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K. L. Luhman

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

At least 19 recordsLinked to original sources
arXiv2026

JWST's Constraints on the Substellar IMF in NGC 2024

A recent study has reported the detection of a turnover in the initial mass function (IMF) below 12 Mjup and the possible detection of a minimum mass near 3 Mjup in the heavily embedded cluster NGC 2024 (<1 Myr), which was based on a sample of brown dwarf candidates in NIRCam images from the James Webb Space Telescope (JWST). I have used those NIRCam data in conjunction with archival spectra from JWST's Near-Infrared Spectrograph (NIRSpec) to constrain the substellar IMF in NGC 2024. I present NIRSpec data for 87 sources, 67 of which are within the NIRCam field. Based on those spectra and data from previous studies (e.g., X-ray emission), I have classified 45 of the NIRSpec targets as members of NGC 2024, 17 of which have spectral types indicative of brown dwarfs (M6.5-L; 13 are within NIRCam). The latter have mass estimates as low as 4 Mjup according to theoretical evolutionary models. There remain a few photometric candidates lacking spectra that extend down to the completeness limit of the images (>=2 Mjup for Ak>=1), so the minimum mass of the IMF has not been detected. It is not possible to measure a reliable substellar IMF from the NIRCam images because of the small number of brown dwarfs encompassed by them, a bias against objects at lower masses due to the high extinctions, and incompleteness at >=10 Mjup due to the saturation limit. Thus, there is no evidence in the JWST data for a turnover below 12jup.

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arXiv2025

JWST Spectra of Brown Dwarf Candidates in the Orion Nebula Cluster

I present an analysis of archival spectra of 200 sources toward the Orion Nebula Cluster (ONC) that were obtained with the Near-Infrared Spectrograph (NIRSpec) on board the James Webb Space Telescope (JWST). I have used these data to assess cluster membership and measure spectral types for the targets. Fifty-three sources are classified as likely cluster members, 24 of which have spectral types that are suggestive of brown dwarfs ($>$M6). Seven of the NIRSpec targets were previously identified as "Jupiter-mass binary objects" (JuMBOs), all of which are background sources rather than brown dwarfs based on the NIRSpec data. The spectral classifications of those objects are consistent with the results of my recent study of the JWST photometry in the ONC, which found that only a few JuMBO components have the colors expected for brown dwarfs, none of which form pairs that have uniquely wide separations or low masses relative to known binary brown dwarfs.

astro-ph.GA↗
arXiv2025

A New Spectral Class of Brown Dwarfs at the Bottom of the IMF in IC 348

In a previous study, we used JWST to identify three new brown dwarfs in the center of a nearby star-forming cluster, IC 348. The faintest object had an estimated mass of 3-4 $M_{\rm Jup}$, making it a contender for the least massive brown dwarf confirmed with spectroscopy. Two of the new brown dwarfs also exhibited absorption features from an unidentified aliphatic hydrocarbon, which were not predicted by atmospheric models and were not previously detected in atmospheres outside of the solar system. We have used JWST to perform a deeper survey for brown dwarfs across a larger field in IC 348. We have identified 39 brown dwarf candidates in NIRCam images and have obtained spectra for 15 of them with NIRSpec, nine of which are classified as substellar members of the cluster. The faintest new members have mass estimates of $\sim2$ $M_{\rm Jup}$, providing a new constraint on the minimum mass of the IMF. Two new members ($\sim2$ and 10 $M_{\rm Jup}$) exhibit large excess emission from circumstellar disks, demonstrating that they harbor the raw materials for planet formation. Finally, eight of the nine new brown dwarfs and one known member that is newly observed with NIRSpec show the aforementioned hydrocarbon features. Among the total of 11 brown dwarfs in IC 348 that have hydrocarbon detections, the features are stronger at fainter magnitudes, indicating that the hydrocarbon is a natural constituent of the atmospheres of the coolest newborn brown dwarfs. We propose a new spectral class "H" that is defined by the presence of the 3.4 $μ$m fundamental band of the hydrocarbon.

astro-ph.GA↗
arXiv2025

The Initial Mass Function of Stars and Brown Dwarfs in the Upper Sco Association

I present infrared spectroscopy of 37 brown dwarf candidates in the Upper Sco association, 35 of which are classified as young and cool, making them likely members. This sample includes many of the faintest spectroscopically confirmed members ($K=16$-17 mag), which should have masses down to $\sim0.007$-0.01 $M_\odot$ for the range of ages in Upper Sco (7-14 Myr). Using my updated membership catalog for Upper Sco, I have estimated the initial mass function (IMF) for a field in the center of the association that encompasses $\sim80$\% of the known members. I have derived IMFs in the same manner for previous membership samples in three other star-forming populations, consisting of IC 348, Taurus, and Chameleon I. When using logarithmic mass bins, the substellar IMFs for Upper Sco and the other young regions are roughly flat down to the completeness limits of $\sim$0.01 $M_\odot$. These IMFs are broadly similar to mass functions recently measured for the solar neighborhood. Finally, I have used W1$-$W2 colors to search for excess emission from circumstellar disks among the late-type objects in my new census of Upper Sco. I measure an excess fraction of 52/200 for members with spectral types of M6.25-M9.5, which is similar to results from previous membership catalogs. For the L-type members, it is difficult to detect the small W2 excess emission produced by typical disks around brown dwarfs because of the large uncertainties in spectral types, which preclude accurate estimates of the photospheric colors. Thus, W2 photometry provides poor constraints on the presence of disks for the L-type members of Upper Sco.

astro-ph.SR↗
arXiv2025

The Membership and Age of the Planet-hosting Young Star IRAS 04125+2902

A transiting planet was recently discovered around a star in the Taurus star-forming region, IRAS 04125+2902, making it one of the youngest known planets. The discovery paper cited two age estimates for IRAS 04125+2902, one based on a comparison to two sets of model isochrones in the Hertzsprung-Russell (H-R) diagram and a second age reported by an earlier study for a putative population in Taurus that includes IRAS 04125+2902 (D4-North). However, the model isochrones in question differ significantly for most temperatures and luminosities of young low-mass stars, and do not reproduce the observed sequences for the TW Hya and 32 Ori associations (10 and 21 Myr). Meanwhile, as found in my previous work, D4-North is a collection of field stars and fragments of several distinct Taurus groups and older associations, so its quoted age is not meaningful. The true parent population for IRAS 04125+2902 is a small group that is ~35 pc behind the L1495 and B209 clouds (B209N). I have analyzed the age of B209N through a comparison to TW Hya and 32 Ori. The M star sequences in the latter two associations have the same shapes, but the sequence for B209N is flatter, indicating that >M4 stars at ages of <10 Myr fade more quickly than stars at earlier types and older ages. For the one member of B209N that is earlier than M4 (IRAS 04125+2902), I estimate an age of 3.0+/-0.4 Myr based on its offsets from TW Hya and 32 Ori, which by happenstance is similar to the value derived through the comparison to model isochrones.

astro-ph.GA↗
arXiv2024

Candidates for Substellar Members of the Orion Nebula Cluster from JWST/NIRCam

In 2022, the James Webb Space Telescope (JWST) obtained 1-5um images of the center of the Orion Nebula Cluster (ONC). I have analyzed these data in an attempt to search for substellar members of the cluster. Using a pair of color-color diagrams, I have identified >200 brown dwarf candidates that lack spectral classifications. Some of the candidates could be protostars (either stellar or substellar) given their very red colors. Based on the age of the ONC and the photometry predicted by theoretical evolutionary models, the faintest candidates could have masses of 1-2 Mjup. This sample of candidates may prove to be valuable for studying various aspects of young brown dwarfs, including their mass function and minimum mass. However, spectroscopy is needed to confirm the membership (via signatures of youth) and late spectral types of the candidates. Finally, I note that most of the "Jupiter-mass binary objects" that have been previously identified with these JWST images are absent from my sample of candidates because their colors are indicative of reddened background sources rather than young brown dwarfs or their photometry is inadequate for assessing their nature because of very low signal-to-noise ratios and/or detections in only a few bands.

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arXiv2024

JWST/NIRSpec Observations of Brown Dwarfs in the Orion Nebula Cluster

We have used the multiobject mode of the Near-Infrared Spectrograph (NIRSpec) on board the James Webb Space Telescope (JWST) to obtain low-resolution 1-5um spectra of 22 brown dwarf candidates in the Orion Nebula Cluster, which were selected with archival images from the Hubble Space Telescope. One of the targets was previously classified as a Herbig-Haro (HH) object and exhibits strong emission in H I, H2, and the fundamental band of CO, further demonstrating that HH objects can have bright emission in that CO band. The remaining targets have late spectral types (M6.5 to early L) and are young based on gravity sensitive features, as expected for low-mass members of the cluster. According to theoretical evolutionary models, these objects should have masses that range from the hydrogen burning limit to 0.003-0.007 Msun. Two of the NIRSpec targets were identified as proplyds in earlier analysis of Hubble images. They have spectral types of M6.5 and M7.5, making them two of the coolest and least massive known proplyds. Another brown dwarf shows absorption bands at 3-5um from ices containing H2O, CO2, OCN-, and CO, indicating that it is either an edge-on class II system or a class I protostar. It is the coolest and least massive object that has detections of these ice features. In addition, it appears to be the first candidate for a protostellar brown dwarf that has spectroscopy confirming its late spectral type.

astro-ph.GA↗
arXiv2024

A Census of the beta Pic Moving Group and Other Nearby Associations with Gaia

I have used the third data release of the Gaia mission to improve the reliability and completeness of membership samples in the beta Pic moving group (BPMG) and other nearby associations with ages of 20-50 Myr (Sco Body, Carina, Columba, chi1 For, Tuc-Hor, IC 2602, IC 2391, NGC 2547). I find that Carina, Columba, and chi1 For are physically related and coeval, and that Carina is the closest fringe of a much larger association. Similarly, Tuc-Hor and IC 2602 form a coeval population that is spatially and kinematically continuous. Both results agree with hypotheses from Gagne et al. (2021). I have used the new catalogs to study the associations in terms of their initial mass functions, X-ray emission, ages, and circumstellar disks. For instance, using the model for Li depletion from Jeffries et al. (2023), I have derived an age of 24.7+0.9/-0.6 Myr for BPMG, which is similar to estimates from previous studies. In addition, I have used infrared photometry from the Wide-field Infrared Survey Explorer to check for excess emission from circumstellar disks among the members of the associations, which has resulted in a dramatic increase in the number of known disks around M stars at ages of 30-50 Myr and a significant improvement in measurements of excess fractions for those spectral types and ages. Most notably, I find that the W3 excess fraction for M0-M6 initially declines with age to a minimum in BPMG <0.015), increases to a maximum in Carina/Columba chi1 For (0.041+0.009/-0.007, 34 Myr), and declines again in the oldest two associations (40-50 Myr). The origin of that peak and the nature of the M dwarf disks at >20 Myr are unclear.

astro-ph.GA↗
arXiv2023

JWST/NIRSpec Observations of the Coldest Known Brown Dwarf

We present 1-5um spectroscopy of the coldest known brown dwarf, WISE J085510.83-071442.5 (WISE 0855), performed with the Near-Infrared Spectrograph (NIRSpec) on board the James Webb Space Telescope (JWST). NIRSpec has dramatically improved the measurement of spectral energy distribution of WISE 0855 in terms of wavelength coverage, signal-to-noise ratios, and spectral resolution. We have performed preliminary modeling of the NIRSpec data using the ATMO 2020 models of cloudless atmospheres, arriving at a best fitting model that has T_eff=285 K. That temperature is ~20 K higher than the value derived by combining our luminosity estimate with evolutionary models (i.e., the radius in the model fit to the SED is somewhat smaller than expected from evolutionary models). Through comparisons to the model spectra, we detect absorption in the fundamental band of CO, which is consistent with an earlier detection in a ground-based spectrum and indicates the presence of vertical mixing. Although PH_3 is expected in Y dwarfs that experience vertical mixing, it is not detected in WISE 0855. Previous ground-based M-band spectroscopy of WISE 0855 has been cited for evidence of H_2O ice clouds, but we find that the NIRSpec data in that wavelength range are matched well by our cloudless model. Thus, clear evidence of H_2O ice clouds in WISE 0855 has not been identified yet, but it may still be present in the NIRSpec data. The physical properties of WISE 0855, including the presence of H_2O clouds, can be better constrained by more detailed fitting with both cloudless and cloudy models and the incorporation of unpublished 5-28um data from the Mid-infrared Instrument on JWST.

astro-ph.SR↗
arXiv2023

JWST/NIRSpec Observations of the Planetary Mass Companion TWA 27B

We present 1-5um spectroscopy of the young planetary mass companion TWA 27B (2M1207B) performed with NIRSpec on board the James Webb Space Telescope. In these data, the fundamental band of CH_4 is absent and the fundamental band of CO is weak. The nondetection of CH_4 reinforces a previously observed trend of weaker CH_4 with younger ages among L dwarfs, which has been attributed to enhanced non-equilibrium chemistry among young objects. The weakness of CO may reflect an additional atmospheric property that varies with age, such as the temperature gradient or cloud thickness. We are able to reproduce the broad shape of the spectrum with an ATMO cloudless model that has T=1300 K, non-equilibrium chemistry, and a temperature gradient reduction caused by fingering convection. However, the fundamental bands of CH_4 and CO are somewhat stronger in the model. In addition, the model temperature of 1300 K is higher than expected from evolutionary models given the luminosity and age of TWA 27B (T=1200 K). Previous models of young L-type objects suggest that the inclusion of clouds could potentially resolve these issues; it remains to be seen whether cloudy models can provide a good fit to the 1-5um data from NIRSpec. TWA 27B exhibits emission in Paschen transitions and the He I triplet at 1.083um, which are signatures of accretion that provide the first evidence of a circumstellar disk. We have used the NIRSpec data to estimate the bolometric luminosity of TWA 27B (log L/L_sun=-4.466+/-0.014), which implies a mass of 5-6 MJup according to evolutionary models.

astro-ph.EP↗
arXiv2023

A Census of the TW Hya Association with Gaia

I have used high-precision photometry and astrometry from the third data release of Gaia to perform a survey for members of the TW Hya association (TWA). I have identified candidate members that appear to share similar kinematics and ages with bona fide members compiled by Gagne et al. (2017) and I have assessed their membership using radial velocities and spectroscopic diagnostics of age from various sources. My new catalog of adopted members contains 67 Gaia sources in 55 systems. The histogram of spectral types for TWA peaks near M5 (~0.15 Msun), resembling the distributions measured for other nearby young associations. The UVW velocities of its members indicate that the association is expanding. The rate of expansion corresponds to an age of 9.6+0.9/-0.8 Myr. In a Gaia color-magnitude diagram, the members of TWA exhibit well-defined sequences of single stars and unresolved binary stars. The combined sequence of low-mass stars in TWA is indicative of an age of 11.4+1.3/-1.2 Myr when compared to the sequence for Upper Centaurus-Lupus/Lower Centaurus-Crux, for which an age of 20 Myr is assumed. Based on these expansion and isochronal ages, I have adopted an age of 10+/-2 Myr for TWA. Finally, I have used mid-infrared photometry from the Wide-field Infrared Survey Explorer to check for excess emission from circumstellar disks among the TWA members. Fourteen members have detected disks, all of which have been reported in previous studies. The fraction of members at <=M6 (>=0.1 Msun) that have full, transitional, or evolved disks is 10/52=0.19+0.08/-0.06. That value is similar to the fraction previously measured for the Upper Sco association, which is roughly coeval with TWA.

astro-ph.GA↗
arXiv2022

A Census of the Taurus Star-forming Region and Neighboring Associations with Gaia

I have used high-precision photometry and astrometry from the third data release of Gaia (DR3) to perform a survey for members of the Taurus star-forming region and young associations in its vicinity. This work has produced a new catalog of 532 adopted members of Taurus, which has only minor changes relative to the previous catalog from Esplin & Luhman 2019. I have used the Gaia astrometry to divide the Taurus members into 13 groups that have distinct kinematics. Meanwhile, I have identified 1378 candidate members of seven associations near Taurus. All of these associations have histograms of spectral types that peak near M5 (~0.15 Msun), resembling other young populations in the solar neighborhood. For the Taurus groups and neighboring associations, I have estimated ages from their sequences of low-mass stars in Hertzsprung-Russell diagrams. Most of the Taurus groups have median ages of ~1-3 Myr while the associations have ages ranging from 13 to 56 Myr. I have used mid-infrared photometry from the Wide-field Infrared Survey Explorer to search for excess emission from circumstellar disks among the candidate members of the associations. Disks are detected for 51 stars, 20 of which are reported for the first time in this work. Some recent studies have proposed that samples of older stars (>=10 Myr) found in the vicinity of Taurus represent a distributed population that is associated with the Taurus cloud complex. However, I find that most of those stars have kinematics that are inconsistent with any relationship with Taurus.

astro-ph.GA↗
arXiv2022

A Census of the 32 Ori Association with Gaia

I have used high-precision photometry and astrometry from the third data release of Gaia (DR3) to identify candidate members of the 32 Ori association. Spectral types and radial velocities have been measured for subsets of the candidates using new and archival spectra. For the candidates that have radial velocity measurements, I have used UVW velocities to further constrain their membership, arriving at a final catalog of 169 candidates. I estimate that the completeness of the survey is ~90% for spectral types of <=M7 (>=0.06 Msun). The histogram of spectral types for the 32 Ori candidates exhibits a maximum at M5 (~0.15 Msun), resembling the distributions measured for other young clusters and associations in the solar neighborhood. The available UVW velocities indicate that the association is expanding, but they do not produce a well-defined kinematic age. Based on their sequences of low-mass stars in color-magnitude diagrams, the 32 Ori association and Upper Centaurus-Lupus/Lower Centaurus-Crux (UCL/LCC) are coeval to within +/-1.2 Myr, and they are younger than the Beta Pic moving group by ~3 Myr, which agrees with results from previous analysis based on the second data release of Gaia. Finally, I have used mid-infrared (IR) photometry from the Wide-field Infrared Survey Explorer to check for excess emission from circumstellar disks among the 32 Ori candidates. Disks are detected for 18 candidates, half of which are reported for the first time in this work. The fraction of candidates at <=M6 that have full, transitional, or evolved disks is 10/149=0.07+0.03/-0.02, which is consistent with the value for UCL/LCC.

astro-ph.GA↗
arXiv2021

A Census of the Stellar Populations in the Sco-Cen Complex

I have used high-precision photometry and astrometry from the early installment of the third data release of Gaia (EDR3) to perform a survey for members of the stellar populations within the Sco-Cen complex, which consist of Upper Sco, UCL/LCC, the V1062 Sco group, Ophiuchus, and Lupus. Among Gaia sources with sigma(pi)<1 mas, I have identified 10,509 candidate members of those populations. I have compiled previous measurements of spectral types, Li equivalent widths, and radial velocities for the candidates, which are available for 3169, 1420, and 1740 objects, respectively. In a subset of candidates selected to minimize field star contamination, I estimate that the contamination is <1% and the completeness is ~90% at spectral types of <=M6-M7 for the populations with low extinction (Upper Sco, V1062 Sco, UCL/LCC). I have used that cleaner sample to characterize the stellar populations in Sco-Cen in terms of their initial mass functions, ages, and space velocities. For instance, all of the populations in Sco-Cen have histograms of spectral types that peak near M4--M5, which indicates that they share similar characteristic masses for their initial mass functions (~0.15-0.2 Msun). After accounting for incompleteness, I estimate that the Sco-Cen complex contains nearly 10,000 members with masses above ~0.01 Msun. Finally, I also present new estimates for the intrinsic colors of young stars and brown dwarfs (<=20 Myr) in bands from Gaia EDR3, the Two Micron All Sky Survey, the Wide-field Infrared Survey Explorer, and the Spitzer Space Telescope.

astro-ph.GA↗
arXiv2021

A Census of the Circumstellar Disk Populations in the Sco-Cen Complex

I have used mid-infrared (IR) photometry from the Wide-field Infrared Survey Explorer (WISE) to perform a census of circumstellar disks among ~10,000 candidate members of the Sco-Cen complex that were recently identified with data from the Gaia mission. IR excesses are detected for more than 1200 of the WISE counterparts that are within the commonly adopted boundary for Sco-Cen, ~400 of which are newly reported in this work. The richest population in Sco-Cen, UCL/LCC, contains the largest available sample of disks (>500) for any population near its age (~20 Myr). UCL/LCC also provides the tightest statistical constraints to date on the disk fractions of low-mass stars for any single age beyond that of Upper Sco (~11 Myr). For Upper Sco and UCL/LCC, I have measured the disk fractions as a function of spectral type. The disk fraction in Upper Sco is higher at later spectral types, which is consistent with the results for previous samples of candidate members. In UCL/LCC, that trend has become more pronounced; the disk fractions in UCL/LCC are lower than those in Upper Sco by factors of ~10, 5.7, and 2.5 at B7-K5.5, K6-M3.5, and M3.75-M6, respectively. The data in UCL/LCC also demonstrate that the disk fraction for low-mass stars remains non-negligible at an age of 20 Myr (0.09+/-0.01). Finally, I find no significant differences in the ages of disk-bearing and diskless low-mass stars in Upper Sco and UCL/LCC based on their positions in color-magnitude diagrams.

astro-ph.GA↗
arXiv2021

Spectroscopy of Candidate Members of the Sco-Cen Complex

We present spectroscopy of 285 previously identified candidate members of populations in the Sco-Cen complex, primarily Ophiuchus, Upper Sco, and Lupus. The spectra are used to measure spectral types and diagnostics of youth. We find that 269 candidates exhibit signatures of youth in our spectra or previous data, which is consistent with their membership in Sco-Cen. We have constructed compilations of candidate members of Ophiuchus, Upper Sco, and Lupus that have spectral classifications and evidence of youth, which contain a total of 2274 objects. In addition, we have used spectra from previous studies to classify three sources in Ophiuchus that have been proposed to be protostellar brown dwarfs: ISO Oph 70, 200, and 203. We measure spectral types of early M from those data, which are earlier than expected for young brown dwarfs based on evolutionary models (>=M6.5) and instead are indicative of stellar masses (~0.6 Msun).

astro-ph.GA↗
arXiv2021

The Initial Mass Function of Low-mass Stars and Brown Dwarfs in the W3 Complex

We have used archival infrared images obtained with the Wide Field Camera 3 on board the Hubble Space Telescope to constrain the initial mass function of low-mass stars and brown dwarfs in the W3 star-forming region. The images cover 438 arcmin$^2$, which encompasses the entire complex, and were taken in the filters F110W, F139M, and F160W. We have estimated extinctions for individual sources in these data from their colors and have dereddened their photometry accordingly. By comparing an area of the images that contains the richest concentration of previously identified W3 members to an area that has few members and is dominated by background stars, we have estimated the luminosity function for members of W3 with masses of 0.03-0.4 $M_\odot$. That luminosity function closely resembles data in typical nearby star-forming regions that have much smaller stellar populations than W3 ($\lesssim$500 vs. several thousand objects). Thus, we do not find evidence of significant variations in the initial mass function of low-mass stars and brown dwarfs with star forming conditions, which is consistent with recent studies of other distant massive star-forming regions.

astro-ph.SR↗
arXiv2020

A Gaia Survey for Young Stars Associated with the Lupus Clouds

I have used high-precision photometry and astrometry from the second data release of the Gaia mission to perform a survey for young stars associated with the Lupus clouds, which have distances of ~160 pc and reside within the Sco-Cen OB association. The Gaia data have made it possible to distinguish Lupus members from most of the stars in other groups in Sco-Cen that overlap with the Lupus clouds, which have contaminated previous surveys. The new catalog of candidate Lupus members should be complete for spectral types earlier than M7 at A_K<0.2 within fields encompassing clouds 1--4. I have used that catalog to characterize various aspects of the Lupus stellar population. For instance, the sequence of low-mass stars in Lupus is ~0.4 mag brighter than the sequence for Upper Sco, which implies an age of ~6 Myr based an adopted age of 10-12 Myr for Upper Sco and the change in luminosity with age predicted by evolutionary models. I also find that the initial mass function in Lupus is similar to that in other nearby star-forming regions based on a comparison of their distributions of spectral types.

astro-ph.SR↗