arXiv subjects

Samir Salim

Publications and source records attributed to Samir Salim.

102 records · Page 6Linked to original sources

arXiv2002

Classifying Luyten Stars Using An Optical-Infrared Reduced Proper Motion Diagram

We present a V-J reduced proper motion (RPM) diagram for stars in the New Luyten Two-Tenths (NLTT) catalog. In sharp contrast to the RPM diagram based on the original NLTT data, this optical-infrared RPM diagram shows distinct tracks for white dwarfs, subdwarfs, and main-sequence stars. It thereby permits the identification of white-dwarf and subdwarf candidates that have a high probability of being genuine.

astro-ph↗

arXiv2002

Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog

We complete construction of a catalog containing improved astrometry and new optical/infrared photometry for the vast majority of NLTT stars lying in the overlap of regions covered by POSS I and by the second incremental 2MASS release, approximately 44% of the sky. The epoch 2000 positions are typically accurate to 130 mas, the proper motions to 5.5 mas/yr, and the V-J colors to 0.25 mag. Relative proper motions of binary components are measured to 3 mas/yr. The false identification rate is ~1% for 11 < V < 18 and substantially less at brighter magnitudes. These improvements permit the construction of a reduced proper motion diagram that, for the first time, allows one to classify NLTT stars into main-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We in turn use this diagram to analyze the properties of both our catalog and the NLTT catalog on which it is based. In sharp contrast to popular belief, we find that NLTT incompleteness in the plane is almost completely concentrated in MS stars, and that SDs and WDs are detected almost uniformly over the sky DEC > -33 deg. Our catalog will therefore provide a powerful tool to probe these populations statistically, as well as to reliably identify individual SDs and WDs.

astro-ph↗

arXiv2002

Improved Astrometry and Photometry for the Luyten Catalog. I. Bright Stars

We outline the construction of an updated version of the New Luyten Two-Tenths (NLTT) catalog of high proper motion stars, which will contain improved astrometry and photometry for the vast majority of the ~59,000 stars in NLTT. The bright end is constructed by matching NLTT stars to Hipparcos, Tycho-2, and Starnet; the faint end by matching to USNO-A and 2MASS. In this first paper, we detail the bright-end matching procedure. We show that for the majority of stars in his catalog, Luyten measured positions accurate to 1" even though he recorded his results much more coarsely. However, there is a long tail of position errors, with one error as large as 11 deg. Proper-motion errors for the stars with small position errors are 24 mas/yr (1 sigma) but deteriorate to 34 mas/yr for stars with inferior positions. NLTT is virtually 100% complete for V<11.5 and |b|>15 deg, but completeness in this magnitude range falls to about 75% at the Galactic plane. Incompleteness near the plane is not uniform, but is rather concentrated in the interval -80<l<20, where the Milky Way is brightest.

astro-ph↗

arXiv2002

Astrometry Survey Missions Beyond the Magnitude Limit

Three planned astrometry survey satellites, FAME, DIVA, and GAIA, all aim at observing magnitude-limited samples. We argue that substantial additional scientific opportunities are within the reach of these mission if they devote a modest fraction of their catalogs to selected targets that are fainter than their magnitude limits. We show that the addition of ~10^6 faint (R>15) targets to the 40 10^6 object FAME catalog can improve the precision of the reference frame by a factor 2.5, to 7 micro-as/yr, increase Galactocentric distance at which halo rotation can be precisely (2 km/s) measured by a factor 4, to 25 kpc, and increase the number of late M dwarfs, L dwarfs, and white dwarfs with good parallaxes by an order of magnitude. In most cases, the candidate quasars, horizontal branch stars, and dim dwarfs that should be observed to achieve these aims are not yet known. We present various methods to identify candidates from these classes, and assess the efficiencies of these methods. The analysis presented here can be applied to DIVA with modest modifications. Application to GAIA should be deferred until the characteristics of potential targets are better constrained.

astro-ph↗

arXiv2001

Searching for Failed Supernovae With Astrometric Binaries

Stars in the mass range 8 Msun<M<30 Msun are thought to end their lives as luminous supernovae that leave behind a neutron star. However, if a substantial fraction of these stars instead ended as black-hole remnants, without producing a supernova (a `failed' supernova), how would one know? We show that, under plausible assumptions, the Hipparcos catalog should contain about 30 f_{fail} astrometric binaries with black-hole companions, where f_{fail} is the fraction of supernovae that fail. Since no black-hole astrometric binaries are found in Hipparcos, one might like to conclude that such failed supernovae are very rare. However, the most important assumption required for this argument, the initial companion mass function (ICMF) of G stars (the majority of Hipparcos stars) in the high-mass companion regime, is without any observational basis. We show how the ICMF of G stars can be measured using the Full-Sky Astrometric Explorer (FAME), thereby permitting an accurate measurement of the rate of supernovae that fail.

astro-ph↗

arXiv2001

M Dwarfs from Hubble Space Telescope Star Counts. IV

We study a sample of about 1400 disk M dwarfs that are found in 148 fields observed with the Wide Field Camera 2 (WFC2) on the Hubble Space Telescope and 162 fields observed with pre-repair Planetary Camera 1 (PC1), of which 95 of the WFC2 fields are newly analyzed. The method of maximum likelihood is applied to derive the luminosity function and the Galactic disk parameters. At first, we use a local color-magnitude relation and a locally determined mass-luminosity relation in our analysis. The results are consistent with those of previous work but with considerably reduced statistical errors. These small statistical errors motivate us to investigate the systematic uncertainties. Considering the metallicity gradient above the Galactic plane, we introduce a modified color-magnitude relation that is a function of Galactic height. The resultant M dwarf luminosity function has a shape similar to that derived using the local color-magnitude relation but with a higher peak value. The peak occurs at $M_V \sim 12$ and the luminosity function drops sharply toward $M_V \sim 14$. We then apply a height-dependent mass-luminosity function interpolated from theoretical models with different metallicities to calculate the mass function. Unlike the mass function obtained using local relations, which has a power-law index $α= 0.47$, the one derived from the height-dependent relations tends to be flat ($α= -0.10$). The resultant local surface density of disk M dwarfs (12.2 +/- 1.6 M_sun pc^{-2}) is somewhat smaller than the one obtained using local relations (14.3 +/- 1.3 M_sun pc^{-2}). Our measurement favors a short disk scale length, H = 2.75 +/- 0.16 (statistical) +/- 0.25 (systematic) kpc.

astro-ph↗

arXiv2000

Nearby Microlensing Events - Identification of the Candidates for the SIM

The Space Interferometry Mission (SIM) is the instrument of choice when it comes to observing astrometric microlensing events where nearby, usually high-proper-motion stars (``lenses''), pass in front of more distant stars (``sources''). Each such encounter produces a deflection in the source's apparent position that when observed by SIM can lead to a precise mass determination of the nearby lens star. We search for lens-source encounters during the 2005-2015 period using Hipparcos, ACT and NLTT to select lenses, and USNO-A2.0 to search for the corresponding sources, and rank these by the SIM time required for a 1% mass measurement. For Hipparcos and ACT lenses, the lens distance and lens-source impact parameter are precisely determined so the events are well characterized. We present 32 candidates beginning with a 61 Cyg A event in 2012 that requires only a few minutes of SIM time. Proxima Centauri and Barnard's star each generate several events. For NLTT lenses, the distance is known only to a factor of 3, and the impact parameter only to 1''. Together, these produce uncertainties of a factor ~10 in the amount of SIM time required. We present a list of 146 NLTT candidates and show how single-epoch CCD photometry of the candidates could reduce the uncertainty in SIM time to a factor of ~1.5.

astro-ph↗

arXiv1999

No nearby counterparts to the moving objects in the Hubble Deep Field

Ibata et al (1999) have recently discovered faint, moving objects in the Hubble Deep Field. The quantity, magnitudes and proper motions of these objects are consistent with old white dwarfs making up the Galactic dark halo. We review a number of ground-based proper motion surveys in which nearby dark halo white dwarfs might be present, if they have the colours and absolute magnitudes proposed. No such objects have been found, whereas we argue here that several times more would be expected than in the Hubble Deep Field. We conclude it is unlikely that hydrogen atmosphere white dwarfs make up a significant fraction of the halo dark matter. No limits can be placed yet on helium atmosphere dwarfs from optical searches.

astro-ph↗

arXiv1999

Stellar Mass Function From SIM Astrometry/Photometry

By combining SIM observations with ground-based photometry, one can completely solve microlensing events seen toward the Galactic bulge. One could measure the mass, distance, and transverse velocity of ~100 lenses to ~5% precision in only ~500 hours of SIM time. Among the numerous applications are 1) measurement of the mass functions (MFs) of the bulge and disk 2) measurement of the relative normalizations of the bulge and disk MFs (and so their relative contribution to the Galactic potential), 3) measurement of the number of bulge white dwarfs and neutron stars (and so the initial MF well above the present turnoff). SIM astrometric measurements are simultaneously photometric measurements. SIM astrometry determines the angular size of the Einstein ring on the sky, and comparison of SIM and ground-based photometry determines the size of the Einstein ring projected onto the observer plane. Only by combining both of these measurements is it possible to completely solve the microlensing events.

astro-ph↗

arXiv1999

Photometric Microlens Parallaxes with SIM

Astrometric measurements of microlensing events can in principle determine both the "parallax" \tilde r_E and the "proper motion" μof an individual event which (combined with the Einstein time scale t_E) in turn yield the mass, distance, and transverse velocity of the lens. We show, however, that the parallax measurements are generically several orders of magnitude less precise than the proper-motion measurements. Fortunately, astrometric measurements by the Space Interferometry Mission (SIM) are simultaneously **photometric** measurements, and since SIM will be in solar orbit, these allow SIM to be used as a classical (photometric) parallax satellite. We show that SIM photometric parallaxes are of comparable precision to its astrometric proper-motion measurements. For I=15 bulge stars, complete solutions with about 5% accuracy in mass, distance, and transverse velocity can be obtained from about 5 hours of observation, 100 to 10,000 times shorter than would be required for a purely astrometric solution of similar precision. Thus, it should be possible to directly measure the mass functions of both the bulge and the inner disk (including both dark and luminous objects) with only a few hundred hours of SIM observations.

astro-ph↗

arXiv1998

Sgr A* ``Visual Binaries'': A Direct Measurement of the Galactocentric Distance

We present a new geometrical method for measuring the distance to the Galactic center (R_0) by solving for the Keplerian orbit of individual stars bound to the black hole associated with the Sgr A* from radial velocity and proper motion measurements. We identify three stars to which the method may be applied, and show that 1-5 % accuracy of R_0 can be expected after 15 years of observing, and 0.5-2 % after 30 years of observing, depending on what the orbital parameters of these three stars turn out to be. Combining the measurements of the three stars with favorable orbital parameters leads to even more precise values. In the example that we present, such combined solution yields 4 % accuracy already by the year 2002. All these estimates assume that annual position measurements will continue to be made with the 2 mas precision recently reported by Ghez et al. The precision of the distance measurement is relatively insensitive to the radial velocity errors, provided that the latter are less than 50 km/s. Besides potentially giving an estimate of R_0 that is better than any currently in use, the greatest advantage of this method is that it is free from systematic errors.

astro-ph↗

arXiv1998

Empirical Calibration of Hubble Space Telescope WFPC2 F606W and F814W Photometry

Using the ground-based V and I photometry of a sample of stars from the Groth Strip, we obtain the first empirical calibration of the F606W and F814W Hubble Space Telescope WFPC2 filters for 0.5<V-I<4.5. We present results in the form of corrections that need to be applied to the two synthetic calibrations currently in use. Both calibrations are found to require corrections to zero points and color-terms.

astro-ph↗