89614163
Jet interactions with a giant molecular cloud in the Galactic centre and ejection of hypervelocity stars
Reproduced with permission from Astronomy & Astrophysics, © 2012 ESO. Published by EDP Sciences
118971395
Variation of the cluster luminosity function across the disk of M51
We study the luminosity function (LF) of the star clusters in M51. Comparing the observed LF with the LF resulting from artificial cluster populations suggests that there exists an upper mass limit for clusters and that this limit and/or the cluster disruption varies with galactocentric distance.
16540380
Observational evidence for a truncation of the star cluster initial mass function at the high mass end
We present the luminosity function (LF) of star clusters in M51 based on HST/ACS observations taken as part of the Hubble Heritage project. The clusters are selected based on their size and with the resulting 5990 clusters we present one of the largest cluster samples of a single galaxy. We find that the LF can be approximated with a double power-law distribution with a break around MV = −8.9. On the bright side the index of the power-law distribution is steeper (α = 2.75) than on the faint-side (α = 1.93), similar to what was found earlier for the “Antennae” galaxies. The location of the bend, however, occurs about 1.6 mag fainter in M51. We confront the observed LF with the model for the evolution of integrated properties of cluster populations of Gieles et al. (2006, A&A, accepted), which predicts that a truncated cluster initial mass function would result in a bend in, and a double power-law behaviour of, the integrated LF. The combination of the large field-of view and the high star cluster formation rate of M51 make it possible to detect such a bend in the LF. Hence, we conclude that there exists a fundamental upper limit to the mass of star clusters in M51. Assuming a power-law cluster initial mass function with exponentional cut-off of the form N dM ∝ M−β exp(−M/MC) dM, we find that MC = 105 M . A direct comparison with the LF of the “Antennae” suggests that there MC = 4 × 105 M .
55226066
Variations in integrated galactic initial mass functions due to sampling method and cluster mass function
Context. Stars are thought to be formed predominantly in clusters. The star clusters are formed according to a cluster initial mass function (CMF) similar to the stellar initial mass function (IMF). Both the IMF and the CMF can be approximated by (broken) powerlaws, which favour low-mass objects. The numerous low-mass clusters will lack high mass stars compared to the underlying IMF, since the most massive star cannot be more massive than its host cluster. If the integrated galactic initial mass function (IGIMF, i.e. the total stellar mass function of all stars in a galaxy) originates from stars formed in star clusters, the IGIMF could be steeper than the IMF in clusters. Aims. We investigate how well constrained this steepening is and how it depends on the choice of sampling method and CMF. We investigate the observability of the IGIMF effect in terms of galaxy photometry and metallicities. Methods. We study various ways to sample the stellar IMF within star clusters and build up the IGIMF from these clusters. We compare analytic sampling to several implementations of random sampling of the IMF and different CMFs. We implement different IGIMFs into the galev evolutionary synthesis package to obtain colours and metallicities for galaxies. Results. Choosing different ways of sampling the IMF results in different IGIMFs. Depending on the lower cluster mass limit and the slope of the cluster mass function, the steepening varies between very strong and negligible.We find the size of the effect is continuous as a function of the power-law slope of the CMF if the CMF extends to masses smaller than the maximum stellar mass. The number of O-stars detected by GAIA will, if some uncertain factors are better understood, help to judge the importance of the IGIMF effect. The impact of different IGIMFs on integrated galaxy photometry is small, within the intrinsic scatter of observed galaxies. Observations of gas fractions and metallicities could rule out at least the most extreme sampling methods, if other sources of error are sufficiently understood. Conclusions. As we still do not understand the details of star formation and the sampling of the stellar IMF in clusters, one sampling method cannot be favoured over another. Also, the CMF at very low cluster masses is not well constrained observationally. These uncertainties therefore need to be taken into account when using an IGIMF, with severe implications for galaxy evolution models and interpretations of galaxy observations.
16163950
ACS imaging of star clusters in M 51 : I. Identification and radius distribution
Context. Size measurements of young star clusters are valuable tools to put constraints on the formation and early dynamical evolution of star clusters. Aims. We use HST/ACS observations of the spiral galaxy M51 in F435W, F555W and F814W to select a large sample of star clusters with accurate effective radius measurements in an area covering the complete disc ofM51.We present the dataset and study the radius distribution and relations between radius, colour, arm/interarm region, galactocentric distance, mass and age. Methods. We select a sample of 7698 (F435W), 6846 (F555W) and 5024 (F814W) slightly resolved clusters and derive their effective radii (Reff) by fitting the spatial profiles with analytical models convolved with the point spread function. The radii of 1284 clusters are studied in detail. Results. We find cluster radii between 0.5 and ∼10 pc, and one exceptionally large cluster candidate with Reff = 21.6 pc. The median Reff is 2.1 pc. We find 70 clusters in our sample which have colours consistent with being old GC candidates and we find 6 new “faint fuzzy” clusters in, or projected onto, the disc of M51. The radius distribution can not be fitted with a power law similar to the one for star-forming clouds. We find an increase in Reff with colour as well as a higher fraction of clusters with B−V >∼ 0.05 in the interarm regions. We find a correlation between Reff and galactocentric distance (RG) of the formReff ∝ R0.12±0.02 G , which is considerably weaker than the observed correlation for old Milky Way GCs. We find weak relations between cluster luminosity and radius: Reff ∝ L0.15±0.02 for the interarm regions and Reff ∝ L−0.11±0.01 for the spiral arm regions, but we do not observe a correlation between cluster mass and radius. Conclusions. The observed radius distribution indicates that shortly after the formation of the clusters from a fractal gas, the radii of the clusters have changed in a non-uniform way. We find tentative evidence suggesting that clusters in spiral arms are more compact.
8133620
ACS imaging of star clusters in M 51 - II. The luminosity function and mass function across the disk
Context. Whether or not there exists a physical upper mass limit for star clusters is as yet unclear. For small cluster samples the mass function may not be sampled all the way to the truncation, if there is one. Data for the rich cluster population in the interacting galaxy M 51 enables us to investigate this in more detail. Aims. Using HST/ACS data, we investigate whether the cluster luminosity function (LF) in M 51 shows evidence for an upper limit to the mass function. The variations of the cluster luminosity function parameters with position on the disk are addressed. Methods. We determine the cluster LF for all clusters in M 51 falling within our selection criteria, as well as for several subsets of the sample. In that way we can determine the properties of the cluster population as a function of galactocentric distance and background intensity. By comparing observed and simulated LFs we can constrain the underlying cluster initial mass function and/or cluster disruption parameters. A physical upper mass limit for star clusters will appear as a bend dividing two power law parts in the LF, if the cluster sample is large enough to sample the full range of cluster masses. The location of the bend in the LF is indicative of the value of the upper mass limit. The slopes of the power laws are an interplay between upper mass limits, disruption times and evolutionary fading. Results. The LF of the cluster population of M 51 is better described by a double power law than by a single power law. We show that the cluster initial mass function is likely to be truncated at the high mass end. We conclude from the variation of the LF parameters with galactocentric distance that both the upper mass limit and the cluster disruption parameters are likely to be a function of position in the galactic disk. At higher galactocentric distances the maximum mass is lower, cluster disruption slower, or both.
13989213
Spitzer observations of M83 and the hot star, H ii region connection
We have undertaken a programme to observe emission lines of [Siv] 10.51, [NeII] 12.81, [Ne III] 15.56, and [S III] 18.71 μm in a number of extragalactic HII regions with the Spitzer Space Telescope. Here we report our results for the nearly face-on spiral galaxy M83. A subsequent paper will present our data and analysis for another substantially face-on spiral galaxy M33. The nebulae selected cover a wide range of galactocentric radii (R G ). The observations were made with the infrared spectrograph in the short wavelength, high dispersion configuration. The above set of four lines is observed cospatially, thus permitting a reliable comparison of the fluxes. From the measured fluxes, we determine the ionic abundance ratios including Ne ++ /Ne + , S 3+ /S ++ and S ++ /Ne + and find that there is a correlation of increasingly higher ionization with larger R G . By sampling the dominant ionization states of Ne and S for Hit II regions, we can approximate the Ne/S ratio by (Ne + + Ne ++ )/(S ++ + S 3+ ). Our findings of ratios that significantly exceed the benchmark Orion Nebula value, as well as a decrease in this ratio with increasing R G , are more likely due to other effects than a true gradient in Ne/S. Two effects that will tend to lower these high estimates and to flatten the gradient are first, the method does not account for the presence of S + and second, S but not Ne is incorporated into grains. Both Ne and S are primary elements produced in α-chain reactions, following C and O burning in stars, making their yields depend very little on the stellar metallicity. Thus, it is expected that Ne/S remains relatively constant throughout a galaxy. We stress that this type of observation and method of analysis does have the potential for accurate measurements of Ne/S, particularly for H II regions that have lower metallicity and higher ionization than those here, such as those in M33. Our observations may also be used to test the predicted ionizing spectral energy distribution (SED) of various stellar atmosphere models. We compare the ratio of fractional ionizations / and / versus / with predictions made from our photoionization models using several of the state-of-the-art stellar atmosphere model grids. The overall best fit appears to be the nebular models using the supergiant stellar atmosphere models of Pauldrach, Hoffmann & Lennon and Sternberg, Hoffmann & Pauldrach. This result is not sensitive to the electron density and temperature range expected for these M83 nebulae. Considerable computational effort has gone into the comparison between data and models, although not all parameter studies have yet been performed on an ultimate level (e.g. in the present paper the stellar atmosphere model abundances have been fixed to solar values). A future paper, with the benefit of more observational data, will continue these studies to further discriminate how the ionic ratios depend on the SED and the other nebular parameters.
14775394
FIVE KEPLER TARGET STARS THAT SHOW MULTIPLE TRANSITING EXOPLANET CANDIDATES
We present and discuss five candidate exoplanetary systems identified with the Kepler spacecraft. These five systems show transits from multiple exoplanet candidates. Should these objects prove to be planetary in nature, then these five systems open new opportunities for the field of exoplanets and provide new insights into the formation and dynamical evolution of planetary systems. We discuss the methods used to identify multiple transiting objects from the Kepler photometry as well as the false-positive rejection methods that have been applied to these data. One system shows transits from three distinct objects while the remaining four systems show transits from two objects. Three systems have planet candidates that are near mean motion commensurabilities—two near 2:1 and one just outside 5:2. We discuss the implications that multi-transiting systems have on the distribution of orbital inclinations in planetary systems, and hence their dynamical histories, as well as their likely masses and chemical compositions. A Monte Carlo study indicates that, with additional data, most of these systems should exhibit detectable transit timing variations (TTVs) due to gravitational interactions, though none are apparent in these data. We also discuss new challenges that arise in TTV analyses due to the presence of more than two planets in a system.
118585956
The Kepler Cluster Study: Stellar Rotation in NGC?6811
We present rotation periods for 71 single dwarf members of the open cluster NGC?6811 determined using photometry from NASA's Kepler mission. The results are the first from The Kepler Cluster Study, which combines Kepler's photometry with ground-based spectroscopy for cluster membership and binarity. The rotation periods delineate a tight sequence in the NGC?6811 color-period diagram from ~1?day at mid-F to ~11?days at early-K spectral type. This result extends to 1?Gyr similar prior results in the ~600?Myr Hyades and Praesepe clusters, suggesting that rotation periods for cool dwarf stars delineate a well-defined surface in the three-dimensional space of color (mass), rotation, and age. It implies that reliable ages can be derived for field dwarf stars with measured colors and rotation periods, and it promises to enable further understanding of various aspects of stellar rotation and activity for cool stars.
40245894
Kepler-36: A Pair of Planets with Neighboring Orbits and Dissimilar Densities
So Close and So Different In our solar system, the rocky planets have very distinct orbits from those of the gas giants. Carter et al. (p. 556, published online 21 June) report on a planetary system where this pattern does not apply, posing a challenge to theories of planet formation. Data from the Kepler space telescope reveal two planets with radically different densities orbiting the same star with very similar orbital periods. One planet has a rocky Earth-like composition and the other is akin to Neptune. The Kepler spacecraft detected a super-Earth and a Neptune-like planet in very tightly spaced orbits around the same star. In the solar system, the planets’ compositions vary with orbital distance, with rocky planets in close orbits and lower-density gas giants in wider orbits. The detection of close-in giant planets around other stars was the first clue that this pattern is not universal and that planets’ orbits can change substantially after their formation. Here, we report another violation of the orbit-composition pattern: two planets orbiting the same star with orbital distances differing by only 10% and densities differing by a factor of 8. One planet is likely a rocky “super-Earth,” whereas the other is more akin to Neptune. These planets are 20 times more closely spaced and have a larger density contrast than any adjacent pair of planets in the solar system.
119199796
Identification of Background False Positives from Kepler Data
ABSTRACT.The Kepler Mission was launched on 2009 March 6 to perform a photometric survey of more than 100,000 dwarf stars to search for Earth-size planets with the transit technique. The reliability of the resulting planetary candidate list relies on the ability to identify and remove false positives. Major sources of astrophysical false positives are planetary transits and stellar eclipses on background stars. We describe several new techniques for the identification of background transit sources that are separated from their target stars, indicating an astrophysical false positive. These techniques use only Kepler photometric data. We describe the concepts and construction of these techniques in detail as well as their performance and relative merits.