41 research outputs found
On the mass segregation of stars and brown dwarfs in Taurus
We use the new minimum spanning tree (MST) method to look for mass
segregation in the Taurus association. The method computes the ratio of MST
lengths of any chosen subset of objects, including the most massive stars and
brown dwarfs, to the MST lengths of random sets of stars and brown dwarfs in
the cluster. This mass segregation ratio (Lambda_MSR) enables a quantitative
measure of the spatial distribution of high-mass and low-mass stars, and brown
dwarfs to be made in Taurus.
We find that the most massive stars in Taurus are inversely mass segregated,
with Lambda_MSR = 0.70 +/- 0.10 (Lambda_MSR = 1 corresponds to no mass
segregation), which differs from the strong mass segregation signatures found
in more dense and massive clusters such as Orion. The brown dwarfs in Taurus
are not mass segregated, although we find evidence that some low-mass stars
are, with an Lambda_MSR = 1.25 +/- 0.15. Finally, we compare our results to
previous measures of the spatial distribution of stars and brown dwarfs in
Taurus, and briefly discuss their implications.Comment: 10 pages, 8 figures, accepted for publication in MNRA
Freezing Resistance in Corn (Zea Mays L.)
Corn (Zea Mays L.), an annual crop, can be severely damaged by late spring frosts. Damage would be especially critical if the growing point were killed. To decrease the possibility that corn may be subjected to freezing conditions, shorter season corn could be planted later in the year. Another possibility, however, would be to develop varieties which could withstand freezing conditions. Due to the problem of spring frosts, the following research was performed with the long range goal of releasing a freeze resistant corn line. The initial stages of the research were to develop a method for screening corn inbred lines for freezing resistance. The method developed was to grow lines in a greenhouse and then subject them to a freezing cycle in the growth chamber. Eighty-eight lines were screened for freezing resistance by this method. In addition, the effects of various water stress conditions on freezing ·of corn inbred lines were investigated. Since the use of a freezing cycle renders susceptible plants useless, a method of screening for freeze resistance without a freezing cycle was also studied. These methods included a relative measure of ice nucleating bacteria, and measurement of various water relationships. The final stages of the research was to begin work with the inheritance of freezing resistance
Testing the universality of star formation - I. Multiplicity in nearby star-forming regions
We have collated multiplicity data for five clusters (Taurus, Chamaeleon I,
Ophiuchus, IC348, and the Orion Nebula Cluster). We have applied the same mass
ratio (flux ratios of delta K <= 2.5) and primary mass cuts (~0.1-3.0 Msun) to
each cluster and therefore have directly comparable binary statistics for all
five clusters in the separation range 62-620 au, and for Taurus, Chamaeleon I,
and Ophiuchus in the range 18-830 au. We find that the trend of decreasing
binary fraction with cluster density is solely due to the high binary fraction
of Taurus, the other clusters show no obvious trend over a factor of nearly 20
in density.
With N-body simulations we attempt to find a set of initial conditions that
are able to reproduce the density, morphology and binary fractions of all five
clusters. Only an initially clumpy (fractal) distribution with an initial total
binary fraction of 73 per cent (17 per cent in the range 62-620 au) is able to
reproduce all of the observations (albeit not very satisfactorily). Therefore,
if star formation is universal the initial conditions must be clumpy and with a
high (but not 100 per cent) binary fraction. This could suggest that most
stars, including M-dwarfs, form in binaries.Comment: Accepted for publication in MNRAS, 19 pages, 22 figure
Spatial differences between stars and brown dwarfs: a dynamical origin?
We use -body simulations to compare the evolution of spatial distributions
of stars and brown dwarfs in young star-forming regions. We use three different
diagnostics; the ratio of stars to brown dwarfs as a function of distance from
the region's centre, , the local surface density of
stars compared to brown dwarfs, , and we compare the global
spatial distributions using the method. From a suite of
twenty initially statistically identical simulations, 6/20 attain
, indicating that dynamical interactions could be responsible for
observed differences in the spatial distributions of stars and brown dwarfs in
star-forming regions. However, many simulations also display apparently
contradictory results - for example, in some cases the brown dwarfs have much
lower local densities than stars (), but their global
spatial distributions are indistinguishable () and the
relative proportion of stars and brown dwarfs remains constant across the
region (). Our results suggest that extreme caution
should be exercised when interpreting any observed difference in the spatial
distribution of stars and brown dwarfs, and that a much larger observational
sample of regions/clusters (with complete mass functions) is necessary to
investigate whether or not brown dwarfs form through similar mechanisms to
stars.Comment: 7 pages, 5 figures, accepted for publication in MNRA
FLAMINGOS Spectroscopy of New Low-Mass Members of the Young Cluster IC 348
We present spectroscopy of candidate stellar and substellar members of the
young cluster IC 348. Using the Florida Multi-Object Imaging Near-Infrared
Grism Observational Spectrometer with the 4 meter telescope at Kitt Peak
National Observatory, we have obtained multi-object moderate-resolution
(R=1000) J- and H-band spectra of 66 infrared sources (H=12-17) toward IC 348,
many of which are difficult to observe spectroscopically at optical wavelengths
(I>20) because they are highly reddened and/or intrinsically cool and red. We
have also observed 19 known cluster members that have optical spectral types
available from previous work. By using these latter sources as the spectral
classification standards, we have identified 14 new members of the cluster with
types of M2-M6 in the sample of 66 new objects. Two additional objects exhibit
types of >M8.5, but cannot be conclusively classified as either field dwarfs or
cluster members with available data. We have estimated extinctions,
luminosities, and effective temperatures for these 16 M-type objects, placed
them on the H-R diagram, and used the evolutionary models of Chabrier & Baraffe
to estimate their masses. If the two candidates at >M8.5 are indeed members,
they should be among the least massive known brown dwarfs in IC 348
(M/M_sun~0.01).Comment: 15 pages, The Astrophysical Journal, 2004, v618 (January 10
Analysis of Bulked and Redundant Accessions of Brassica Germplasm Using Assignment Tests of Microsatellite Markers
This study was conducted to determine if Brassica germplasm bulks created and maintained by the USDA-ARS North Central Plant Introduction Station (NCRPIS) were made with genetically indistinguishable component accessions and to examine newly identified putative duplicate accessions to determine if they can be bulked. Using ten microsatellite primer pairs, we genotyped two bulks of B. rapa L. ssp. dichotoma (Roxb.) Hanelt comprising four accessions and three bulks of B. rapa L. ssp. trilocularis (Roxb.) Hanelt comprising fourteen accessions, as well as four pairs of putatively duplicate accessions of B.␣napus L. Assignment tests on ten individual plants per accession were conducted using a model-based clustering method to arrive at probabilities of likelihood of accession assignment. The assignment tests indicated that one of the two bulks of B. rapa ssp. dichotoma involves genetically heterogeneous accessions. It was observed in the B. rapassp. trilocularis bulks that the component accessions could be differentiated into groups, with misassignments observed most frequent within groups. In B. napus, only one of the four pairs of putative duplicates showed significant genetic differentiation. The other three pairs of putative duplicates lack differences and support the creation of bulks. The results of the assignment tests were in agreement with cluster analyses and tests of population differentiation. Implications of these results in terms of germplasm management include the maintenance and/or re-creation of someBrassica germplasm bulks by excluding those accessions identified as being unique in this study
A search for mass segregation of stars and brown dwarfs in \rho\ Ophiuchi
We apply two different algorithms to search for mass segregation to a recent
observational census of the rho Ophiuchi star forming region. Firstly, we apply
the Lambda_MSR method, which compares the minimum spanning tree (MST) of a
chosen subset of stars to MSTs of random subsets of stars in the cluster, and
determine the mass segregation ratio, Lambda_MSR. Secondly, we apply the
m-Sigma method, which calculates the local stellar surface density around each
star and determines the statistical significance of the average surface density
for a chosen mass bin, compared to the average surface density in the whole
cluster. Using both methods, we find no indication of mass segregation (normal
or inverse) in the spatial distribution of stars and brown dwarfs in rho
Ophiuchi. Although rho Ophiuchi suffers from high visual extinction, we show
that a significant mass segregation signature would be detectable, albeit
slightly diluted, despite dust obscuration of centrally located massive stars.Comment: 8 pages, 5 figures, accepted for publication in MNRA
New H2 collision-induced absorption and NH3 opacity and the spectra of the coolest brown dwarfs
We present new cloudy and cloudless model atmospheres for brown dwarfs using
recent ab initio calculations of the line list of ammonia (NH3) and of the
collision-induced absorption of molecular hydrogen (H2). We compare the new
synthetic spectra with models based on an earlier description of the H2 and NH3
opacities. We find a significant improvement in fitting the nearly complete
spectral energy distribution of the T7p dwarf Gliese 570D and in near infrared
color-magnitude diagrams of field brown dwarfs. We apply these new models to
the identification of NH3 absorption in the H band peak of very late T dwarfs
and the new Y dwarfs and discuss the observed trend in the NH3-H spectral
index. The new NH3 line list also allows a detailed study of the medium
resolution spectrum of the T9/T10 dwarf UGPS J072227.51-054031.2 where we
identify several specific features caused by NH3.Comment: 37 pages, 13 figures. Accepted for publication in the Astrophysical
Radiation-Hydrodynamic Simulations of the Formation of Orion-Like Star Clusters I. Implications for the Origin of the Initial Mass Function
One model for the origin of typical galactic star clusters such as the Orion
Nebula Cluster (ONC) is that they form via the rapid, efficient collapse of a
bound gas clump within a larger, gravitationally-unbound giant molecular cloud.
However, simulations in support of this scenario have thus far have not
included the radiation feedback produced by the stars; radiative simulations
have been limited to significantly smaller or lower density regions. Here we
use the ORION adaptive mesh refinement code to conduct the first ever
radiation-hydrodynamic simulations of the global collapse scenario for the
formation of an ONC-like cluster. We show that radiative feedback has a
dramatic effect on the evolution: once the first ~10-20% of the gas mass is
incorporated into stars, their radiative feedback raises the gas temperature
high enough to suppress any further fragmentation. However, gas continues to
accrete onto existing stars, and, as a result, the stellar mass distribution
becomes increasingly top-heavy, eventually rendering it incompatible with the
observed IMF. Systematic variation in the location of the IMF peak as star
formation proceeds is incompatible with the observed invariance of the IMF
between star clusters, unless some unknown mechanism synchronizes the IMFs in
different clusters by ensuring that star formation is always truncated when the
IMF peak reaches a particular value. We therefore conclude that the global
collapse scenario, at least in its simplest form, is not compatible with the
observed stellar IMF. We speculate that processes that slow down star
formation, and thus reduce the accretion luminosity, may be able to resolve the
problem.Comment: 17 pages, 13 figures, emulateapj format, ApJ in press; simulation
movies available at http://www.ucolick.org/~krumholz/publications.htm
Masses, Radii, and Cloud Properties of the HR 8799 Planets
The near-infrared colors of the planets directly imaged around the A star HR
8799 are much redder than most field brown dwarfs of the same effective
temperature. Previous theoretical studies of these objects have concluded that
the atmospheres of planets b, c, and d are unusually cloudy or have unusual
cloud properties. Some studies have also found that the inferred radii of some
or all of the planets disagree with expectations of standard giant planet
evolution models. Here we compare the available data to the predictions of our
own set of atmospheric and evolution models that have been extensively tested
against observations of field L and T dwarfs, including the reddest L dwarfs.
Unlike some previous studies we require mutually consistent choices for
effective temperature, gravity, cloud properties, and planetary radius. This
procedure thus yields plausible values for the masses, effective temperatures,
and cloud properties of all three planets. We find that the cloud properties of
the HR 8799 planets are not unusual but rather follow previously recognized
trends, including a gravity dependence on the temperature of the L to T
spectral transition--some reasons for which we discuss. We find the inferred
mass of planet b is highly sensitive to whether or not we include the H and K
band spectrum in our analysis. Solutions for planets c and d are consistent
with the generally accepted constraints on the age of the primary star and
orbital dynamics. We also confirm that, like in L and T dwarfs and solar system
giant planets, non-equilibrium chemistry driven by atmospheric mixing is also
important for these objects. Given the preponderance of data suggesting that
the L to T spectral type transition is gravity dependent, we present an
exploratory evolution calculation that accounts for this effect. Finally we
recompute the the bolometric luminosity of all three planets.Comment: 52 pages, 12 figures, Astrophysical Journal, in press. v2 features
minor editorial updates and correction