Typical Dwarf Research Articles

The luminosity-iameter relations for globular clusters and dwarf spheroidal galaxies

Abstract It is shown that globular clusters and the dwarf spheroidal companions of the Galaxy have a different distribution of flattening values and appear to occupy adjacent regions of the Mv versus log Rh plane that can be separated by what will be referred to as the Shapley line. Surprisingly, typical dwarf spheroidal companions to the Milky Way system are fainter than the average Galactic globular cluster.

How accurately can we age-date solar-type dwarfs using activity/rotation diagnostics?

AbstractIt is well established that activity and rotation diminishes during the life of sun-like main sequence (~F7-K2V) stars. Indeed, the evolution of rotation and activity among these stars appears to be so deterministic that their rotation/activity diagnostics are often utilized as estimators of stellar age. A primary motivation for the recent interest in improving the ages of solar-type field dwarfs is in understanding the evolution of debris disks and planetary systems. Reliable isochronal age-dating for field, solar-type main sequence stars is very difficult given the observational uncertainties and multi-Gyr timescales for significant structural evolution. Observationally, significant databases of activity/rotation diagnostics exist for field solar-type field dwarfs (mainly from chromospheric and X-ray activity surveys).But how well can we empirically age-date solar-type field stars using activity/rotation diagnostics?Here I summarize some recent results for F7-K2 dwarfs from an analysis by Mamajek & Hillenbrand (2008), including an improved “gyrochronology” [Period(color, age)] calibration, improved chromospheric (R′HK) and X-ray (log(LX/Lbol)) activity vs. rotation (via Rossby number) relations, and a chromospheric vs. X-ray activity relation that spans four orders of magnitude in log(LX/Lbol). Combining these relations, one can produce predicted chromospheric and X-ray activity isochrones as a function of color and age for solar type dwarfs.

OGLE-TR-211 – a new transiting inflated hot Jupiter from the OGLE survey and ESO LP666 spectroscopic follow-up program

We present results of the photometric campaign for planetary and low-luminosity object transits conducted by the OGLE survey in the 2005 season (Campaign #5). About twenty of the most promising candidates discovered in these data were subsequently verified spectroscopically with the VLT/FLAMES spectrograph. One of the candidates, OGLE-TR-211, reveals clear changes of radial velocity with a small amplitude of 82 m/s, varying in phase with photometric transit ephemeris. Further analysis confirms the planetary nature of this system. Follow-up precise photometry of OGLE-TR-211 with VLT/FORS, together with radial velocity spectroscopy, supplemented with high-resolution, high S/N VLT/UVES spectra allowed us to derive parameters of the planet and host star. OGLE-TR-211b is a hot Jupiter orbiting an F7-8 spectral type dwarf star with a period of 3.68 days. The mass of the planet is equal to 1.03±0.20 MJup, while its radius RJup. The radius is about 20% larger than the typical radius of hot Jupiters of similar mass. OGLE-TR-211b is, then, another example of inflated hot Jupiters – a small group of seven exoplanets with large radii and unusually low densities – objects that are a challenge to the current models of exoplanets.

Infrared Spectroscopy of Short-Period Cataclysmic Variables

We present near-infrared spectra for five dwarf novae with short orbital periods. We performed fittings of the spectral energy distributions (SEDs) by assuming a power-law distribution for the accretion disk and using template spectra of late-type dwarfs for the secondary star. Two are WZ Sge–or WX Cet-type stars, whose spectra show water absorption features, and one of them shows K$\;$ I and Na$\;$ I absorption lines in the $J$-band. We find that the secondary contributions to the overall SED are larger in WZ Sge stars than in ER UMa stars. The SED fitting suggests M9 and L1 type dwarf companions for our WZ Sge systems, and an M2 dwarf for our ER UMa star. We discuss the direct relationship between the high temperatures of secondary stars and high mass-transfer rates in ER UMa stars. SDSS J013701.06–091234.9 is the only object showing a spectrum dominated by the secondary component. The spectrum strongly suggests that the evolutionary path of SDSS J013701.06–091234.9 is different from that of ordinary CVs, and this object is a candidate of EI Psc-like systems.

K–H2 quasi-molecular absorption detected in the T-dwarf $\mathsf{\varepsilon}$ Indi Ba

Context. T-type dwarfs present a broad and shallow absorption feature centred around 6950 A in the blue wing of the K doublet at 0.77 μ m which resembles in depth and shape the satellite absorption predicted by detailed collisional broadening profiles. In our previous work, the position of the predicted line satellite was however somewhat too blue compared to the observed feature. Aims. In this paper, we investigate whether new calculations of the energy surfaces of the potentials in the K–H2 system, including spin-orbit coupling, result in a closer coincidence of the satellite with the observed position. We also investigate the extent to which CaH absorption bands contribute to the feature and at what T eff these respective opacity sources predominate. Methods. We present model atmospheres and synthetic spectra, including gravitational settling for an improved description of depth-dependent abundances of refractory elements, and based on new K–H2 line profiles using improved interaction potentials.Results. By comparison with a high signal-to-noise optical spectrum of the T1 dwarf e Indi Ba, we find that these new models do reproduce the observed feature, while CaH does not contribute for the atmospheric parameters considered. We also find that CaH is settled out so deep into the atmosphere that even turbulent vertical mixing would appear insufficient to bring significant amounts of CaH to the observable photosphere in dwarfs of later type than ~L5. Conclusions. We conclude that previous identification of the feature at this location in the spectra of T dwarfs as well as the latest L dwarfs with CaH was erroneous, as expected on physical grounds: calcium has already condensed onto grains in early L dwarfs and thus should have settled out of the photosphere in cooler brown dwarfs. This finding revokes one of the observational verifications for the cloud-clearing theory assumption: a gradual clearing of the cloud cover in early T dwarfs.

On the Spectral Evolution of Cool, Helium‐Atmosphere White Dwarfs: Detailed Spectroscopic and Photometric Analysis of DZ Stars

We present a detailed analysis of a large spectroscopic and photometric sample of DZ white dwarfs based on our latest model atmosphere calculations. We revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz (12 stars) and analyze 147 new DZ white dwarfs discovered in the SDSS. The inclusion of metals and hydrogen in our model atmosphere calculations leads to different atmospheric parameters than those derived from pure helium models. Calcium abundances are found in the range from log(Ca/He) = -12 to -8. We also find that fits of the coolest objects show peculiarities, suggesting that our physical models may not correctly describe the conditions of high atmospheric pressure encountered in the coolest DZ stars. We find that the mean mass of the 11 DZ stars with trigonometric parallaxes, M = 0.63 M☉, is significantly lower than that obtained from pure helium models, M = 0.78 M☉, and in much better agreement with the mean mass of other types of white dwarfs. We determine hydrogen abundances for 27% of the DZ stars in our sample, while only upper limits are obtained for objects with low-S/N spectroscopic data. We confirm with a high level of confidence that the accretion rate of hydrogen is at least 2 orders of magnitude smaller than that of metals (and up to 5 in some cases) to be compatible with the observations. We find a correlation between the hydrogen abundance and the effective temperature, suggesting for the first time empirical evidence of a lower temperature boundary for the hydrogen screening mechanism. Finally, we speculate on the possibility that the DZA white dwarfs could be the result of the convective mixing of thin hydrogen-rich atmospheres with the underlying helium convection zone.

IGR J00234+6141: a new INTEGRAL source identified as an intermediate polar

Context. Following an extensive survey of the galactic plane by the INTEGRAL satellite, new hard X-ray sources are discovered with a significant fraction of Cataclysmic Variables (CVs) among them. Aims. We report here the identification of one of these hard X-ray sources, IGR J00234+6141, as an accreting magnetic white dwarf of intermediate polar type. Methods. We analyse the high energy emission of the INTEGRAL source using all available data and provide complementary optical photometric and spectroscopic data obtained in August and October 2006, respectively. Results. Based on a refined INTEGRAL position, we confirm the proposed optical identification. We clearly detect the presence of a 564 s periodic optical modulation that we identify as the rotation of the white dwarf. The analysis of the optical spectrum also demonstrates that the emission lines show a modulation in radial velocity with an orbital period of Porb = (4.033 ± 0.005) h. Conclusions. The two periodicities indicate that IGR J00234+6141 is a magnetic CV of the intermediate polar type. This is one of the faintest and hardest sources of this type detected by INTEGRAL. This confirms earlier conclusions that intermediate polars contribute significantly to the population of galactic X-ray sources and represent a significant fraction of the high energy background.

A search for ultra-compact dwarf galaxies in the Centaurus galaxy cluster

Aim: To extend the investigations of ultra-compact dwarf galaxies (UCDs) beyond the well studied Fornax and Virgo clusters. Methods: We measured spectroscopic redshifts of about 400 compact object candidates with 19.2 < V < 22.4 mag in the central region of the Centaurus galaxy cluster (d=43Mpc), using VIMOS@VLT. The luminosity range of the candidates covers that of bright globular clusters (GCs) and of UCDs in Fornax and Virgo. Results: We confirm the cluster membership of 27 compact objects, covering an absolute magnitude range -12.2 < M_V < -10.9 mag. We do not find counterparts to the two very large and bright UCDs in Fornax and Virgo with M_V=-13.5 mag, possibly due to survey incompleteness. The compact objects' distribution in magnitude and space is consistent with that of the GC population. Their kinematics and spatial distribution associate them to the central galaxies rather than to the overall cluster potential. The compact objects have a mean metallicity consistent with that of the metal-rich globular cluster sub-population. Compact objects with high S/N spectra exhibit solar [alpha/Fe] abundances, consistent with typical dwarf elliptical galaxy values and unlike galactic bulge globular clusters. HST based size estimates for a sub-sample of eight compact objects reveal the existence of one very large object with half-light radius r_h around 30 pc, having M_V=-11.6 mag (~10^7 M_sun). This source shows super-solar [alpha/Fe] abundances. Seven further sources are only marginally larger than typical GCs with r_h in the range 4 to 10 pc. Conclusions: We consider the largest compact object found to be the only bona-fide UCD detected in our study. In order to improve our understanding of UCDs in Centaurus, a significant increase of our survey completeness is necessary.

Chemical Properties of Star-Forming Dwarf Galaxies

Recent studies of the near-infrared (NIR) properties of dwarf irregular galaxies (dIs) and blue compact dwarfs (BCDs) have provided improved estimates for the NIR luminosity of old stellar populations in these galaxies. These can be used to derive gas fractions, and thereby to evaluate how BCDs have evolved with respect to dIs. Oxygen abundances have been derived for four BCDs in the Virgo Cluster from a run at Gemini-North in 2003. Combining these new abundances with published values, we study the correlations among the metallicity, Ks luminosity, gas mass, baryonic mass, and gas fraction. Within errors, the two types of dwarfs appear to share a common relation between the oxygen abundance and the luminosity in Ks. The correlation between metallicity and the gas fraction is the same for BCDs as for dIs, indicating that BCD evolution has been similar to dIs. Since dIs appear to have evolved as isolated systems, the BCD bursts are unlikely to be a consequence of gas infall or merging.

Discovery of a nearby L–T transition object in the Southern Galactic plane*

We present the discovery of 2MASS J11263991-5003550identified as part of an ongoing survey to discover ultra-cool dwarfs in the Southern Galactic Plane, using data from the 2MASS and SuperCOSMOS Sky Surveys. Strong FeH and H2O features in the near-infrared JH-band spectrum reveal characteristics seen in both mid-L, and L-T transition type dwarfs. We suggest these may be attributable to holes in the condensate cloud layers in the atmosphere of a single sub-stellar object, but cannot at present completely rule out the role of binarity as the possible cause. We also identify this object as a blue L dwarf, and explore the similar observable characteristics of these objects with those of the L-T transition. From this comparison we suggest that the temperature (and thus spectral type) at which the condensate cloud later begins to break-up/rain out, may be highly sensitive to small variations in metallicity. However, the JH-band spectrum of this object does not resemble that of the known L subdwarfs, and therefore extreme metal deficiency may not in fact be responsible for the discordant features. We estimate a spectral type of L9±1, and measure a large proper-motion of µ(tot)=1. ′′ 65 ± 0. 03 yr −1 . Also, a spectrophotometric distance of 8.2pc is estimated, possibly making this object the nearest easily observable single L-T transition object in the southern hemisphere.

The Solar Neighborhood. XIX. Discovery and Characterization of 33 New Nearby White Dwarf Systems

We present medium resolution spectroscopy and multi-epoch VRI photometry for 21 new nearby (< 50 pc) white dwarf systems brighter than V ~ 17. Of the new systems, ten are DA (including a wide double degenerate system with two DA components), eight are DC, two are DZ, and one is DB. In addition, we include multi-epoch VRI photometry for eleven known white dwarf systems that do not have trigonometric parallax determinations. Using model atmospheres relevant for various types of white dwarfs (depending on spectral signatures), we perform spectral energy distribution modeling by combining the optical photometry with the near-infrared JHK from the Two Micron All-Sky Survey to derive physical parameters (i.e., effective temperature and distance estimates). We find that twelve new and six known white dwarf systems are estimated to be within the NStars and Catalog of Nearby Stars horizons of 25 pc. Coupled with identical analyses of the 56 white dwarf systems presented in Paper XIX of this series, a total of 20 new white dwarf systems and 18 known white dwarf systems are estimated to be within 25 pc. These 38 systems of the 88 total studied represent a potential 34% increase in the 25 pc white dwarf population (currently known to consist of 110 systems with trigonometric parallaxes of varying qualities). We continue an ongoing effort via CTIOPI to measure trigonometric parallaxes for the systems estimated to be within 25 pc to confirm proximity and further fill the incompleteness gap in the local white dwarf population. Another 38 systems (both new and known) are estimated to be between 25 and 50 pc and are viable candidates for ground-based parallax efforts wishing to broaden the horizon of interest.

Physical and Spectral Characteristics of the T8 and Later Type Dwarfs

We use newly observed and published near-infrared spectra, together with synthetic spectra obtained from model atmospheres, to derive physical properties of three of the latest type T dwarfs. A new R ≈ 1700 spectrum of the T7.5 dwarf HD 3651B, together with existing data, allows a detailed comparison to the well-studied and very similar dwarf Gl 570D. We find that HD 3651B has both higher gravity and higher metallicity than Gl 570D, with best-fit atmospheric parameters of Teff = 820-830 K, log g = 5.4-5.5, [m/H] = +0.2, and Kzz = 104 cm2 s-1. Its age is 8-12 Gyr, and its implied mass is 60-70 MJ. We perform a similar analysis of the T8 and T7.5 dwarfs 2MASS J09393548-2448279 and 2MASS J11145133-2618235 using published data, comparing them to the well-studied T8, 2MASS J04151954-0935066. We find that these two dwarfs have effectively the same Teff as the reference dwarf, and similar or slightly higher gravities, but lower metallicities. The derived parameters are Teff = 725-775 K and [m/H] = -0.3; log g = 5.3 - 5.45 for 2MASS J09393548-2448279 and log g = 5.0 - 5.3 for 2MASS J11145133-261823. The age and mass are ~10 Gyr and 60 MJ for 2MASS J09393548-2448279, and ~5 Gyr and 40 MJ for 2MASS J11145133-261823. A serious limitation to such analyses is the incompleteness of the line lists for transitions of CH4 and NH3 at λ ≤ 1.7 μm, which are also needed for synthesizing the spectrum of the later, cooler, Y type. Spectra of Saturn and Jupiter, and of laboratory CH4 and NH3 gas, suggest that NH3 features in the Y and J bands may be useful as indicators of the next spectral type, and not features in the H and K bands, as previously thought. However, until cooler objects are found, or the line lists improve, large uncertainties remain, as the abundance of NH3 is likely to be significantly below the chemical equilibrium value. Moreover, inclusion of laboratory NH3 opacities in our models predicts band shapes that are discrepant with existing data. It is possible that the T spectral class will have to be extended to temperatures around 400 K, when water clouds condense in the atmosphere and dramatically change the spectral energy distribution of the brown dwarf.

The Incidence of Dwarf Novae in Large Area Transient Searches

Understanding and quantifying the contribution of known classes of transient and variable sources is an important lesson to be learned from the manifold of precursor programs of the near-future large synoptic sky survey programs such as SkyMapper, Pan-STARRS, and LSST. With this goal in mind, we undertook photometric and spectroscopic follow-up observations of four recently reported unidentified transients. For two sources, WFI J132813.7-214237 and WFI J161953.3+031909, we show that unfortunate coincidences led to their previous designation as transients. While the former is now interpreted as the spatial coincidence of a solar system object with a faint background star, the latter is merely a cataclysmic variable unfortunately caught in and out of eclipse. The third candidate, ROTSE3 J160213.1-021311.7, is identified as an SU UMa type dwarf nova with quiescent brightness of R ~ 22.7 and an outburst amplitude of ~5 mag. The fourth event, SDSS-SN 15207, similarly shows evidence for a dwarf nova origin. Our main conclusion is that cataclysmic variables in their various avatars will contribute moderately to the population of transient objects.

Evidence for a Planetary Companion around a Nearby Young Star

We report evidence for a planetary companion around the nearby young star HD 70573. The star is a G type dwarf located at a distance of 46 pc with age estimation between 20 and 300 Myrs. We carried out spectroscopic observations of this star with FEROS at the 2.2 m MPG/ESO telescope at La Silla. Our spectroscopic analysis yields a spectral type of G1-1.5V and an age of about 100 Myrs. Variations in stellar radial velocity of HD 70573 have been monitored since December 2003 until January 2007. The velocity accuracy of FEROS within this period is about 10 m/s. HD 70573 shows a radial velocity variation with a period of 852 +/- 12 days and a semi-amplitude of 149 +/- 6 m/s. The period of this variation is significantly longer than its rotational period, which is 3.3 days. Based on the analysis of the Ca II K emission line, Halpha and Teff variation as stellar activity indicators as well as the lack of a correlation between the bisector velocity span and the radial velocity, we can exclude the rotational modulation and non-radial pulsations as the source of the long-period radial velocity variation. Thus, the presence of a low-mass companion around the star provides the best explanation for the observed radial velocity variation. Assuming a primary mass m1=1.0 +/- 0.1 Msun for the host star, we calculated a minimum mass of the companion m2sini of 6.1 Mjup, which lies in the planetary mass regime, and an orbital semi-major axis of 1.76 AU. The orbit of the planet has an eccentricity of e=0.4. The planet discovery around the young star HD 70573 gives an important input for the study of debris disks around young stars and their relation to the presence of planets.

분화용 및 가로수형 무궁화 품종의 광합성 특성

무궁화 품종별 생장특성을 조사한 결과 수고는 불새 품종이 가장 높았고, 소양 품종이 가장 작은 값을 나타내었으며, 분지와 잎의 수는 백단심<TEX>${\times}$</TEX>경북1 품종이 가장 많았다. 무궁화 품종별 꽃 특성을 조사한 결과 선덕과 불새 품종의 화폭과 화고는 가장 높은 값을 보였으며, 소양 품종이 가장 작은 값을 나타내었다. 전반적으로 엽록소 함량(a, b, a+b)의 품종별 차이는 선덕과 불새 품종에서 가장 근소한 것으로 나타났다. 백단심<TEX>${\times}$</TEX>경북1과 소양 품종은 불새와 선덕 품종에 비하여 광보상점과 광포화점이 낮았으며, 암호흡 및 광합성능력 그리고 순량지수율도 모두 낮은 것으로 분석되었다. 이러한 결과로 비추어 볼 때, 무궁화 품종 백단심<TEX>${\times}$</TEX>경북1과 소양은 분화용으로서의 개발 가능성이 높은 것으로 평가된다. This study was conducted to find characteristics of photosynthesis for dwarf and street tree cultivars of Hibiscus syriacus L. Characteristics of growth and flowering on some cultivars were investigated. The photosynthetic capacity for the cultivars was also measured. Bulsae showed the best height increase; Soyang represented the lowest growth. Baektanshim <TEX>${\times}$</TEX> Kyungbuk1 had the largest number of branches and leaves. Sundeok and Bulsae were the best in terms of height and width of flower, Soyang showed the smallest flowers. According to the analysis of chlorophyll content (chlorophyll a, b, total) for the cultivars, there was little difference between Sundeok and Bulsae. Based on light response curves for cultivars, it was found that Baektanshim <TEX>${\times}$</TEX> Kyungbuk1 and Soyang had lower light compensation and light saturation points than Bulsae and Sundeok. Cultivars Baektanshim <TEX>${\times}$</TEX> Kyungbuk1 and Soyang were also low in dark respiration, photosynthetic capacity, and net apparent quantum yield. In conclusion, it is recommended that cultivars Baektanshim <TEX>${\times}$</TEX> Kyungbuk1 and Soyang might be used as a dwarf type due to photosynthetic capacity.

Discovery of photospheric argon in very hot central stars of planetary nebulae and white dwarfs

Context. We report the first discovery of argon in hot evolved stars and white dwarfs. We have identified the Ar vii 1063.55 A line in some of the hottest known (Teff = 95 000−110 000 K) central stars of planetary nebulae and (pre-) white dwarfs of various spectral type. Aims. We determine the argon abundance and compare it to theoretical predictions from stellar evolution theory as well as from diffusion calculations. Methods. We analyze high-resolution spectra taken with the Far Ultraviolet Spectroscopic Explorer. We use non-LTE line-blanketed model atmospheres and perform line-formation calculations to compute synthetic argon line profiles. Results. We find a solar argon abundance in the H-rich central star NGC 1360 and in the H-deficient PG 1159 star PG 1424+535. This confirms stellar evolution modeling that predicts that the argon abundance remains almost unaffected by nucleosynthesis. For the DAO-type central star NGC 7293 and the hot DA white dwarfs PG 0948+534 and RE J1738+669 we find argon abundances that are up to three orders of magnitude smaller than predictions of calculations assuming equilibrium of radiative levitation and gravitational settling. For the hot DO white dwarf PG 1034+001 the theoretical overprediction amounts to one dex. Conclusions. Our results confirm predictions from stellar nucleosynthesis calculations for the argon abundance in AGB stars. The argon abundance found in hot white dwarfs, however, is another drastic example that the current state of equilibrium theory for trace elements fails to explain the observations quantitatively.

WX Ceti: a closer look at its behaviour in quiescence and outburst

Context. WX Cet is a dwarf nova with rare outbursts of large amplitude. Aims. We compile the available data of WX Cet, compare the results with other SU UMa stars, and discuss our findings in the context of current theories of superhumps and superoutbursts to progress with our understanding of SU UMa stars. Methods. We analyse all recorded outbursts of WX Cet, based on the AAVSO archive and other published sources, and present new CCD photometry during two recent superoutbursts, including the determination of the corresponding periodicities. We perform numerical disc instability model calculations and compare its predictions with the observations. Results. WX Cet is a SU UMa type dwarf nova with a superoutburst cycle of 880 days on average, and short eruptions every 200 days. It seems that the outburst cycle length increased by nearly a factor of 2 during the past 70 years. According to our numerical simulations, this can be explained in the context of the disc instability model by assuming enhanced mass transfer during outburst and a decreasing mean mass transfer rate during the last decades. Using the data available, we refine the orbital period of WX Cet to 0.0582610 ± 0.0000002 days and interpret the orbital hump found in quiescence as emission from the hot spot. During two recent superoutbursts in July 2001 and December 2004 we observed superhumps, with a rather large positive period derivative of u Ps/Ps = 1.6 × 10 −4 , present only during the first 9 days of a superoutburst. Afterwards and during decline from the “plateau” phase, a constant superhump period of about 0.05922 days was observed. Late superhumps are present for at least 12 days after the decline from the “plateau”, with a period of 0.05927 days. We find this phenomenology difficult to interpret in the context of the standard explanation for superhumps, i.e. the thermal tidal instability model. Conclusions. We interpret the long-term light curve of WX Cet as the result of a significantly decreasing mean mass transfer rate. Highlighting the complexity of the observed superhump light curves, we emphasise the importance of WX Cet for a proper understanding of the SU UMa star outburst physics and the evolution of ultra-short period cataclysmic variables.

The star formation history of dwarf galaxies: First results of the MAGPOP-ITP

The MAGPOP-ITP is a large observational project of the EU MAGPOP Network. In order to study the star formation history of dwarf galaxies, a total of 60 nights was allocated on the 4 large La Palma telescopes in the framework of an International Time Programme. The programme comprises continuum, Hα and near-infrared imaging and optical spectroscopy of quiescent and star-forming dwarfs in the field and in the Virgo Cluster. All galaxies in the sample are selected to have imaging in the UV (by GALEX) and in the optical (by SDSS), and many have additional data available in the GOLDMine database.The aim of the programme is to study the star formation of dwarf galaxies in function of their environment and dwarf type. In this contribution, I will focus on the results obtained for dwarf elliptical galaxies (dEs) using long-slit optical spectroscopy obtained with the NOT. We derive mean luminosity-weighted ages and metallicities using Lick indices for 25 dEs.The dEs show a large spread in both ages and metallicities, but overall they are younger and less metal-rich than normal ellipticals (Es). The [α/Fe] ratios of the dEs are compatible with solar ratios, or are even slightly underabundant, pointing at prolonged star formation histories for dEs.

Star formation history and evolution of gas-rich dwarf galaxies in the Centaurus A group

We analyse the properties of three unusual dwarf galaxies in the Centaurus A group discovered with the HIPASS survey. From their optical morphology, they appear to be low surface brightness dwarf spheroidals, yet they are gas rich (MH i/LB > 1) with gas-mass-to-stellar light ratios larger than typical dwarf irregular galaxies. Therefore these systems appear different from any dwarfs of the Local Group. They should be favoured hosts for starburst, whereas we find a faint star formation region in only one object. We have obtained 21-cm data and Hubble Space Telescope photometry in V and I bands, and have constructed colour magnitude diagrams (CMDs) to investigate their stellar populations and to set a constraint on their age. From the comparison of the observed and model CMDs, we infer that all three galaxies are at least older than 2 Gyr (possibly even as old as 10 Gyr) and remain gas rich because their star formation rates have been very low (≲10−3 M⊙ yr−1) throughout. In such systems, star formation appears to have been sporadic and local, though one object (HIPASS J1321−31) has a peculiar red plume in its CMD suggesting that many of its stars were formed in a ‘miniburst’ 300–500 Myr ago. The question of why there are no similar dwarf galaxies in the Local Group remains open.

Infrared Spectroscopy of the Ultra-Low-Mass Binary Oph 162225-240515

Binary properties are an important diagnostic of the star and brown dwarf formation processes. While wide binaries appear to be rare in the substellar regime, recent observations have revealed Ophiuchus 162225-240515 (2MASS J16222521-2405139) as a likely young ultra-low-mass binary with an apparent separation of ≈240 AU. Here we present low-resolution near-infrared spectra of the pair from NTT/SOFI (R ~ 600) and VLT/ISAAC (R ~ 1400), covering the 1.0-2.5 μm spectral region. By comparing them to model atmospheres from Chabrier & Baraffe and Burrows et al., we confirm the surface temperatures to be TA = 2350 ± 150 K and TB = 2100 ± 100 K for the two components of the binary, consistent with earlier estimates from optical spectra. Using gravity-sensitive K I features, we find the surface gravity to be significantly lower than field dwarfs of the same spectral type, providing the best evidence so far that these objects are indeed young. However, we find that models are not sufficiently reliable to infer accurate ages/masses from surface gravity. Instead, we derive masses of MA = 13MJ and MB = 10MJ for the two objects using the well-constrained temperatures and assuming an age of 1-10 Myr, consistent with the full range of ages reported for the Oph region.