arXiv subjects

K. L. Luhman

Publications and source records attributed to K. L. Luhman.

At least 91 records · Page 5Linked to original sources

arXiv2006

Discovery of a Young Substellar Companion in Chamaeleon

During an imaging survey of the Chamaeleon I star-forming region with the ACS aboard HST, we have discovered a candidate substellar companion to the young low-mass star CHXR 73 (~2 Myr, M=0.35 Msun). We measure a projected separation of 1.3+/-0.03" for the companion, CHXR 73 B, which corresponds to 210 AU at the distance of the cluster. A near-infrared spectrum of this source obtained with CorMASS at the Magellan II telescope exhibits strong steam absorption that confirms its late-type nature (>=M9.5). In addition, the gravity-sensitive shapes of the H- and K-band continua demonstrate that CHXR 73 B is a young, pre-main-sequence object rather than a field star. The probability that CHXR 73A and B are unrelated members of Chamaeleon I is ~0.001. We estimate the masses of CHXR 73 B and other known substellar companions in young clusters with a method that is consistent with the dynamical measurements of the eclipsing binary brown dwarf 2M 0535-0546, which consists of a comparison of the bolometric luminosities of the companions to the values predicted by the evolutionary models of Chabrier & coworkers and Burrows & coworkers. We arrive at mass estimates of 0.003-0.004, 0.024+/-0.012, 0.011+0.01/-0.003, and 0.012+0.008/-0.005 Msun for 2M 1207-3932 B, GQ Lup B, DH Tau B, and CHXR 73 B, respectively. Thus, DH Tau B and CHXR 73 B appear to be the least massive companions to stars outside the solar system that have been detected in direct images, and may have masses that are within the range observed for extrasolar planetary companions (M<=0.015 Msun). However, because these two objects (as well as 2M 1207-3932 B) probably did not form within circumstellar disks around their primaries, we suggest that they should be viewed as brown dwarf companions rather than planets.

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arXiv2006

The Spatial Distribution of Brown Dwarfs in Taurus

By combining photometry from the 2MASS Point Source Catalog and the USNO-B1.0 Catalog with optical and infrared spectroscopy, I have performed a search for young brown dwarfs in an area of 225 deg^2 encompassing all of the Taurus star-forming region (1 Myr). From this work, I have discovered 22 new members of Taurus, five of which were independently found by Guieu and coworkers. Sixteen of these new members have spectral types later than M6 and thus are likely to be brown dwarfs according to the theoretical evolutionary models of Chabrier and Baraffe. After adding these new members to the previously known members of Taurus, I have compared the spatial distributions of stars and brown dwarfs across the entire region. I find no statistically significant difference between these two distributions. Taurus does not contain the large, extended population of brown dwarfs that has been predicted by some embryo ejection models for the formation of brown dwarfs. However, these results are consistent with other ejection models, as well as models in which stars and brown dwarfs share a common formation mechanism.

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arXiv2006

A Survey for New Members of Taurus with the Spitzer Space Telescope

We present the results of a search for new members of the Taurus star-forming region using the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope}. With IRAC images of 29.7 deg^2 of Taurus at 3.6, 4.5, 5.8, and 8.0 um, we have identified sources with red mid-infrared colors indicative of disk-bearing objects and have obtained optical and infrared spectra of 23 of these candidate members. Through this work, we have discovered 13 new members of Taurus, two of which have spectral types later than M6 and thus are likely to be brown dwarfs according to the theoretical evolutionary models of Chabrier and Baraffe. This survey indicates that the previous census of Taurus has a completeness of ~80% for members with disks. The new members that we have found do not significantly modify the previously measured distributions of Taurus members as a function of position, mass, and extinction. For instance, we find no evidence for a population of highly reddened brown dwarfs (A_K~2) that has been missed by previous optical and near-infrared surveys, which suggests that brown dwarf disks are not significantly more flared than disks around stars. In addition to the new members, we also present IRAC photometry for the 149 previously known members that appear within this survey, which includes 27 objects later than M6.

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arXiv2005

Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk

Using the Hubble Space Telescope, the 4 m Blanco telescope at the Cerro Tololo Inter-American Observatory, and the Spitzer Space Telescope, we have performed deep imaging from 0.8 to 8 um of the southern subcluster in the Chamaeleon I star-forming region. In these data, we have discovered an object, Cha 110913-773444, whose colors and magnitudes are indicative of a very low-mass brown dwarf with a circumstellar disk. In a near-infrared spectrum of this source obtained with the Gemini Near-Infrared Spectrograph, the presence of strong steam absorption confirms its late-type nature (>=M9.5) while the shapes of the H- and K-band continua and the strengths of the Na I and K I lines demonstrate that it is a young, pre-main-sequence object rather than a field dwarf. A comparison of the bolometric luminosity of Cha 110913-773444 to the luminosities predicted by the evolutionary models of Chabrier and Baraffe and Burrows and coworkers indicates a mass of 8+7/-3 M_Jup, placing it fully within the mass range observed for extrasolar planetary companions (M<=15 M_Jup). The spectral energy distribution of this object exhibits mid-infrared excess emission at >5 um, which we have successfully modeled in terms of an irradiated viscous accretion disk with M'<=10e-12 M_sun/year. Cha 110913-773444 is now the least massive brown dwarf observed to have a circumstellar disk, and indeed is one of the least massive free-floating objects found to date. These results demonstrate that the raw materials for planet formation exist around free-floating planetary-mass bodies.

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arXiv2005

Observations of IC348: The Disk Population at 2-3 Million Years

We present near and mid-infrared photometry obtained with the Spitzer Space Telescope of 300 known members of the IC348 cluster. We merge this photometry with existing ground-based optical and near-infrared photometry in order to construct optical-infrared spectral energy distributions (SEDs) for all the cluster members and present a complete atlas of these SEDs. We employ these observations to both investigate the frequency and nature of the circumstellar disk population in the cluster. The observations are sufficiently sensitive to enable the first detailed measurement of the disk frequency for very low mass stars at the peak of the stellar IMF. Using measurements of infrared excess between 3.6 and 8 microns we find the total frequency of disk-bearing stars in the cluster to be 50 +/- 6%. However, only 30 +/- 4% of the member stars are surrounded by optically thick, primordial disks, while the remaining disk-bearing stars are surrounded by what appear to be optically thin,anemic disks. The disk fraction appears to be a function of spectral type and stellar mass. The disk longevity and thus conditions for planet formation appear to be most favorable for stars which are of comparable mass to the sun. The optically thick disks around later type (> M4) stars appear to be less flared than the disks around earlier type stars. This may indicate a greater degree of dust settling and a more advanced evolutionary state for the late M disk population. Finally we find that the presence of an optically thick dust disk is correlated with gaseous accretion as measured by the strength of Halpha emission. These results suggest that it is more likely for dust disks to persist in the absence of active gaseous accretion than for active accretion to persist in the absence of dusty disks.

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arXiv2005

Spitzer/IRAC Photometry of the Eta Chameleontis Association

We present IRAC 3.6, 4.5, 5.8 and 8 micron photometry for the 17 A, K and M type members of the Eta Chameleontis association. These data show infrared excesses toward six of the 15 K and M stars, indicating the presence of circumstellar disks around 40% of the stars with masses of 0.1-1 solar mass. The two A-stars show no infrared excesses. The excess emission around one of the stars is comparable to the median excess for classical T Tauri stars in the Taurus association; the remaining five show comparatively weak excess emission. Taking into account published Halpha spectroscopy that shows that five of the six stars are accreting, we argue that the disks with weak mid-infrared excesses are disks in which the inner disks have been largely depleted of small grains by grain growth, or, in one case, the small grains have settled to the midplane. This suggests that Eta Cha has a much higher fraction of disks caught in the act of transitioning into optically thin disks than that measured in younger clusters and associations.

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arXiv2005

The Age of AB Dor

We have derived a new age estimate for the nearby young star AB Dor and have investigated the resulting implications for testing theoretical evolutionary models with the data reported by Close and coworkers for the low-mass companion AB Dor C. Using color-magnitude diagrams, we find that the AB Dor moving group is roughly coeval with the Pleiades (100-125 Myr) and is clearly older than IC 2391 (35-50 Myr). In fact, based on a comparison of the kinematics of AB Dor and the Pleiades, we suggest that the stars identified by Zuckerman and coworkers as members of a moving group with AB Dor are remnants of the large scale star-formation event that formed the Pleiades. Using the age of 50+50/-20 Myr adopted by Close, the luminosity predicted by the models of Chabrier and Baraffe for AB Dor C is larger than the value reported by Close, but is still within the quoted uncertainties. Meanwhile, the agreement is good when our age estimate for AB Dor C is adopted. Thus, we find no evidence in the data presented by Close for AB Dor C to suggest that previous studies using the models of Chabrier and Baraffe and bolometric luminosity as the mass indicator have significantly underestimated the masses of young low-mass stars and brown dwarfs.

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arXiv2005

The Disk Fractions of Brown Dwarfs in IC 348 and Chamaeleon I

Using the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope, we have obtained mid-infrared photometry for 25 and 18 low-mass members of the IC 348 and Chamaeleon I star-forming clusters, respectively (>M6, M<=0.08 Msun). We find that 42+/-13% and 50+/-17% of the two samples exhibit excess emission indicative of circumstellar disks. In comparison, the disk fractions for stellar members of these clusters are 33+/-4% and 45+/-7% (M0-M6, 0.7 Msun>=M>=0.1 Msun). The similarity of the disk fractions of stars and brown dwarfs is consistent with a common formation mechanism and indicates that the raw materials for planet formation are available around brown dwarfs as often as around stars.

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arXiv2005

Discovery of a Wide Low-mass Binary System in Upper Scorpius

Using the near-infrared spectrometer SpeX and its slit-viewing camera at the IRTF, I have resolved a low-mass member of the Upper Scorpius OB association into a double star. From K-band images of the pair, DENIS-P J161833.2-251750.4 A and B, I measure a separation of 0.96" and a magnitude difference of dK=0.42 mag. I present resolved 0.8-2.5 micron spectroscopy of the two objects, both of which exhibit signatures of youth in the shape of their H- and K-band continua, demonstrating that both are members of Upper Scorpius rather than field stars. In addition, through a comparison to optically-classified pre-main-sequence objects, I derive a spectral type near M5 for each component, corresponding to a mass of ~0.15 Msun with the evolutionary models of Chabrier and Baraffe. The probability that this pair is composed of unrelated M-type members of Upper Scorpius is ~10^-5. When added to the recent discoveries of other wide, easily disrupted low-mass binaries, this new system further establishes that the formation of low-mass stars and brown dwarfs does not require ejection from multiple systems. These observations also indicate that wide low-mass binaries can form in OB associations as well as in smaller clusters where the previously known wide pairs have been found. Thus, the available data show no perceptible effect of star-forming environment on the prevalence of loosely bound low-mass systems.

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arXiv2005

Testing Theoretical Evolutionary Models with AB Dor C and the Initial Mass Function

We assess the constraints on the evolutionary models of young low-mass objects that are provided by the measurements of the companion AB Dor C by Close and coworkers and by a new comparison of model-derived IMFs of star-forming regions to the well-calibrated IMF of the solar neighborhood. After performing an independent analysis of Close's imaging and spectroscopic data for AB Dor C, we find that AB Dor C is not detected at a significant level (SN 1.2) in the SDI images when one narrow-band image is subtracted from another, but that it does appear in the individual SDI frames as well as the images at JHK. Using the age of 75-150 Myr for AB Dor from Luhman, Stauffer, & Mamajek, the luminosity predicted by the models of Chabrier & Baraffe is consistent with the value that we estimate. We measure a spectral type of M6+/-1 from the K-band spectrum of AB Dor C, which is earlier than the value of M8+/-1 from Close and is consistent with the model predictions when a dwarf temperature scale is adopted. In a test of these models at much younger ages, we show that the low-mass IMFs that they produce for star-forming regions are similar to the IMF of the solar neighborhood. If the masses of the low-mass stars and brown dwarfs in these IMFs of star-forming regions were underestimated by a factor of two as suggested by Close, then the IMF characterizing the current generation of Galactic star formation would have to be radically different from the IMF of the solar neighborhood.

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arXiv2005

Spitzer Identification of the Least Massive Known Brown Dwarf with a Circumstellar Disk

Using the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope, we have obtained mid-infrared photometry of the least massive known brown dwarf in the Chamaeleon I star-forming region. For this young brown dwarf, OTS 44, we have constructed a spectral energy distribution (SED) from 0.8-8 um by combining the measurements at 3.6, 4.5, 5.8, and 8.0 um from IRAC with ground-based photometry at I, J, H, and K. The resulting SED for OTS 44 exhibits significant excess emission longward of 3 um relative to the SED expected from the photosphere of the brown dwarf. We have successfully modeled the source of this excess emission in terms of an irradiated viscous accretion disk with M'<=10^-10 M_sun/year. With a spectral type of M9.5 and a mass of ~15 M_Jup, OTS 44 is now the coolest and least massive brown dwarf observed to have a circumstellar disk. These measurements demonstrate that disks exist around brown dwarfs even down to the deuterium burning mass limit and the approximate upper mass limit of extrasolar planetary companions.

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arXiv2005

Measuring Accretion in Young Substellar Objects: Approaching the Planetary Mass Regime

We present observations of Halpha emission line profiles taken at Magellan Observatory for a sample of 39 young low-mass stars and brown dwarfs in the Taurus and Chamaeleon I star forming regions. We have identified 11 new substellar accretors, more than tripling the number of known brown dwarfs with measurable accretion activity. These include the lowest-mass objects yet seen with accretion, with masses down to ~0.015 Msun. Using models of Halpha emission produced in magnetospheric accretion flows, the most widely applicable primary calibrator now available, we determine the first estimates of mass accretion rates for objects at such extremely low masses. For the six objects with masses < 0.03 Msun, we find accretion rates of ~ 5x10^{-12} Msun/yr, among the smallest yet measured. These new results continue the trend of decreasing mass accretion rate with decreasing (sub)stellar mass that we have noted previously for samples of more massive objects; the overall correlation is Mdot \~ M^{2.1}, and now extends over a mass range of over two orders of magnitude. Finally, the absence of a discontinuity in the distribution of accretion rates with mass tends to suggest that stars and brown dwarfs share similar formation histories.

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arXiv2005

An HST Search for Substellar Companions in the Young Cluster IC 348

We present the results of a HST WFPC2 search for substellar companions to members of the star-forming cluster IC 348. The detection limits for companions to low-mass stars and brown dwarfs in the PC images are dm_791=0, 2.5, and 5.5 at separations of 0.05", 0.1", and 0.3", respectively, which correspond to M_2/M_1=1, 0.3, and 0.1 at 15, 30, and 90 AU. Meanwhile, for heavily saturated solar-mass primaries in the WFC images, the limits are dm_791=0 and 6 (M_2/M_1=1 and 0.04) at 0.2" and 0.4". The sky limiting magnitude of m_791~26 at large separations from a primary corresponds to a mass of ~0.006 M_sun. For the two faintest candidate companions appearing in these data, we have obtained 0.8-2.5 um spectra with SpeX at the IRTF, which we classify as cluster members with spectral types near M6, corresponding to masses of ~0.1 M_sun. Thus, no probable substellar companions are detected in this survey. The frequencies of stellar and substellar companions within 0.4-5" (120-1600 AU) from low-mass stars in IC 348 agree with measurements in the field. The factor of ~3-10 deficiency in brown dwarfs relative to stars among companions at wide separations in IC 348 and across the much larger range of separations probed for field stars is equal within the uncertainties to the deficiency in brown dwarfs in measurements of mass functions of isolated objects. In other words, when defined relative to stars, the brown dwarf "desert" among companions is also present among isolated objects, which is expected if stellar and substellar companions form in the same manner as their free-floating counterparts. Meanwhile, among the 14 substellar primaries in our survey of IC 348, no companions are detected. This absence of wide binary brown dwarfs is statistically consistent with the frequency of wide binary stars in IC 348.

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arXiv2004

New Brown Dwarfs and an Updated Initial Mass Function in Taurus

I have performed a search for young low-mass stars and brown dwarfs (BDs) in 2 regions encompassing a total area of 4 deg^2 in the Taurus star-forming region, discovering 15 new members of Taurus. In addition, I present 7 new members outside of these areas from the initial stage of a survey of all of Taurus. These 22 objects exhibit spectral types of M4.5-M9.25 and masses of 0.3-0.015 M_sun according to the theoretical evolutionary models of Baraffe and Chabrier, 7 of which are likely to be BDs. Emission in H(alpha), He I, Ca II, [O I], and [S II] and excess emission in optical and near-IR bands among some of these objects suggest the presence of accretion, outflows, and circumstellar disks. The results from the 4 deg^2 survey have been combined with previous studies of Taurus to arrive at an IMF for a total area of 12.4 deg^2. As in the previous IMFs for Taurus, the updated IMF peaks at a higher mass (0.8 M_sun) than the mass functions in IC 348 and Orion (0.1-0.2 M_sun). Meanwhile, the deficit of BDs in Taurus appears to be less significant (x1.4-1.8) than found in earlier studies (x2) because of a slightly higher BD fraction in the new IMF for Taurus and a lower BD fraction in the new spectroscopic IMF for the Trapezium from Slesnick and coworkers. The spatial distribution of the low-mass stars and BDs discovered in the two new survey areas closely matches that of the more massive members. Thus, on the degree size scales (~3 pc) probed to date, there is no indication that BDs form through ejection.

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arXiv2004

A Survey for Low-Mass Stars and Brown Dwarfs in the Eta Cha and Eps Cha Young Associations

I present the results of a search for new low-mass stars and brown dwarfs in the Eta Cha and Eps Cha young associations. Within radii of 1.5 and 0.5 deg surrounding Eta Cha and Eps Cha, respectively, I have constructed color-magnitude diagrams from DENIS and 2MASS photometry and have obtained spectra of the candidate low-mass members therein. The five candidates in Eta Cha are classified as four field M dwarfs and one carbon star. No new members are found in this survey, which is complete for M_sun=0.015-0.15 according to the evolutionary models of Chabrier and Baraffe. Thus, an extended population of low-mass members is not present in Eta Cha out to four times the radius of the known membership. Meanwhile, the three candidate members of Eps Cha are classified as young stars, and thus likely members of the association, based on Li absorption and gravity-sensitive absorption lines. These new sources have spectral types of M2.25, M3.75, and M5.75, corresponding to masses of 0.45, 0.25, and 0.09 M_sun by the models of Chabrier and Baraffe. For one of these stars, intense H(alpha) emission, forbidden line emission, and strong K-band excess emission suggest the presence of accretion, an outflow, and a disk, respectively. This young star is also much fainter than expected for an association member at its spectral type, which could indicate that it is seen in scattered light. No brown dwarfs are detected in Eps Cha down to the completeness limit of 0.015 M_sun. The absence of brown dwarfs in these associations is statistically consistent with the mass functions measured in star-forming regions, which exhibit only ~2 and ~1 brown dwarfs for stellar samples at the sizes of the Eta Cha and Eps Cha associations.

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arXiv2004

Spectroscopic Confirmation of the Least Massive Known Brown Dwarf in Chamaeleon

We present spectroscopy of two candidate substellar members of the Chamaeleon I star-forming region. The candidates, which were identified photometrically by Oasa, Tamura, & Sugitani, have been observed at 1-2.5 micron during commissioning of the Gemini Near-Infrared Spectrograph. The late-type nature of one of the candidates, OTS 44, is confirmed through the detection of strong steam absorption bands. The other object, OTS 7, exhibits no late-type features and is likely a background star or galaxy. The gravity-sensitive shape of the H- and K-band continua demonstrate that OTS 44 is a young, pre-main-sequence object rather than a field dwarf. We measure a spectral type of M9.5 for OTS 44 based on a comparison of its spectrum to data for optically-classified young late-type objects. Because OTS 44 is the coolest and faintest object with confirmed membership in Chamaeleon I, it is very likely the least massive known member of the cluster. By comparing the position of OTS 44 on the H-R diagram to the evolutionary models of Chabrier & Baraffe, we infer a mass of ~0.015 M_sun. Although this estimate is uncertain by at least a factor of two, OTS 44 is nevertheless one of the least massive free-floating brown dwarfs confirmed spectroscopically to date.

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arXiv2004

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).

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arXiv2004

The First Discovery of a Wide Binary Brown Dwarf

I present observations of a new faint double, 2MASS J11011926-7732383AB, toward the Chamaeleon I star-forming region. From optical and near-infrared images of the pair, I measure a separation of 1.44" and extract RIJHK_s photometry of the components (I_A=17.21, dI=1.07, K_s A=11.97, dK_s=0.84). I use resolved optical spectroscopy to derive spectral types of M7.25 and M8.25 for the A and B components, respectively. Based on the strengths of gravity-sensitive features in these data, such as the Na I and K I absorption lines, I conclude that these objects are young members of Chamaeleon I rather than field stars. The probability that this pair is composed of unrelated late-type members of Chamaeleon I is low enough (~5x10^-5) to definitively establish it as a binary system. After estimating extinctions, effective temperatures, and bolometric luminosities for the binary components, I place them on the H-R diagram and infer their masses with the evolutionary models of Chabrier and Baraffe, arriving at substellar values of 0.05 and 0.025 M_sun. The projected angular separation of this system corresponds to 240 AU at the distance of Chamaeleon I, making it the first known binary brown dwarf with a separation greater than 20 AU. This demonstration that brown dwarfs can form in fragile, easily disrupted configurations is direct evidence that the formation of brown dwarfs does not require ejection from multiple systems or other dynamical effects. It remains possible that ejection plays a role in the formation of some brown dwarfs, but it is not an essential component according to these observations.

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