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Register a new userNICMOS/HST Observations of the Embedded Cluster Associated with Mon R2: Constraining the sub-stellar Initial Mass Function
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We have analyzed HST/NICMOS2 F110W, F160W, F165M, and F207M band images covering the central 1x1 of the cluster associated with Mon R2 in order to constrain the Initial Mass Function (IMF) down to 20 Mjup. The flux ratio between the F165M and F160W bands was used to measure the strength of the water band absorption feature and select a sample of 12 out of the total sample of 181 objects that have effective temperatures between 2700 K and 3300 K. These objects are placed in the HR diagram together with sources observed by Carpenter et al. (1997) to estimate an age of ~1 Myr for the low mass cluster population. By constructing extinction limited samples, we are able to constrain the IMF and the fraction of stars with a circumstellar disk in a sample that is 90% complete for both high and low mass objects. For stars with estimated masses between 0.1 Msun and 1.0 Msun for a 1 Myr population with Av < 19 mag, we find that 27+-9% have a near-infrared excess indicative of a circumstellar disk. The derived fraction is similar to, or slightly lower than, the fraction found in other star forming regions of comparable age. We constrain the number of stars in the mass interval 0.08-1.0Msun to the number of objects in the mass interval 0.02-0.08 Msun by forming the ratio, R**=N(0.08-1Msun)/N(0.02-0.08Msun) for objects in an extinction limited sample complete for Av < 7 mag. The ratio is found to be R^**=2.2+-1.3 assuming an age of 1 Myr, consistent with the similar ratio predicted by the system IMF proposed by Chabrier (2003). The ratio is similar to the ratios observed towards the Orion Nebula Cluster and IC 348 as well as the ratio derived in the 28 square degree survey of Taurus by Guieu et al. (2006).
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We present HST/NICMOS photometry, and low-resolution K-band spectra of the GLIMPSE9 stellar cluster. The newly obtained color-magnitude diagram shows a cluster sequence with H-Ks =1 mag, indicating an interstellar extinction Aks=1.6pm0.2 mag. The spectra of the three brightest stars show deep CO band-heads, which indicate red supergiants with spectral type M1-M2. Two 09-B2 supergiants are also identified, which yield a spectrophotometric distance of 4.2pm0.4 kpc. Presuming that the population is coeval, we derive an age between 15 and 27 Myr, and a total cluster mass of 1600pm400 Msun, integrated down to 1 Msun. In the vicinity of GLIMPSE9 are several HII regions and SNRs, all of which (including GLIMPSE 9) are probably associated with a giant molecular cloud (GMC) in the inner galaxy. GLIMPSE9 probably represents one episode of massive star formation in this GMC. We have identified several other candidate stellar clusters of the same complex.
Prior to the detection of black holes (BHs) via the gravitational waves (GWs) they generate at merger, the presence of BHs was inferred in X-ray binaries, mostly via dynamical measurements, with masses in the range between $sim 5-20~M_odot$. The LIGO discovery of the first BHs via GWs was surprising in that the two BHs that merged had masses of $35.6^{+4.8}_{-3.0}$ and $30.6^{+3.0}_{-4.4},M_odot$, which are both above the range inferred from X-ray binaries. With 20 BH detections from the O1/O2 runs, the distribution of masses remains generally higher than the X-ray inferred one, while the effective spins are generally lower, suggesting that, at least in part, the GW-detected population might be of dynamical origin rather than produced by the common evolution of field binaries. Here we perform high-resolution N-body simulations of a cluster of isolated BHs with a range of initial mass spectra and upper mass cut-offs, and study the resulting binary mass spectrum resulting from the dynamical interactions. Our clusters have properties similar to those of the massive remnants in an OB association $sim 10 , mathrm{Myr}$ after formation. We perform a likelihood analysis for each of our dynamically-formed binary population against the data from the O1 and O2 LIGO/Virgo runs. We find that an initial mass spectrum $M_{rm BH}propto M^{-2.35}$ with an upper mass cutoff $M_{rm max}sim 50M_odot$ is favored by the data, together with a slight preference for a merger rate that increases with redshift.
We present an analysis of NICMOS photometry and low-resolution grism spectroscopy of low-mass stars and sub-stellar objects in the young star-forming region NGC 1333. Our goal is to constrain the ratio of low-mass stars to sub- stellar objects down to 20 Mjup in the cluster as well as constrain the cluster IMF down to 30 Mjup in combination with a previous survey of NGC 1333 by Wilking et al. Our survey covers 4 fields of 51.2 x 51.2, centered on brown dwarf candidates previously identified in Wilking et al. We extend previous work based on the use of a water vapor index for spectral typing to wavelengths accessible with NICMOS on the HST. Spectral types were derived for the 14 brightest objects in our fields, ranging from <M0 - M8, which at the age of the cluster (0.3 Myr) corresponds to a range in mass of >0.25 - 0.02 Msun. In addition to the spectra, we present an analysis of the color-magnitude diagram using pre-main sequence evolutionary models of DAntona & Mazzitelli. Using an extinction-limited sample, we derive the ratio of low-mass stars to brown dwarfs. Comparisons of the observed ratio to that expected from the field IMF of Chabrier indicate that the two results are consistent. We combine our data with that of Wilking et al. to compute the ratio of intermediate-mass stars (0.1 - 1.0 Msun) to low-mass objects (0.03 - 0.1 Msun) in the cluster. We also report the discovery of a faint companion to the previously confirmed brown dwarf ASR 28, as well as a possible outflow surrounding ASR 16. If the faint companion is confirmed as a cluster member, it would have a mass of ~ 5 Mjup (mass ratio 0.15) at a projected distance of 350 AU, similar to 2MASS 1207-3923 B.
We present an analysis of NICMOS observations of the embedded cluster associated with NGC 2024. An analysis of the cluster color-magnitude diagram (CMD) using the models of DAntona & Mazzitelli (1997) and Baraffe et al. (1998) indicates that the ratio of intermediate mass (1.0 to 10.0 M_sun) to low mass (0.1 to 1.0 M_sun) stars is consistent with the stellar initial mass function (IMF) for the field. In addition to the CMD analysis, we present results on the multiplicity of stars in the region. Three companions (in a sample of 95 potential primaries) were found, with angular separations between 0.4 and 1.0, translating to a projected linear separation of 184 AU to 460 AU for an estimated distance of 460 pc. The completeness of binary detections is assessed using recovery fractions calculated by a series of tests using artificially generated companions to potential primaries in the data frames. We find that the binary fraction in NGC 2024 is consistent with that of Duquennoy & Mayor (1991) for solar neighborhood stars over the range of separations and companion masses appropriate for our survey.
The low-mass end of the stellar Initial Mass Function (IMF) is constrained by focusing on the baryon-dominated central regions of strong lensing galaxies. We study in this letter the Einstein Cross (Q2237+0305), a z=0.04 barred galaxy whose bulge acts as lens on a background quasar. The positions of the four quasar images constrain the surface mass density on the lens plane, whereas the surface brightness (H-band NICMOS/HST imaging) along with deep spectroscopy of the lens (VLT/FORS1) allow us to constrain the stellar mass content, for a range of IMFs. We find that a classical single power law (Salpeter IMF) predicts more stellar mass than the observed lensing estimates. This result is confirmed at the 99% confidence level, and is robust to systematic effects due to the choice of population synthesis models, the presence of dust, or the complex disk/bulge population mix. Our non-parametric methodology is more robust than kinematic estimates, as we do not need to make any assumptions about the dynamical state of the galaxy or its decomposition into bulge and disk. Over a range of low-mass power law slopes (with Salpeter being Gamma=+1.35) we find that at a 90% confidence level, slopes with Gamma>0 are ruled out.
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