arXiv subjects

C. A. Nelson

Publications and source records attributed to C. A. Nelson.

21 records · Page 2Linked to original sources

arXiv2000

The MACHO Project: Microlensing Results from 5.7 Years of LMC Observations

We report on our search for microlensing towards the Large Magellanic Cloud (LMC). Analysis of 5.7 years of photometry on 11.9 million stars in the LMC reveals 13 - 17 microlensing events. This is significantly more than the $\sim$ 2 to 4 events expected from lensing by known stellar populations. The timescales ($\that$) of the events range from 34 to 230 days. We estimate the microlensing optical depth towards the LMC from events with $2 < \that < 400$ days to be 1.2 ^{+0.4}_ {-0.3} \ten{-7}$, with an additional 20% to 30% of systematic error. The spatial distribution of events is mildly inconsistent with LMC/LMC disk self-lensing, but is consistent with an extended lens distribution such as a Milky Way or LMC halo. Interpreted in the context of a Galactic dark matter halo, consisting partially of compact objects, a maximum likelihood analysis gives a MACHO halo fraction of 20% for a typical halo model with a 95% confidence interval of 8% to 50%. A 100% MACHO halo is ruled out at the 95% C.L. for all except our most extreme halo model. Interpreted as a Galactic halo population, the most likely MACHO mass is between $ 0.15 \msun$ and $ 0.9 \msun$, depending on the halo model, and the total mass in MACHOs out to 50 kpc is found to be 9+4-3 10^{10} msun, independent of the halo model. These results are marginally consistent with our previous results, but are lower by about a factor of two. Besides a larger data set, this work also includes an improved efficiency determination, improved likelihood analysis, and more thorough testing of systematic errors, especially with respect to the treatment of potential backgrounds to microlensing, such as supernovae in galaxies behind the LMC. [Abridged]

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arXiv1999

The Distance to the LMC via the Eclipsing Binary HV 2274

We present UBV photometry of the eclipsing binary Harvard Variable 2274 in the Large Magellanic Cloud (LMC). The stellar parameters of this binary system were calculated by Guinan et al. (1998a) who gave both a reddening towards HV 2274 of E(B-V) = 0.083 +/- 0.006 and a distance modulus to the LMC, mu_{LMC} = 18.42 +/- 0.007. The reddening of this system was also determined by Udalski et al. (1998) who found E(B-V) = 0.149 +\- 0.015. With Udalski et al. (1998) B and V photometry Guinan et al.(1998b) obtained E(B-V) = 0.120 +\- 0.009 and mu_{LMC} = 18.30 +\- 0.07. Using our UBV photometry, we derive a reddening of E(B-V)= 0.088 +\- 0.025, consistent with the original value of Guinan et al. (1998a) and supporting a longer distance modulus to the LMC of about mu_{LMC} = 18.40 +\- 0.07. We stress the uncertainties inherent in ground-based UBV photometry and the concomitant uncertainties in determining distances based upon such photometry.

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arXiv1999

Calibration of the MACHO Photometry Database

The MACHO Project is a microlensing survey that monitors the brightnesses of 60 million stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud, and Galactic bulge. Our database presently contains about 80 billion photometric measurements, a significant fraction of all astronomical photometry. We describe the calibration of MACHO two-color photometry and transformation to the standard Kron-Cousins V and R system. Calibrated MACHO photometry may be properly compared with all other observations on the Kron-Cousins standard system, enhancing the astrophysical value of these data. For 9 million stars in the LMC bar, independent photometric measurements of 20,000 stars with V < 18 mag in field-overlap regions demonstrate an internal precision sigV = 0.021, sigR = 0.019, and sig(V-R) = 0.028 mag. The accuracy of the zero-point in this calibration is estimated to be +-0.035 mag for stars with colors in the range -0.1 < (V-R) < 1.2 mag. A comparison of calibrated MACHO photometry with published photometric sequences and new Hubble Space Telescope observations shows agreement. The current calibration zero-point uncertainty for the remainder of the MACHO photometry database is estimated to be +- 0.10 mag in V or R and +-0.04 mag in (V-R). We describe the first application of calibrated MACHO photometry data: the construction of a color-magnitude diagram used to calculate our experimental sensitivity to detect microlensing in the LMC.

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