Showing papers on "Blue dwarf published in 2000"
TL;DR: In this article, photometry, far red spectra, and spectral classifications for an additional 67 L dwarfs discovered by the Two Micron All Sky Survey are presented, and a table of all known L and T dwarfs believed to lie within 25 pc of the Sun.
Abstract: We present JHKs photometry, far red spectra, and spectral classifications for an additional 67 L dwarfs discovered by the Two Micron All Sky Survey. One of the goals of this new search was to locate more examples of the latest L dwarfs. Of the 67 new discoveries, 17 have types of L6 or later. Analysis of these new discoveries shows that Hα emission has yet to be convincingly detected in any L dwarf later than type L4.5, indicating a decline or absence of chromospheric activity in the latest L dwarfs. Further analysis shows that 16 (and possibly four more) of the new L dwarfs are lithium brown dwarfs and that the average line strength for those L dwarfs showing lithium increases until type ~L6.5 V, then declines for later types. This disappearance may be the first sign of depletion of atomic lithium as it begins to form into lithium-bearing molecules. Another goal of the search was to locate nearer, brighter L dwarfs of all subtypes. Using absolute magnitudes for 17 L dwarf systems with trigonometric parallax measurements, we develop spectrophotometric relations to estimate distances to the other L dwarfs. Of the 67 new discoveries, 21 have photometric distances placing them within 25 pc of the Sun. A table of all known L and T dwarfs believed to lie within 25 pc—53 in total — is also presented. Using the distance measurement of the coolest L dwarf known, we calculate that the gap in temperature between L8 and the warmest known T dwarfs is less than 350 K and probably much less. If the transition region between the two classes spans a very small temperature interval, this would explain why no transition objects have yet been uncovered. This evidence, combined with model fits to low-resolution spectra of late M and early L dwarfs, indicates that L-class objects span the range 1300 K Teff 2000 K. The near-infrared color-color diagram shows that L dwarfs fall along a natural, redder extension of the well-known M dwarf track. These near-infrared colors get progressively redder for later spectral types, with the L dwarf sequence abruptly ending near (J-H, H-Ks, J-Ks) ≈ (1.3, 0.8, 2.1).
762 citations
TL;DR: In this article, a review of detached binaries consisting of white dwarfs with either other white- dwarfs or low-mass main-sequence stars in tight orbits around them is presented.
50 citations
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9 citations
1 Jan 2000
TL;DR: In this paper, the observed properties of pulsating white dwarfs are reviewed and examples of some of the applications of seismology to these stars demonstrate how they can provide useful constraints on stellar evolution theory.
Abstract: I briefly review the observed properties of pulsating white dwarfs. Examples of some of the applications of seismology to these stars demonstrate how they can provide useful constraints on stellar evolution theory.
5 citations
TL;DR: In this article, the authors used the Hubble Space Telescope observations of the eclipsing dwarf nova OY Carinae after its 1992 April superoutburst to isolate ultraviolet spectra of the white dwarf, the accretion disk, and the bright spot.
Abstract: Hubble Space Telescope observations of the eclipsing dwarf nova OY Carinae after its 1992 April superoutburst are used to isolate ultraviolet spectra (1150-2500 A at 9.2 A FWHM resolution) of the white dwarf, the accretion disk, and the bright spot. The white dwarf spectra have a Stark-broadened photospheric Lyα absorption feature but are veiled by a forest of absorption features that we attribute to absorption by intervening disk material (a curtain). All the spectral fits required supersonic turbulence in the curtain material with Mach numbers of 6-8. All curtain temperatures were between 10,000 and 11,000 K. There was a curtain temperature increase ~3 months after the superoutburst. We find that the white dwarf temperature changed from 19,700 K just 27 days after the end of the superoutburst to 18,000 K roughly 3 months after the superoutburst; the exponential (e-folding) decay time of the white dwarf temperature was 66 days. We present evidence that the heating of the white dwarf was more extensive during the superoutburst than the normal outburst. The thermal response of the OY Car white dwarf to outburst heating is compared with WZ Sagittae, VW Hydri (the most similar dwarf nova to OY Car), and the cooling timescales of other dwarf novae after superoutburst. The measured cooling timescales of the five systems with superoutbursts appear to be shorter the longer the orbital period (accretion rate). Possible implications are discussed. There is evidence of a disk flux variation, independent of the effect of white dwarf cooling, which suggests a possible contradiction of the disk instability model. To establish this, however, data are required throughout a quiescent cycle.
3 citations
TL;DR: In this paper, the status of two WEET campaigns carried out on the objects is summarized and the frequency stability of specific modes of two pulsating white dwarfs is provided.
Abstract: campaign to monitor the frequency stability of specific modes of two pulsating white dwarfs is provided. In addition, the status of two W E T campaigns carried out on the objects is summarized. K e y words : stars: pulsating white dwarfs stars: individual: L 19-2, E C 20058-5234
3 citations
1 Jan 2000
1 citations
TL;DR: In this article, the authors proposed a morphological classification of blue compact dwarf galaxies based on the Hubble sequence and showed that the structure of BCDGs is more complex, probably the results of mergers, and/or slow accretion.
Abstract: Blue Compact dwarf galaxies are young starburst dwarf galaxies. A morphological classification has been proposed in the early 80'sbased on the Hubble sequence. On one hand, the classification resembles the one of the normal galaxies: elliptical-like and irregular-like. It was thought that the morphology reflects the dynamics. BCDGs shall be rotation supported and claims for `spiral arms' have been made. On the other hand, recent observations both in the IR and optical ranges suggest that the structure of BCDGs is more complex, probably the results of mergers, and/or slow accretion.
1 Jun 2000
Posted Content•
TL;DR: This article showed that EF Eri has one of the coldest white dwarfs in cataclysmic variables, implying a cooling age t_cool>=10e9 yrs or accretional heating at a rate as given by gravitational radiation.
Abstract: Low-state spectrophotometry of the short-period polar EF Eridani (Porb=81 min) found the system at V=18.0 with no trace of the companion (Wheatley and Ramsay 1998). We show that the lack of such spectral features implies that the companion to the white dwarf in EF Eri has a spectral type later than M9 and is either a transition object at the brink of hydrogen burning or a brown dwarf. The optical low state spectrum indicates a temperature of the white dwarf of Teff=9500+-500K. This is one of the coldest white dwarfs in cataclysmic variables, implying a cooling age t_cool>=10e9 yrs or accretional heating at a rate as given by gravitational radiation. The large age of the system excludes a warm brown dwarf as companion. EF Eri has either just passed through the period minimum of cataclysmic variable stars or has started mass transfer from an old brown dwarf secondary.
TL;DR: In this article, the authors identify the common properties and trends PWDs exhibit as a class and find that pulsators of low mass have higher luminosity, suggesting the range of instability is highly mass-dependent.
Abstract: One of the least understood aspects of white dwarf evolution is the process by which they are formed. The initial stages of white dwarf evolution are characterized by high luminosity, high effective temperature, and high surface gravity, making it difficult to constrain their properties through traditional spectroscopic observations. We are aided, however, by the fact that many Hand He-deficient pre-white dwarfs (PWDs) are multiperiodic g-mode pulsators. These stars fall into two classes, the variable planetary nebula nuclei (PNNV) and the “naked” GW Vir stars. Pulsations in PWDs provide a unique opportunity to probe their interiors, which are otherwise inaccesible to direct observation. Until now, however, the nature of the pulsation mechanism, the precise boundaries of the instability strip, and the mass distribution of the PWDs were complete mysteries. These problems must be addressed before we can apply knowledge of pulsating PWDs to improve understanding of white dwarf formation. This paper lays the groundwork for future theoretical investigations of these stars. In recent years, Whole Earth Telescope observations led to determination of mass and luminosity for the majority of the GW Vir pulsators. With these observations, we identify the common properties and trends PWDs exhibit as a class. We find that pulsators of low mass have higher luminosity, suggesting the range of instability is highly mass-dependent. The observed trend of decreasing periods with decreasing luminosity matches a decrease in the maximum (standing-wave) g-mode period across the instability strip. We show that the red edge can be caused by the lengthening of the driving timescale beyond the maximum sustainable period. This result is general for ionization-based
1 Jan 2000
TL;DR: In this paper, the number and physical properties of binary Be stars with white dwarfs taking into account cooling of the compact object and discuss the formation of such systems were studied. But the number of binary systems containing a Be star paired with a white dwarf in our galaxy is very large − 46% of all Be stars formed as a result of binary evolution must have a white dwarfish as companion.
Abstract: Using the “Scenario Machine” we study the number and physical properties of binary Be stars with white dwarfs taking into account cooling of the compact object and discuss the formation of such systems. According to our calculations the number of binary systems containing a Be star paired with a white dwarf in our galaxy is very large − 46% of all Be stars formed as a result of binary evolution must have a white dwarf as companion. These white dwarfs should be very hot with a surface temperature lying in the range from 30,000 to 60,000 K.
Posted Content•
TL;DR: In this paper, the basic properties of old white dwarf stars and the ongoing efforts to detect this population directly are reviewed. But the authors focus on the line of sight towards the Large Magellanic Cloud (LMC).
Abstract: A popular interpretation of recent microlensing studies of the line of sight towards the Large Magellanic Cloud invokes a population of old white dwarf stars in the Galactic halo Below I review the basic properties of old white dwarf stars and the ongoing efforts to detect this population directly
TL;DR: In this paper, a synthetic spectral analysis of five IUE NEWSIPS archival spectra of the peculiar, very active dwarf nova CN Orionis taken at the system's lowest flux levels during quiescence was carried out.
Abstract: We have carried out a synthetic spectral analysis of five IUE NEWSIPS archival spectra of the peculiar, very active dwarf nova CN Orionis taken at the system's lowest flux levels during quiescence. AAVSO data indicate the visual magnitude V ~ 14 for CN Ori at this time. On the assumption that the white dwarf contributes significantly to the far-UV light of the system, we have computed a two-parameter grid of synthetic, high-gravity spectra in LTE with solar composition using TLUSTY195 and SYNSPEC42 and carried out fits of these pure photospheric models to the far-UV continuum and narrow absorption line spectra. We find that the far-UV spectrum in quiescence is well represented by a hot (Teff = 30,000 K, log g = 7) white dwarf with probable subsolar silicon abundance and all other observed metal transitions at essentially their solar values. For comparison, we have fitted optically thick accretion disk models to the same spectra for Mwd = 0.80 M⊙ and disk inclination angle i = 60° for accretion rates = 10–10.5 and 10–9.5 M⊙ yr–1. We compare the Teff of CN Ori, one of a handful of dwarf novae above the period gap with a model photospheric analysis, to all other dwarf novae with white dwarfs of known Teff derived using white dwarf model atmospheres. We find evidence that the white dwarfs in dwarf novae above the period gap are hotter than the ones below the gap. This trend suggests more heated, younger degenerates above the gap and is consistent with the overall empirical evidence that mass transfer rates in dwarf novae above the period gap are higher than mass transfer rates in systems below the gap.
TL;DR: The authors showed that the I−Z color is not a good temperature indicator for objects between L1V and L5V, such objects having colours that overlap with mid M dwarfs.
Abstract: The I−Z colour has recently been shown to be a good temperature indicator for M dwarfs. We present the first IZ photometry of a small sample of L dwarfs ranging in spectral type from L0.5V to L6.0V. We find that the I−Z colour is not a good temperature indicator for objects between L1V and L5V, such objects having colours that overlap with mid M dwarfs. We attribute this to the reduction in the strength of the TiO and VO bands in the L dwarfs, which are the dominant opacity source in the I band for late M dwarfs. Beyond L5V, I−Z appears to be a reasonable indicator. This has important implications for the planning of optical surveys for cool objects in clusters and the field. For example, I−Z will cease to be a good method of identifying brown dwarfs in the Pleiades below around 0.04 M⊙, and at around 0.075 M⊙ in the Hyades and Praesepe.
TL;DR: In this article, it was shown that the redistribution of chemical elements ensuing from the crystallization of C/O white dwarfs provides between 10% and 20% of the total energy released during the process, depending on the internal chemical composition.
Abstract: The evolution of white dwarfs is a cooling process that depends on the energy stored in the core and on the way in which it is transferred through the envelope. In this paper we show that despite some (erroneous) claims, the redistribution of chemical elements ensuing from the crystallization of C/O white dwarfs provides between the 10% and the 20% of the total energy released during the crystallization process, depending on the internal chemical composition, which is not at all negligible, given the present state of the white dwarf cooling theory.
TL;DR: In this article, color-magnitude diagrams and color-color diagrams for white dwarfs with pure hydrogen atmospheres down to an effective temperature Teff = 1500 K were presented.
Abstract: We present new cooling sequences, color-magnitude diagrams, and color-color diagrams for cool white dwarfs with pure hydrogen atmospheres down to an effective temperature Teff = 1500 K. We include a more detailed treatment of the physics of the fully ionized interior, particularly an improved discussion of the thermodynamics of the temperature-dependent ion-ion and ion-electron contributions of the quantum, relativistic electron-ion plasma. The present calculations also incorporate accurate boundary conditions between the degenerate core and the outermost layers as well as updated atmosphere models including the H2-H2 induced-dipole absorption. We examine the differences on the cooling time of the star arising from uncertainties in the initial carbon-oxygen profile and the core-envelope L-Tc relation. The maximum time delay due to crystallization-induced chemical fractionation remains substantial, from ~1.0 Gyr for 0.5 and 1.2 M☉ white dwarfs to ~1.5 Gyr for 0.6-0.8 M☉ white dwarfs, even with initial stratified composition profiles, and cannot be ignored in detailed white dwarf cooling calculations. These cooling sequences provide theoretical support to search for or identify old disk or halo hydrogen-rich white dwarfs by characterizing their mass and age from their observational signatures.
TL;DR: In this article, the authors presented a single-star photometry for the blue dwarf galaxy UGCA 290, whose morphology is intermediate between classic iE blue compact dwarf galaxies and blue dwarf galaxies that exhibit no red background sheet of older stars.
Abstract: We present Hubble Space Telescope/Wide Field Planetary Camera 2 single-star photometry for the blue dwarf galaxy UGCA 290, whose morphology is intermediate between classic iE blue compact dwarf galaxies and blue dwarf galaxies that exhibit no red background sheet of older stars. The color-magnitude diagram of this galaxy in V and I, extending over 6 mag, is remarkably similar to that of the star-forming region in the iE blue compact dwarf galaxy VII Zw 403. There is no evidence for gaps in its star formation history over the last billion years, and the color of its red giant branch indicates a very metal-poor stellar population. From the magnitude of the tip of the red giant branch, we derive a distance of 6.7 Mpc, more than twice the distance estimated from the brightest blue supergiants.
TL;DR: In this paper, the authors estimate the possible contribution of halo white dwarf binaries to the low-frequency (10-5 Hz < f < 10-1 Hz) gravitational wave background, assuming the fraction of white dwarfs in binaries is the same in the halo as in the disk.
Abstract: The possibility that Galactic halo MACHOs are white dwarfs has recently attracted much attention. Using the known properties of white dwarf binaries in the Galactic disk as a model, we estimate the possible contribution of halo white dwarf binaries to the low-frequency (10-5 Hz < f < 10-1 Hz) gravitational wave background. Assuming the fraction of white dwarfs in binaries is the same in the halo as in the disk, we find the confusion background from halo white dwarf binaries could be 5 times stronger than the expected contribution from Galactic disk binaries, dominating the response of the proposed space-based interferometer LISA. Low-frequency gravitational wave observations will be the key to discovering the nature of the dark MACHO binary population.
TL;DR: In this paper, the amplitude spectra from four observing campaigns conducted on GW Librae during 1997 and 1998 were presented, showing that the star's pulsation spectrum is highly unstable on timescales of months and exhibits clusters of signals very closely spaced in frequency with separations on the order of 1 /iHz.
Abstract: The dwarf nova GW Librae is the first cataclysmic variable discovered to have a primary in the white dwarf instability strip, making it the only non-radially pulsating star known to be accreting. The primaries of CVs are notoriously difficult to study because they are embedded in hot, bright accretion discs. Applying the techniques of asteroseismology to GW Librae could give us an unprecedented look at a white dwarf that has undergone ~ 10 years of accretion. However, an accreting white dwarf could have a non-spherical internal structure, rendering the standard (spherically symmetric) solution to white dwarf pulsations invalid for its study. This paper presents amplitude spectra from four observing campaigns conducted on GW Librae during 1997 and 1998. The star's pulsation spectrum is highly unstable on timescales of months and exhibits clusters of signals very closely spaced in frequency, with separations on the order of 1 /iHz. 232 L. van Zyl, B. Warner, D. O'Donoghue et al.
University of Hawaii at Manoa1, Princeton University2, Pennsylvania State University3, Johns Hopkins University4, University of Texas at Austin5, University of Washington6, Fermilab7, New Mexico State University8, Eötvös Loránd University9, University of Tokyo10, University of Chicago11, United States Department of the Navy12, University of Michigan13
TL;DR: In this article, the authors report the discovery of three cool brown dwarfs that fall in the effective temperature gap between the latest L dwarfs currently known, with no methane absorption bands in the 1-2.5 µm range, and the previously known methane (T) dwarfs, whose spectra are dominated by methane and water.
Abstract: We report the discovery of three cool brown dwarfs that fall in the effective temperature gap between the latest L dwarfs currently known, with no methane absorption bands in the 1-2.5 µm range, and the previously known methane (T) dwarfs, whose spectra are dominated by methane and water. The newly discovered objects were detected as very red objects in the Sloan Digital Sky Survey imaging data and have JHK colors between the red L dwarfs and the blue Gl 229B-like T dwarfs. They show both CO and CH(4) absorption in their near-infrared spectra in addition to H(2)O, with weaker CH(4) absorption features in the H and K bands than those in all other methane dwarfs reported to date. Due to the presence of CH(4) in these bands, we propose that these objects are early T dwarfs. The three form part of the brown dwarf spectral sequence and fill in the large gap in the overall spectral sequence from the hottest main-sequence stars to the coolest methane dwarfs currently known.
TL;DR: It appears that a significant fraction of the local dark matter halo is in the form of very old, cool, white dwarf stars, found in a systematic proper-motion survey.
Abstract: We present the discovery and spectroscopic identification of two very high proper-motion ancient white dwarf stars, found in a systematic proper-motion survey. Their kinematics and apparent magnitude clearly indicate that they are halo members, while their optical spectra are almost identical to the recently identified cool halo white dwarf WD 0346+246. Canonical stellar halo models predict a white dwarf volume density that is 2 orders of magnitude less than the ρ ~ 7 × 10-4 M☉ pc-3 inferred from this survey. With the caveat that the sample size is very small, it appears that a significant fraction, ~10%, of the local dark matter halo is in the form of very old, cool, white dwarfs.
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TL;DR: In this article, spectroscopic, photometric and astrometric observations of four bright L dwarfs identified in the course of the 2MASS near-infrared survey are presented.
Abstract: We present spectroscopic, photometric and astrometric observations of four bright L dwarfs identified in the course of the 2MASS near-infrared survey. Our spectroscopic data extend to wavelengths shortward of 5000 A in the L0 dwarf 2MASS J0746+2000 and the L4 dwarf 2MASS J0036+1840, allowing the identification of absorption bands due to MgH and CaOH. The atomic resonance lines Ca I λ4227 and Na I λλ5890/5896 are extremely strong, with the latter having an equivalent width of 240 A in the L4 dwarf. By spectral type L5, the D lines extend over ~1000 A and absorb a substantial fraction of the flux emitted in the V band, with a corresponding effect on the (V-I) broadband color. The K I resonance doublet at 7665/7699 A increases in equivalent width from spectral type M3 to M7, but decreases in strength from M7 to L0 before broadening substantially at later types. These variations are likely driven by dust formation in these cool atmospheres.
Journal Article•
TL;DR: In this article, the authors presented accurate and detailed mass-radius relations for white dwarf (WD) models with helium, carbon, oxygen, silicon and iron cores, and with and without a hydrogen envelope, by using a fully updated stel- lar evolutionary code.
Abstract: The purpose of this work is to present accurate and detailed mass-radius relations for white dwarf (WD) models with helium, carbon, oxygen, silicon and iron cores, and with and without a hydrogen envelope, by using a fully updated stel- lar evolutionary code. We considered masses from 0.15 M to 0.5 M for the case of helium core, from 0.45 M to 1.2 M for carbon, oxygen and silicon cores, and from 0.45 M to 1.0 M for the case of an iron core. In view of the recent measure- ments made by Hipparcos that strongly suggest the existence of WDs with an iron-dominated core, we focus our attention mainly on the finite-temperature, mass-radius relations for WD models with iron interiors. In addition, we explore the effects of gravitational, chemical and thermal diffusion on low mass he- lium white dwarf models with hydrogen and helium envelopes.
TL;DR: An infrared spectrum of an extremely cool white dwarf that is consistent with the new models is reported, determining the star's temperature to be 3,500 ± 200 K, making it the coolest known white dwarf.
Abstract: White dwarfs are the remnant cores of stars that initially had masses of less than 8 solar masses. They cool gradually over billions of years, and have been suggested1,2 to make up much of the ‘dark matter’ in the halo of the Milky Way. But extremely cool white dwarfs have proved difficult to detect, owing to both their faintness and their anticipated similarity in colour to other classes of dwarf stars. Recent improved models3,4,5 indicate that white dwarfs are much more blue than previously supposed, suggesting that the earlier searches may have been looking for the wrong kinds of objects. Here we report an infrared spectrum of an extremely cool white dwarf that is consistent with the new models. We determine the star's temperature to be 3,500 ± 200 K, making it the coolest known white dwarf. The kinematics of this star indicate that it is in the halo of the Milky Way, and the density of such objects implied by the serendipitous discovery of this star is consistent with white dwarfs dominating the dark matter in the halo.
TL;DR: In this article, the effects of diffusion on the structure and evolution of low-mass helium white dwarfs are explored, and it is shown that diffusion causes hydrogen to float and the other elements to sink over time scales shorter than evolutionary time-scales.
Abstract: This paper is aimed at exploring the effects of diffusion on the structure and evolution of low-mass helium white dwarfs. To this end, we solve the multicomponent flow equations describing gravitational settling and chemical and thermal diffusion. The diffusion calculations are coupled to an evolutionary code in order to follow the cooling of low-mass, helium core white dwarf models having envelopes made up of a mixture of hydrogen and helium, as recently suggested by detailed evolutionary calculations for white dwarf progenitors in binary systems. We find that diffusion causes hydrogen to float and the other elements to sink over time-scales shorter than evolutionary time-scales. This produces a noticeable change in the structure of the outer layers, making the star inflate. Thus, in order to compute accurately the mass–radius relation for low-mass helium white dwarfs we need to account for the diffusion processes during (at least) the white dwarf stages of the evolution of these objects. This should be particularly important when studying the general characteristics of binary systems containing a helium white dwarf and a pulsar.
In addition, we present an analytic, approximate model for the outer layers of the white dwarf aimed at interpreting the physical reasons for the change in the surface gravity for low-mass white dwarfs induced by diffusion.
TL;DR: In this article, an analysis of the evolution of the amplitude spectrum over many seasons of the DA pulsating white dwarf G29-38 has been performed, showing that neither beating nor resonant mode coupling can account for the observed appearance and disappearance of modes, although some of them clearly grow while others get damped.
Abstract: An analysis of the evolution of the amplitude spectrum over many seasons of the DA pulsating white dwarf G29-38 has been performed. Neither beating nor resonant mode coupling can account for the observed appearance and disappearance of modes, although some of them clearly grow while others get damped. Therefore some unknown non-adiabatic, non-linear process has to be invoked that affects both the mode selection mechanism and the driving efficiency on a time-scale as short as a day.
TL;DR: In this article, three nights of blue and red spectroscopy of the extreme-amplitude dwarf nova GW Lib at quiescence were performed, and the orbital period is 79.4 minutes and the semiamplitude of the Hβ emission-line velocity curve is 40.0 km s-1.
Abstract: Three nights of blue and red spectroscopy of the extreme-amplitude dwarf nova GW Lib at quiescence show that the orbital period is 79.4 minutes and the semiamplitude of the Hβ emission-line velocity curve is 40.0 km s-1. The emission lines are very narrow and are surrounded by absorption troughs. The spectra are roughly consistent with that of a white dwarf of temperature near 11,000 K contributing 100% of the light at a distance of 114 pc. The short orbital period and low are consistent with the general properties of extreme-amplitude dwarf novae.
TL;DR: In this paper, the discovery of two binary M dwarfs in the immediate solar neighbourhood using the Two Micron All Sky Survey (2MASS) was reported, one is an M6.5 companion to the nearby G star HD 86728 (Gl 376) and the other is a M8 companion with 14.5 arcsec separation.
Abstract: We report the discovery of two binary M dwarf systems in the immediate solar neighbourhood using the Two Micron All Sky Survey (2MASS). The first is an M6.5 companion to the nearby G star HD 86728 (Gl 376). The known properties of HD 86728 indicate that the M dwarf (Gl 376B) is old, metal-rich and only 14.9 parsec away. The M dwarf is highly active, with both Hα and X-ray emission. Thus, Gl 376B offers the opportunity to study an old, bright, active M dwarf with known metallicity, age and luminosity. We show that it is probable that Gl 376B is itself an unresolved pair. The other system consists of an M6.5 and an M8 dwarf with 14.5 arcsec separation. We estimate a distance of ∼16 parsec for this very low-mass pair. Stronger activity is observed in the M6.5 dwarf, supporting evidence that chromospheric activity is weakening near the hydrogen-burning limit.