Far‐Infrared Properties of M Dwarfs (original) (raw)
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Spitzer 24 mum Survey for Dust Disks around Hot White Dwarfs
Astron J, 2011
Two types of dust disks around white dwarfs (WDs) have been reported: small dust disks around cool metal-rich WDs consisting of tidally disrupted asteroids and a large dust disk around the hot central WD of the Helix planetary nebula (PN) possibly produced by collisions among Kuiper-Belt-like objects. To search for more dust disks of the latter type, we have conducted a Spitzer MIPS 24 μm survey of 71 hot WDs or pre-WDs, among which 35 are central stars of PNe (CSPNs). Nine of these evolved stars are detected and their 24 μm flux densities are at least two orders of magnitude higher than their expected photospheric emission. Considering the bias against the detection of distant objects, the 24 μm detection rate for the sample is gsim15%. It is striking that seven, or ~20%, of the WD and pre-WDs in known PNe exhibit 24 μm excesses, while two, or 5%-6%, of the WDs not in PNe show 24 μm excesses and they have the lowest 24 μm flux densities. We have obtained follow-up Spitzer Infrared Spectrograph spectra for five objects. Four show clear continuum emission at 24 μm, and one is overwhelmed by a bright neighboring star but still shows a hint of continuum emission. In the cases of WD 0950+139 and CSPN K 1-22, a late-type companion is present, making it difficult to determine whether the excess 24 μm emission is associated with the WD or its red companion. High-resolution images in the mid-infrared are needed to establish unambiguously the stars responsible for the 24 μm excesses.
Astronomy and Astrophysics, 2018
Context. Being the most numerous and oldest stars in the galaxy, M dwarfs are objects of great interest for exoplanet searches. The presence of molecules in their atmosphere complicates our understanding of their atmospheric properties. But great advances have recently been made in the modeling of M dwarfs due to the revision of solar abundances. Aims. We aim to determine stellar parameters of M dwarfs using high resolution spectra (R ∼ 90 000) simultaneously in the visible and the near-infrared. The high resolution spectra and broad wavelength coverage provide an unique opportunity to understand the onset of dust and cloud formation at cool temperatures. Furthermore, this study will help in understanding the physical processes which occur in a cool atmospheres, particularly, the redistribution of energy from the optical to the near-infrared. Methods. The stellar parameters of M dwarfs in our sample have been determined by comparing the high resolution spectra both in the optical and in the near-infrared simultaneously observed by CARMENES with the synthetic spectra obtained from the BT-Settl model atmosphere. The detailed spectral synthesis of these observed spectra both in the optical and in the near-infrared helps to understand the missing continuum opacity. Results. For the first time, we derive fundamental stellar parameters of M dwarfs using the high resolution optical and near-infrared spectra simultaneously. We determine T eff , log g and [M/H] for 292 M dwarfs of spectral type M0 to M9, where the formation of dust and clouds are important. The derived T eff for the sample ranges from 2300 to 4000 K, values of log g ranges from 4.5 ≤ log g ≤ 5.5 and the resulting metallicity ranges from −0.5 ≤ [M/H] ≤ +0.5. We have also explored the possible differences in T eff , log g and [M/H] by comparing them with other studies of the same sample of M dwarfs.
Infrared photometry of late-M, L, and T dwarfs
We present ZJHKL ′ M ′ photometry of a sample of 58 late-M, L, and T dwarfs, most of which are identified from the Sloan Digital Sky Survey and the Two Micron All-Sky Survey. Near-infrared spectra and spectral classifications for most of this sample are presented in a companion paper by Geballe et al. We derive the luminosities of 18 dwarfs in the sample with known parallaxes, and the results imply -2that the effective temperature range for the L dwarfs in our sample is approximately 2200-1300 K and for the T dwarfs 1300-800 K. We obtained new photometric data at the United Kingdom Infrared Telescope for: 42 dwarfs at Z, 34 dwarfs at JHK, 21 dwarfs at L ′ , as well as M ′ data for two L dwarfs and two T dwarfs. The M ′ data provide the first accurate photometry for L and T dwarfs in this bandpass -for a T2 and a T5 dwarf, we find K-M ′ = 1.2 and 1.6, respectively. These colors are much bluer than predicted by published models suggesting that CO may be more abundant in these objects than expected, as has been found for the T6 dwarf Gl 229B. We also find that K-L ′ increases monotonically through most of the M, L, and T subclasses, but it is approximately constant between types L6 and T5, restricting its usefulness as a temperature indicator. The degeneracy is probably due to the onset of CH 4 absorption at the blue edge of the L ′ bandpass. The JHK colors of L dwarfs show significant scatter, suggesting that the fluxes in these bandpasses are sensitive to variations in photospheric dust properties. The H − K colors of the later T dwarfs also show some scatter which we suggest is due to variations in pressure-induced H 2 opacity, which is sensitive to gravity and metallicity. lists the UKIRT Z magnitudes and dates of observation for 42 dwarfs in the sample. To calibrate the non-standard Z photometry, we derived transformations between the SDSS z ′ , Cousins I, UKIRT J and Z filters using A0 stars, which are defined as having zero color, and provisional z ′ standards . In doing so, we transformed the z ′
SpitzerObservations of Nearby M Dwarfs
The Astrophysical Journal, 2006
We present Spitzer IRAC and MIPS observations for a sample of eight M dwarfs: six dMe, one dM, and one sdMe star. All of our targets are found to have SEDs which are fitted within the error bars by a purely photospheric spectrum out to 24µm. We find no evidence for IR excess. None of our targets is detected in the MIPS 70 and 160µm bands. The estimated ages for all are >10 Myr, suggesting that enough disk dissipation has occurred within the inner several AU of the star. For four of these, Mullan et al. (1989) had reported IRAS detections at 12µm, although the reported fluxes were below the 5-σ IRAS detection limit (∼0.2 Jy). Mullan et al. also pointed out that V-K colors in dMe stars are larger than those in dM stars, possibly because of the presence of a chromosphere. Here we suggest that metallicity effects provide a better explanation of the V-K data. For reasons of observational selection, our targets are not the most active flare stars known, but being dMe stars indicates the presence of a chromosphere. Scaling from Houdebine's model of the AU Mic chromosphere, we have computed the free-free infrared excesses for a range of densities. Our Spitzer 24µm data shows that the chromospheres in two of our targets are less dense than in AU Mic by a factor of 10 or more. This is consistent with the fact that our sample includes the less active flare stars. Our models also indicate that the chromospheric contribution to the observed AU Mic emission at submillimeter wavelengths is only about 2%.
Mid‐Infrared Spectra of Dust Debris around Main‐Sequence Stars
The Astrophysical Journal Supplement Series, 2004
We report spectra obtained with the Spitzer Space Telescope in the λ = 14-35 µm range of 19 nearby main-sequence stars with infrared excesses. The six stars with strong dust emission show no recognizable spectral features, suggesting that the bulk of the emitting particles have diameters larger than 10 µm. If the observed dust results from collisional grinding of larger solids, we infer minimum masses of the parent body population between 0.004 M ⊕ and 0.06 M ⊕. We estimate grain production rates of ∼10 10 g s −1 around λ Boo and HR 1570; selective accretion of this matter may help explain their peculiar surface abundances.
New constraints on the formation and settling of dust in the atmospheres of young M and L dwarfs
Astronomy & Astrophysics, 2014
Context. Gravity modifies the spectral features of young brown dwarfs (BDs). A proper characterization of these objects is crucial for the identification of the least massive, and latest-type objects in star-forming regions, and to explain the origin(s) of the peculiar spectro-photometric properties of young directly imaged extrasolar planets and BD companions. Aims. We obtained medium-resolution (R∼1500-1700) near-infrared (1.1-2.5 µm) spectra of seven young M9.5-L3 dwarfs classified at optical wavelengths. We aim to empirically confirm the low surface gravity of the objects in the near-infrared. We also test whether self-consistent atmospheric models correctly represent the formation and the settling of dust clouds in the atmosphere of young late-M and L dwarfs. Methods. We used ISAAC (Infrared Spectrometer And Array Camera) at VLT (Very Large Telescope) to obtain the spectra of the targets. We compared them to those of mature and young BDs, and young late-type companions to nearby stars with known ages, in order to identify and study gravity-sensitive features. We computed spectral indices weakly sensitive to the surface gravity to derive near-infrared spectral types. Finally, we found the best fit between each spectrum and synthetic spectra from the BT-Settl 2010 and 2013 atmospheric models. Using the best fit, we derived the atmospheric parameters of the objects and identify which spectral characteristics the models do not reproduce. Results. We confirmed that our objects are young BDs and we found near-infrared spectral types in agreement with the ones determined at optical wavelengths. The spectrum of the L2γ dwarf 2MASSJ232252.99-615127.5 reproduces well the spectrum of the planetary mass companion 1RXS J160929.1-210524b. BT-Settl models fit the spectra and the 1-5 µm spectral energy distribution of the L0-L3 dwarfs for temperatures between 1600-2000 K. But the models fail to reproduce the shape of the H band, and the nearinfrared slope of some of our targets. This fact, and the best fit solutions found with super-solar metallicity are indicative of a lack of dust, in particular at high altitude, in the cloud models.
The spectroscopic study of M8.5-M9.5 stars and brown dwarfs
EPJ Web of Conferences, 2013
We present high-resolution spectra analysis of the three late-M dwarfs LP944-20, SIPS J2045-6332 and DENIS-P J0021.0-4244. The stellar spectra were observed with Very Large Telescope/Ultraviolet-Visual Echelle Spectrograph (VLT/UVES) in optical and near-infrared regions. The effective temperatures T eff and log g was defined by comparing observed and theoretical energy distributions for the investigated objects. Synthetic spectra were calculated for PHOENIX atmosphere models-COND and DUSTY, as well as for Semi-empirical atmosphere model. We discuss the influence of the effects associated with dust in stellar atmosphere on the energy distribution in the stellar spectra.
Spitzer 24 um Survey for Dust Disks around Hot White Dwarfs
2011
Two types of dust disks around white dwarfs (WDs) have been reported: small dust disks around cool metal-rich WDs consisting of tidally disrupted asteroids, and a large dust disk around the hot central WD of the Helix planetary nebula (PN) possibly produced by collisions among Kuiper Belt-like objects. To search for more dust disks of the latter type, we have conducted a Spitzer MIPS 24 µm survey of 71 hot WDs or pre-WDs, among which 35 are central stars of PNe (CSPNs). Nine of these evolved stars are detected and their 24 µm flux densities are at least two orders of magnitude higher than their expected photospheric emission. Considering the bias against detection of distant objects, the 24 µm detection rate for the sample is 15%. It is striking that seven, or ∼20%, of the WD and pre-WDs in known PNe exhibit 24 µm excesses, while two, or 5-6%, of the WDs not in PNe show 24 µm excesses and they have the lowest 24 µm flux densities. We have obtained follow-up Spitzer IRS spectra for five objects. Four show clear continuum emission at 24 µm, and one is overwhelmed by a bright neighboring star but still show a hint of continuum emission. In the cases of WD 0950+139 and CSPN K 1-22, a late-type companion is present, making it difficult to determine whether the excess 24 µm emission is associated with the
Spitzer 24 μm survey for dust disks around hot white dwarfs
2011
Two types of dust disks around white dwarfs (WDs) have been reported: small dust disks around cool metal-rich WDs consisting of tidally disrupted asteroids, and a large dust disk around the hot central WD of the Helix planetary nebula (PN) possibly produced by collisions among Kuiper Belt-like objects. To search for more dust disks of the latter type, we have conducted a Spitzer MIPS 24 µm survey of 71 hot WDs or pre-WDs, among which 35 are central stars of PNe (CSPNs). Nine of these evolved stars are detected and their 24 µm flux densities are at least two orders of magnitude higher than their expected photospheric emission. Considering the bias against detection of distant objects, the 24 µm detection rate for the sample is 15%. It is striking that seven, or ∼20%, of the WD and pre-WDs in known PNe exhibit 24 µm excesses, while two, or 5-6%, of the WDs not in PNe show 24 µm excesses and they have the lowest 24 µm flux densities. We have obtained follow-up Spitzer IRS spectra for five objects. Four show clear continuum emission at 24 µm, and one is overwhelmed by a bright neighboring star but still show a hint of continuum emission. In the cases of WD 0950+139 and CSPN K 1-22, a late-type companion is present, making it difficult to determine whether the excess 24 µm emission is associated with the
We present new JHK photometry on the MKO-NIR system and JHK spectroscopy for a large sample of L and T dwarfs. Photometry has been obtained for 71 dwarfs and spectroscopy for 56. The sample comprises newly identified very red objects from the Sloan Digital Sky Survey (SDSS) and known dwarfs from the SDSS and the Two Micron All Sky Survey (2MASS). Spectral classification has been carried out using four previously defined indices (from Geballe et al. 2002, G02) that measure the strengths of the near infrared water and methane bands. We identify 9 new L8-9.5 dwarfs and 14 new T dwarfs from SDSS, including the latest yet found by SDSS, the T7 dwarf SDSS J175805.46+463311.9. We classify 2MASS J04151954−0935066 as T9, the latest and coolest dwarf found to date.