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.
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.
118630350
DETECTION OF POTENTIAL TRANSIT SIGNALS IN THE FIRST THREE QUARTERS OF Kepler MISSION DATA
We present the results of a search for potential transit signals in the first three quarters of photometry data acquired by the Kepler mission. The targets of the search include 151,722 stars which were observed over the full interval and an additional 19,132 stars which were observed for only one or two quarters. From this set of targets we find a total of 5392 detections which meet the Kepler detection criteria: those criteria are periodicity of signal, an acceptable signal-to-noise ratio, and a composition test which rejects spurious detections which contain non-physical combinations of events. The detected signals are dominated by events with relatively low signal-to-noise ratio and by events with relatively short periods. The distribution of estimated transit depths appears to peak in the range between 40 and 100 parts per million, with a few detections down to fewer than 10 parts per million. The detections exhibit signal-to-noise ratios from 7.1σ, which is the lower cutoff for detections, to over 10,000σ, and periods ranging from 0.5 days, which is the lower cutoff used in the procedure, to 109 days, which is the upper limit of achievable periods given the length of the data set and the criteria used for detections. The detected signals are compared to a set of known transit events in the Kepler field of view which were derived by a different method using a longer data interval; the comparison shows that the current search correctly identified 88.1% of the known events. A tabulation of the detected transit signals, examples which illustrate the analysis and detection process, a discussion of future plans and open, potentially fruitful, areas of further research are included.
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.
119267981
Kepler Mission Design, Realized Photometric Performance, and Early Science
The Kepler Mission, launched on 2009 March 6, was designed with the explicit capability to detect Earth-size planets in the habitable zone of solar-like stars using the transit photometry method. Results from just 43 days of data along with ground-based follow-up observations have identified five new transiting planets with measurements of their masses, radii, and orbital periods. Many aspects of stellar astrophysics also benefit from the unique, precise, extended, and nearly continuous data set for a large number and variety of stars. Early results for classical variables and eclipsing stars show great promise. To fully understand the methodology, processes, and eventually the results from the mission, we present the underlying rationale that ultimately led to the flight and ground system designs used to achieve the exquisite photometric performance. As an example of the initial photometric results, we present variability measurements that can be used to distinguish dwarf stars from red giants.
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.
3207172
The Maximum Mass of Star Clusters
When an universal untruncated star cluster initial mass function (CIMF) described by a power-law distribution is assumed, the mass of the most massive star cluster in a galaxy (M_max) is the result of the size-of-sample (SoS) effect. This implies a dependence of M_max on the total number of star clusters (N). The SoS effect also implies that M_max within a cluster population increases with equal logarithmic intervals of age. This is because the number of clusters formed in logarithmic age intervals increases (assuming a constant cluster formation rate). This effect has been observed in the SMC and LMC. Based on the maximum pressure (P_int) inside molecular clouds, it has been suggested that a physical maximum mass (M_max[phys]) should exist. The theory predicts that M_max[phys] should be observable, i.e. lower than M_max that follows from statistical arguments, in big galaxies with a high star formation rate. We compare the SoS relations in the SMC and LMC with the ones in M51 and model the integrated cluster luminosity function (CLF) for two cases: 1) M_max is determined by the SoS effect and 2) M_max=M_max[phys]=constant. The observed CLF of M51 and the comparison of the SoS relations with the SMC and LMC both suggest that there exists a M_max[phys] of 5*10^5 M_sun in M51. The CLF of M51 looks very similar to the one observed in the ``Antennae'' galaxies. A direct comparison with our model suggests that there M_max[phys]=2*10^6 M_sun.
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.
119116927
Disentangling galaxy environment and host halo mass
The properties of both observed galaxies and dark matter haloes in simulations depend on their environment. The term “environment” has, however, been used to describe a wide variety of measures that may or may not correlate with each other. Popular measures of environment include, for example, the distance to the N th nearest neighbour, the number density of objects within some distance, or, for the case of galaxies only, the mass of the host dark matter halo. Here we use results from the Millennium simulation and a semi-analytic model for galaxy formation to quantify the relations between different measures of environment and halo mass. We show that the environmental parameters used in the observational literature are in effect measures of halo mass, even if they are measured for a fixed stella r mass. The strongest correlation between environmental density and halo mass arises when the number of objects is counted out to a distance of 1.5 ‐ 2 times the virial radius of the host h alo and when the galaxies/haloes are required to be relatively bright/massive. For observational studies this virial radius is no t easily determined, but the number of neighbours out to 1 ‐ 2 h −1 Mpc gives a similarly strong correlation with halo mass. For the distance to the N th nearest neighbour the (anti-)correlation with halo mass is nearly as strong provided N > 2. We demonstrate that this environmental parameter becomes insensitive to halo mass if it is constructed from dimensionless quantities. This can be achieved by scaling the minimum luminosity/mass of neighbours to that of the object that the environment is determined for and by dividing the distance to a length scale associated with either the neighbour or the galaxy under consideration. We show how such a halo mass independent environmental parameter can be defin ed for both observational and numerical studies. The results presented here will help fut ure studies to disentangle the effects of halo mass and external environment on the properties of galaxies and dark matter haloes.
14521886
Galactic consequences of clustered star formation
If all stars form in clusters and both stars and clusters follow a power-law distribution which favours the creation of low-mass objects, the numerous low-mass clusters will be deficient in high-mass stars. Therefore, the stellar mass function integrated over the entire galaxy (the integrated galactic initial mass function; IGIMF) will be steeper at the high-mass end than the underlying stellar IMF. We show how the steepness of the IGIMF depends on the sampling method and on the assumptions made regarding the star cluster mass function. We also investigate the O-star content, integrated photometry and chemical enrichment of galaxies that result from several IGIMFs compared to more standard IMFs.
118651823
The correlation structure of dark matter halo properties
We investigate the correlation between nine different dark matter halo properties using a rank correlation analysis and a Principal Component Analysis for a sample of haloes spanning five orders of magnitude in mass. We consider mass and dimensionless measures of concentration, age, relaxedness, sphericity, triaxiality, substructure, spin and environment, where the latter is defined in a way that makes it insensitive to mass. We find that concentration is the most fundamental property. Except for environment, all parameters are strongly correlated with concentration. Concentration, age, substructure, mass, sphericity and relaxedness can be considered a single family of parameters, albeit with substantial scatter. In contrast, spin, environment and triaxiality are more independent, although spin does correlate strongly with substructure and both spin and triaxiality correlate substantially with concentration. Although mass sets the scale of a halo, all other properties are more sensitive to concentration.