distant. It has been found that most O-stars are multiples. HD 150 136 is the nearest system to Earth with >100 M_sol, and provides a unique opportunity to study an extremely massive system. Recently, evidence for the existence of a third component in HD 150 136, in addition to the tight spectroscopic binary that forms the main component, was found in spectroscopic observations. Our aim was to image and obtain astrometric and photometric measurements of this component using long baseline optical interferometry to further constrain the nature of this component. We observed HD150136 with the near-infrared instrument AMBER attached to the ESO VLT Interferometer. The recovered closure phases are robust to systematic errors and provide unique information on the source asymmetry. Therefore, they are of crucial relevance for both image reconstruction and model fitting of the source structure. The third component in HD 150 136 is clearly detected in the high-quality data from AMBER. It is located at a projected angular distance of 7.3 mas, or about 13 AU at the line-of-sight distance of HD 150 136, at a position angle of 209 degrees East of North, and has a flux ratio of 0.25 with respect to the inner binary. We resolved the third component of HD 150 136 in J, H and K filters. The luminosity and color of the tertiary agrees with the predictions and shows that it is also an O main-sequence star. The small measured angular separation indicates that the tertiary may be approaching the periastron of its orbit. These results, only achievable with long baseline near infrared interferometry, constitute the first step towards the understanding of the massive star formation mechanisms.

Density distributions of outflow-driven turbulence

Protostellar jets and outflows are signatures of star formation and promising mechanisms for driving supersonic turbulence in molecular clouds. We quantify outflow-driven turbulence through three-dimensional numerical simulations using an isothermal version of the robust total variation diminishing code. We drive turbulence in real-space using a simplified spherical outflow model, analyse the data through density probability distribution functions (PDF), and investigate the Core Formation Rate per free-fall time (CFR_ff). The real-space turbulence driving method produces a negatively skewed density PDF possessing an enhanced tail on the low-density side. It deviates from the log-normal distributions typically obtained from Fourier-space turbulence driving at low densities, but can provide a good fit at high-densities, particularly in terms of mass weighted rather than volume weighted density PDF. Due to this fact, we suggest that the CFR_ff determined from a Fourier-driven turbulence model could be comparable to that of our particular real-space driving model, which has a ratio of solenoidal to compressional components from the resulting turbulence velocity fields of ~0.6.

EFFECTS OF CIRCUMNUCLEAR DISK GAS EVOLUTION ON THE SPIN OF CENTRAL BLACK HOLES

Mass and spin are the only two parameters needed to completely characterize black holes in General Relativity. However, the interaction between black holes and their environment is where complexity lies, as the relevant physical processes occur over a large range of scales. That is particularly relevant in the case of super-massive black holes (SMBHs), hosted in galaxy centers, and surrounded by swirling gas and various generations of stars. These compete with the SMBH for gas consumption and affect both dynamics and thermodynamics of the gas itself. How the behavior of such fiery environment influence the angular momentum of the gas accreted onto SMBHs, and, hence, black-hole spins is uncertain. We explore the interaction between SMBHs and their environment via first 3D sub-parsec resolution simulations (ranging from 0.1 pc to 1 kpc scales) that study the evolution of the SMBH spin by including the effects of star formation, stellar feedback, radiative transfer, and metal pollution according to the proper stellar yields and lifetimes. This approach is crucial to investigate the impact of star formation processes and feedback effects on the angular momentum of the material that could accrete on the central hole. We find that star formation and feedback mechanisms can locally inject significant amounts of entropy in the surrounding medium, and impact on the inflow inclination angles and Eddington fractions. As a consequence, the resulting trends show upper-intermediate equilibrium values for the spin parameter, a, of about 0.6 - 0.9, corresponding to radiative efficiencies \epsilon = 9% - 15%. These results suggest that star formation feedback taking place in the circum-nuclear disk during the in-fall cannot induce alone very strong chaotic trends in the gas flow, quite independently from the different numerical parameters.