Dwarf Host Research Articles

K-band spectroscopic metallicities and temperatures of M-dwarf stars

I present the metallicity and effective temperature techniques developed for M dwarf stars by Rojas-Ayala et al. (2010, 2012). These techniques are based on absorption features present in the modest resolution K-band spectra (R ∼2700) of M dwarfs and have been calibrated using FGK+M dwarf pairs and synthetic atmosphere models. The H2O-K2 index seems to overestimate the effective temperatures of M dwarfs when compared to interferometric measurements. The metallicity distribution of the M dwarf host candidates by the Kepler Mission hints that jovian-size planets form preferentially around solar and super- solar metallicity environments, while small rocky planet host exhibit a wide range of metallicities, just like in their solar-type counterparts.

WASP-52b, WASP-58b, WASP-59b, and WASP-60b: Four new transiting close-in giant planets

We present the discovery of four new transiting hot jupiters, detected mainly from SuperWASP-North and SOPHIE observations. These new planets, WASP-52b, WASP-58b, WASP-59b, and WASP-60b, have orbital periods ranging from 1.7 to 7.9 days, masses between 0.46 and 0.94 M_Jup, and radii between 0.73 and 1.49 R_Jup. Their G1 to K5 dwarf host stars have V magnitudes in the range 11.7-13.0. The depths of the transits are between 0.6 and 2.7%, depending on the target. With their large radii, WASP-52b and 58b are new cases of low-density, inflated planets, whereas WASP-59b is likely to have a large, dense core. WASP-60 shows shallow transits. In the case of WASP-52 we also detected the Rossiter-McLaughlin anomaly via time-resolved spectroscopy of a transit. We measured the sky-projected obliquity lambda = 24 (+17/-9) degrees, indicating that WASP-52b orbits in the same direction as its host star is rotating and that this prograde orbit is slightly misaligned with the stellar equator. These four new planetary systems increase our statistics on hot jupiters, and provide new targets for follow-up studies.

WTS: A near-infrared transit survey

The WFCAM Transit Survey is a transiting planet survey running on the United Kingdom Infrared Telescope targeting M dwarf stars in the near infrared. The survey has been operating since 2007 and gathering photometric time series of about 15000M dwarf stars brighter than J = 17mag. We identified and followed-up planet candidates from the most complete field and found two hot Jupiters around non-M dwarf hosts (WTS-1b & WTS-2b) but found no planets around the M dwarfs. 1. OBSERVATIONS AND DATA QUALITY The WFCAM Transit Survey (WTS) is an ongoing campaign survey on the United Kingdom Infrared Telescope (UKIRT) operating in the near-infrared, in J-band since 2007. The WTS targets 4 superfields, two in the winter and two in the summer sky and each covering 1.6 square degrees in size. A field is made up of 8 pawprints with slightly overlapping regions at the edges. The WTS operates as a backup program of the large UKIRT surveys (such as the UKIDSS surveys and UHS). The WTS runs in queue mode using 1 or 0.5 hour long observing blocks resulting 4 or 2 epochs per block respectively. The exposure time is 90s. Table 1 summarizes main properties of the fields and the current number of epochs. We expect to complete 1 2-3 fields before the closure of UKIRT in 2013. The main science goal to identify planets orbiting around M dwarfs or earlier stars. Other science topics includes but not limited to characterizing of low mass eclipsing binaries; studying M dwarf variability in the J band; identifying high proper motion sources e.g. cool white dwarfs, brown dwarfs, TNOs. Figure 1 summarizes our light curve noise properties in the 19hr field. The saturation appears at J = 13mag. We reach 3-4mmag precision at the bright end and at J = 17mag where the sky noise dominates we have a precision of 2%. Mainly because of follow-up possibilities, we limit our main science goal of finding planets to stellar sources brighter than J = 17magnitude, but for other science topics we used fainter sources, too. For reference we show the signal depth for a few star-planet systems. In Figure 2 we present a basic sensitivity diagram, showing the effect of the irregular observing pattern and number of epochs in the different fields. We used a simple simulation of a Neptune sized planet transiting around an M0 star. The simulation provides us an unbiased sample of different transit lengths and timings derived from random geometry of inclination (we allow grazing systems, too).

The luminosity function of diverse satellite galaxy systems

The high-resolution, SPH galaxies of the McMaster Unbiased Galaxy Survey (MUGS) are used to examine the satellite systems of sixteen model host galaxies. Each galaxy has a different mass, angular momentum and merger history that yield a rich set of satellite luminosity functions. With new observations of distant satellite systems, we can compare these luminosity functions to satellite systems beyond the Local Group. We find that the luminosity functions of our simulations compare well to observations when the luminosity functions are scaled according to host mass. We use the recently-found relationship between dwarf satellites and host mass in distant satellite systems (Trentham & Tully 2009) to normalize a theoretical, complete luminosity function for the Milky Way (Koposov et al. 2008). The luminosity function of satellites, expressed as a function of the host mass, is given by dN/dM_V= 3.5 x M_host^(0.91) x 10^(0.1 x M_V-10.2), where host mass is given in M_sun. The mass of a host galaxy can be used to predict the number of dwarf satellites and even when considering spiral and elliptical hosts separately this relation holds.

On the association between core-collapse supernovae and H ii regions

Previous studies of the location of core-collapse supernovae (ccSNe) in their host galaxies have variously claimed an association with HII regions; no association; or an association only with hydrogen-deficient ccSNe. Here, we examine the immediate environments of 39 ccSNe whose positions are well known in nearby (<15 Mpc), low inclination (<65 degrees) hosts using mostly archival, continuum-subtracted H-alpha ground-based imaging. We find that 11 out of 29 hydrogen-rich ccSNe are spatially associated with HII regions (38 +/- 11%), versus 7 out of 10 hydrogen-poor ccSNe (70 +/- 26%). Similar results from Anderson et al. led to an interpretation that the progenitors of type Ib/c ccSNe are more massive than those of type II ccSNe. Here, we quantify the luminosities of HII region either coincident with, or nearby to the ccSNe. Characteristic nebulae are long-lived (~20 Myr) giant HII regions rather than short-lived (~4 Myr) isolated, compact HII regions. Therefore, the absence of a HII region from most type II ccSNe merely reflects the longer lifetime of stars with <12 Msun than giant HII regions. Conversely, the association of a HII region with most type Ib/c ccSNe is due to the shorter lifetime of stars with >12 Msun stars than the duty cycle of giant HII regions. Therefore, we conclude that the observed association between certain ccSNe and HII provides only weak constraints upon their progenitor masses. Nevertheless, we do favour lower mass progenitors for two type Ib/c ccSNe that lackassociated nebular emission, a host cluster or a nearby giant HII region. Finally, we also reconsider the association between long Gamma Ray Bursts and the peak continuum light from their (mostly) dwarf hosts, and conclude that this is suggestive of very high mass progenitors, in common with previous studies.

MICROLENSING BINARIES WITH CANDIDATE BROWN DWARF COMPANIONS

Brown dwarfs are important objects because they may provide a missing link between stars and planets, two populations that have dramatically different formation history. In this paper, we present the candidate binaries with brown dwarf companions that are found by analyzing binary microlensing events discovered during 2004 - 2011 observation seasons. Based on the low mass ratio criterion of q < 0.2, we found 7 candidate events, including OGLE-2004-BLG-035, OGLE-2004-BLG-039, OGLE-2007-BLG-006, OGLE-2007-BLG-399/MOA-2007-BLG-334, MOA-2011-BLG-104/OGLE-2011-BLG-0172, MOA-2011-BLG-149, and MOA-201-BLG-278/OGLE-2011-BLG-012N. Among them, we are able to confirm that the companions of the lenses of MOA-2011-BLG-104/OGLE-2011-BLG-0172 and MOA-2011-BLG-149 are brown dwarfs by determining the mass of the lens based on the simultaneous measurement of the Einstein radius and the lens parallax. The measured mass of the brown dwarf companions are (0.02 +/- 0.01) M_Sun and (0.019 +/- 0.002) M_Sun for MOA-2011-BLG-104/OGLE-2011-BLG-0172 and MOA-2011-BLG-149, respectively, and both companions are orbiting low mass M dwarf host stars. More microlensing brown dwarfs are expected to be detected as the number of lensing events with well covered light curves increases with new generation searches.

THE FLAT TRANSMISSION SPECTRUM OF THE SUPER-EARTH GJ1214b FROM WIDE FIELD CAMERA 3 ON THEHUBBLE SPACE TELESCOPE

Capitalizing on the observational advantage offered by its tiny M dwarf host, we present HST/WFC3 grism measurements of the transmission spectrum of the super-Earth exoplanet GJ1214b. These are the first published WFC3 observations of a transiting exoplanet atmosphere. After correcting for a ramp-like instrumental systematic, we achieve nearly photon-limited precision in these observations, finding the transmission spectrum of GJ1214b to be flat between 1.1 and 1.7 microns. Inconsistent with a cloud-free solar composition atmosphere at 8.2 sigma, the measured achromatic transit depth most likely implies a large mean molecular weight for GJ1214b's outer envelope. A dense atmosphere rules out bulk compositions for GJ1214b that explain its large radius by the presence of a very low density gas layer surrounding the planet. High-altitude clouds can alternatively explain the flat transmission spectrum, but they would need to be optically thick up to 10 mbar or consist of particles with a range of sizes approaching 1 micron in diameter.

Formation of massive globular clusters with heavy element abundance spread in the Galactic building blocks

ABSTRACT A growing number of recent observations have revealed that the Galactic globular cluster (GC) ω Cen is not the only GC that shows abundance spread in heavy elements (e.g. Fe). In order to understand the origin of the Galactic GCs with heavy element abundance spread (HEAS), we investigate the formation processes of massive GCs (MGCs) with masses larger than 106 M⊙ in gas-rich dwarf galaxies interacting and merging with the very young Galaxy. We find that massive and compact stellar clumps with masses larger than 106 M⊙, which can be regarded as progenitors of MGCs, can form from massive gas clumps that are developed through merging of gaseous regions initially at different radii and thus with different metallicities. Therefore, it is inevitable that MGCs formed in dwarfs have HEAS. The abundance spread in each individual MGC depends on the radial metallicity gradient of the host dwarf such that it can be larger for the steeper metallicity gradient. For example, MGCs formed in a dwarf with a central metallicity of [Fe/H] =−1.1 and the radial gradient of ∼−0.2 dex kpc−1 can have the abundance spread of Δ[Fe/H] ∼ 0.2. The simulated MGCs appear to be significantly flattened owing to their dissipative formation from gas discs of their host dwarfs. Based on these results, we discuss possibly diverse formation mechanisms for the Galactic GCs such as M22, M54, NGC 2419, ω Cen and Terzan 5.

On the origin of the stellar halo and multiple stellar populations in the globular cluster NGC 1851

We propose that the observed stellar halo around the globular cluster (GC) NGC 1851 is evidence for its formation in the central region of its defunct host dwarf galaxy. We numerically investigate the long-term dynamical evolution of a nucleated dwarf galaxy embedded in a massive dark matter halo under the strong tidal field of the Galaxy. The dwarf galaxy is assumed to have a stellar nucleus (or a nuclear star cluster) that could be the progenitor for NGC 1851. We find that although the dark matter halo and the stellar envelope of the host dwarf of NGC 1851 can be almost completely stripped during its orbital evolution around the Galaxy, a minor fraction of stars in the dwarf can remain trapped by the gravitational field of the nucleus. The stripped nucleus can be observed as NGC 1851 with no/little dark matter whereas stars around the nucleus can be observed as a diffuse stellar halo around NGC 1851. The simulated stellar halo has a symmetric distribution with a power-law density slope of ~ -2 and shows no tidal tails within ~200pc from NGC 1851. We show that two GCs can merge with each other to form a new nuclear GC embedded in field stars owing to the low stellar velocity dispersion of the host dwarf. This result makes no assumption on the ages and/or chemical abundances of the two merging GCs. Thus the observed stellar halo and characteristic multiple stellar populations in NGC 1851 suggest that NGC 1851 could have formed initially in the central region of an ancient dwarf galaxy.

ACCRETION OF A TERRESTRIAL-LIKE MINOR PLANET BY A WHITE DWARF

We present optical and infrared characterization of the polluted DAZ white dwarf GALEX J193156.8+011745. Imaging and spectroscopy from the ultraviolet to the thermal infrared indicates that the white dwarf hosts excess infrared emission consistent with the presence of an orbiting dusty debris disk. In addition to the five elements previously identified, our optical echelle spectroscopy reveals chromium and manganese and enables restrictive upper limits on several other elements. Synthesis of all detections and upper limits suggests that the white dwarf has accreted a differentiated parent body. We compare the inferred bulk elemental composition of the accreted parent body to expectations for the bulk composition of an Earth-like planet stripped of its crust and mantle and find relatively good agreement. At least two processes could be important in shaping the final bulk elemental composition of rocky bodies during the late phases of stellar evolution: irradiation and interaction with the dense stellar wind.

ASTROPHYSICAL PARAMETERS AND HABITABLE ZONE OF THE EXOPLANET HOSTING STAR GJ 581

GJ 581 is an M dwarf host of a multiplanet system. We use long-baseline interferometric measurements from the CHARA Array, coupled with trigonometric parallax information, to directly determine its physical radius to be $0.299 \pm 0.010 R_{\odot}$. Literature photometry data are used to perform spectral energy distribution fitting in order to determine GJ 581's effective surface temperature $T_{\rm EFF}=3498 \pm 56$ K and its luminosity $L=0.01205 \pm 0.00024 L_{\odot}$. From these measurements, we recompute the location and extent of the system's habitable zone and conclude that two of the planets orbiting GJ 581, planets d and g, spend all or part of their orbit within or just on the edge of the habitable zone.

WANDERING BLACK HOLES IN BRIGHT DISK GALAXY HALOS

We perform SPH+N-body cosmological simulations of massive disk galaxies, including a formalism for black hole seed formation and growth, and find that satellite galaxies containing supermassive black hole seeds are often stripped as they merge with the primary galaxy. These events naturally create a population of "wandering" black holes that are the remnants of stripped satellite cores; galaxies like the Milky Way may host 5 -- 15 of these objects within their halos. The satellites that harbor black hole seeds are comparable to Local Group dwarf galaxies such as the Small and Large Magellanic Clouds; these galaxies are promising candidates to host nearby intermediate mass black holes. Provided that these wandering black holes retain a gaseous accretion disk from their host dwarf galaxy, they give a physical explanation for the origin and observed properties of some recently discovered off-nuclear ultraluminous X-ray sources such as HLX-1.

CORE-COLLAPSE SUPERNOVAE FROM THE PALOMAR TRANSIENT FACTORY: INDICATIONS FOR A DIFFERENT POPULATION IN DWARF GALAXIES

We use the first compilation of 72 core-collapse supernovae (SNe) from the Palomar Transient Factory (PTF) to study their observed subtype distribution in dwarf galaxies compared to giant galaxies. Our sample is the largest single-survey, untargeted, spectroscopically classified, homogeneous collection of core-collapse events ever assembled, spanning a wide host-galaxy luminosity range (down to M_r ~ -14 mag) and including a substantial fraction (>20%) of dwarf (M_r >= -18 mag) hosts. We find more core-collapse SNe in dwarf galaxies than expected and several interesting trends emerge. We use detailed subclassifications of stripped-envelope core-collapse SNe and find that all Type I core-collapse events occurring in dwarf galaxies are either SNe Ib or broad-lined SNe Ic (SNe Ic-BL), while "normal" SNe Ic dominate in giant galaxies. We also see a significant excess of SNe IIb in dwarf hosts. We hypothesize that in lower metallicity hosts, metallicity-driven mass loss is reduced, allowing massive stars that would have appeared as "normal" SNe Ic in metal-rich galaxies to retain some He and H, exploding as Ib/IIb events. At the same time, another mechanism allows some stars to undergo extensive stripping and explode as SNe Ic-BL (and presumably also as long-duration gamma-ray bursts). Our results are still limited by small-number statistics, and our measurements of the observed N(Ib/c)/N(II) number ratio in dwarf and giant hosts (0.25_{-0.15}^{+0.3} and 0.23_{-0.08}^{+0.11}, respectively; 1 sigma uncertainties) are consistent with previous studies and theoretical predictions. As additional PTF data accumulate, more robust statistical analyses will be possible, allowing the evolution of massive stars to be probed via the dwarf-galaxy SN population.

Accreted versusin situMilky Way globular clusters

Here we examine the Milky Way's GC system to estimate the fraction of accreted versus in situ formed GCs. We first assemble a high quality database of ages and metallicities for 93 Milky Way GCs from literature deep colour-magnitude data. The age-metallicity relation for the Milky Way's GCs reveals two distinct tracks -- one with near constant old age of ~12.8 Gyr and the other branches to younger ages. We find that the latter young track is dominated by globular clusters associated with the Sagittarius and Canis Major dwarf galaxies. Despite being overly simplistic, its age-metallicity relation can be well represented by a simple closed box model with continuous star formation. The inferred chemical enrichment history is similar to that of the Large Magellanic Cloud, but is more enriched, at a given age, compared to the Small Magellanic Cloud. After excluding Sagittarius and Canis Major GCs, several young track GCs remain. Their horizontal branch morphologies are often red and hence classified as Young Halo objects, however they do not tend to reveal extended horizontal branches (a possible signature of an accreted remnant nucleus). Retrograde orbit GCs (a key signature of accretion) are commonly found in the young track. We also examine GCs that lie close to the Fornax-Leo-Sculptor great circle defined by several satellite galaxies. We find that several GCs are consistent with the young track and we speculate that they may have been accreted along with their host dwarf galaxy, whose nucleus may survive as a GC. Finally, we suggest that 27-47 GCs (about 1/4 of the entire system), from 6-8 dwarf galaxies, were accreted to build the Milky Way GC system we seen today.

Dwarf Galaxies in Voids: Dark Matter Halos and Gas Cooling

Galaxy surveys have shown that luminous galaxies are mainly distributed in large filaments and galaxy clusters. The remaining large volumes are virtually devoid of luminous galaxies. This is in concordance with the formation of the large‐scale structure in the universe as derived from cosmological simulations. However, the numerical results indicate that cosmological voids are abundantly populated with dark matter haloes which may in principle host dwarf galaxies. Observational efforts have in contrast revealed that voids are apparently devoid of dwarf galaxies. We investigate the formation of dwarf galaxies in voids by hydrodynamical cosmological simulations. Due to the cosmic ultraviolet background radiation low‐mass haloes show generally a reduced baryon fraction. We determine the characteristic mass below which dwarf galaxies are baryon deficient. We show that the circular velocity below which the accretion of baryons is suppressed is approximately 40 kms−1. The suppressed baryon accretion is caused by the photo‐heating due to the UV background. We set up a spherical halo model and show that the effective equation of the state of the gas in the periphery of dwarf galaxies determines the characteristic mass. This implies that any process which heats the gas around dwarf galaxies increases the characteristic mass and thus reduces the number of observable dwarf galaxies.

Transit detection limits for sub-stellar and terrestrial companions to white dwarfs

The SuperWASP project is a ground-based ultra wide angle search for extra-solar planetary transits that has successfully detected 15 previously unknown planets in the last two years. We have used SuperWASP photometric data to investigate the transit characteristics of and detection limits for brown dwarfs, gas giants and terrestrial companions in orbit around white dwarfs. The relatively small size of a white dwarf host star (approximately 1 Earth radius), implies that any sub-stellar or gas giant companion will completely eclipse it, while terrestrial bodies smaller than the Moon will produce relatively large (> 1%) transits, detectable in good S/N light-curves. We performed extensive simulations using SuperWASP photometric data and we found that for Gaussian random noise we are sensitive to companions as small as the Moon. Our sensitivity drops in the presence of co-variant noise structure, nevertheless Earth-size bodies remain readily detectable in relatively low S/N data. We searched for eclipses and transit signals in a sample of 174 WASP targets, resulting from a cross-correlation of the McCook & Sion catalogue and the SuperWASP data archive. This study found no evidence for sub-stellar or planetary companions in close orbits around our sample of white dwarfs.

LOW-MASS SEYFERT 2 GALAXIES IN THE SLOAN DIGITAL SKY SURVEY

We describe a sample of low-mass Seyfert 2 galaxies selected from the Sloan Digital Sky Survey (SDSS), with a median absolute magnitude of Mg = –19.0 mag. These galaxies are Type 2 counterparts to the Seyfert 1 galaxies with intermediate-mass black holes identified by Greene & Ho. Spectra obtained with the Echellette Spectrograph and Imager at the Keck Observatory are used to determine the central stellar velocity dispersions and to examine the emission-line properties. Overall, the stellar velocity dispersions are low (~40-90 km s–1), and we find 12 objects having σ < 60 km s–1, a range where very few Seyfert 2 galaxies were previously known. The sample follows the correlation between stellar velocity dispersion and FWHM([O III]) seen in more massive Seyfert galaxies, indicating that the narrow-line FWHM values are largely determined by virial motion of gas in the central regions of the host galaxies, but the [O III] emission lines exhibit a higher incidence of redward asymmetries and double-peaked profiles than what is found in typical Seyfert samples. Using estimates of the black hole masses and active galactic nucleus (AGN) bolometric luminosities, we find that these galaxies are typically radiating at a high fraction of their Eddington rate, with a median L bol/L Edd = 0.4. We identify one galaxy, SDSS J110912.40+612346.7, as a Type 2 analog of the nearby dwarf Seyfert 1 galaxy NGC 4395, with a nearly identical narrow-line spectrum and a dwarf spiral host of only Mg = –16.8 mag. The close similarities between these two objects suggest that the obscuring torus of AGN unification models may persist even at the lowest luminosities seen among Seyfert galaxies, below L bol = 1041 ergs s–1. Spectropolarimetry observations of four objects do not reveal any evidence for polarized broad-line emission, but SDSS J110912.40+612346.7 has a continuum polarization significantly in excess of the expected Galactic foreground polarization, a possible indication of scattered light from a hidden nucleus. Forthcoming observations of this sample, including X-ray and mid-infrared spectroscopy, can provide new tests of the obscuring torus model for active galaxies at low luminosities.

Characterizing the Near‐UV Environment of M Dwarfs

We report the results of our HST snapshot survey with the ACS HRC PR200L prism, designed to measure the near-UV emission in a sample of nearby M dwarfs. 33 stars were observed, spanning the mass range from 0.1 - 0.6 solar masses (T_eff ~ 2200K - 4000K) where the UV energy distributions vary widely between active and inactive stars. These observations provide much-needed constraints on models of the habitability zone and the atmospheres of possible terrestrial planets orbiting M dwarf hosts, and will be useful in refining the target selection for future space missions such as TPF. We compare our data with a new generation of M dwarf atmospheric models and discuss their implication for the chromospheric energy budget. These NUV data will also be valuable in conjunction with existing optical, FUV and X-ray data to explore unanswered questions regarding the dynamo generation and magnetic heating in low-mass stars.

SN 2005ap: A Most Brilliant Explosion

We present unfiltered photometric observations with ROTSE-III and optical spectroscopic follow-up with HET and the Keck telescope of the most luminous supernova yet identified, SN 2005ap. The spectra taken about 3 days before and 6 days after maximum light show narrow emission lines (likely originating in the dwarf host) and absorption lines at a redshift of z = 0.2832, which puts the peak unfiltered magnitude at -22.7 ± 0.1 absolute. Broad P Cygni features corresponding to Hα, C III, N III, and O III are further detected with a photospheric velocity of ~20,000 km s-1. Unlike other highly luminous supernovae such as 2006gy and 2006tf that show slow photometric evolution, the light curve of SN 2005ap indicates a 1-3 week rise to peak followed by a relatively rapid decay. The spectra also lack the distinct emission peaks from moderately broadened (FWHM ~2000 km s-1) Balmer lines seen in SN 2006gy and SN 2006tf. We briefly discuss the origin of the extraordinary luminosity from a strong interaction as may be expected from a pair instability eruption or a GRB-like engine encased in a H/He envelope.

Dissipative Transformation of Nonnucleated Dwarf Galaxies into Nucleated Systems

Recent photometric observations by the Hubble Space Telescope have revealed the physical properties of stellar galactic nuclei in nucleated dwarf galaxies in the Virgo cluster of galaxies. In order to elucidate the formation processes of nucleated dwarfs, we numerically investigate gasdynamics, star formation, and chemical evolution within the central 1 kpc of gas disks embedded within the galactic stellar components of nonnucleated dwarfs. We find that high-density, compact stellar systems can be formed in the central regions of dwarfs as a result of dissipative, repeated merging of massive stellar and gaseous clumps developed from nuclear gaseous spiral arms as a consequence of local gravitational instability. The central stellar components are found to have stellar masses that are typically 5% of their host dwarfs' and show very flattened shapes, rotational kinematics, and central velocity dispersions significantly smaller than those of their host dwarfs. We also find that more massive dwarfs can develop more massive, more metal-rich, and higher density stellar systems in their central regions, because star formation and chemical enrichment proceed more efficiently owing to the less dramatic suppression of star formation by supernova feedback effects in more massive dwarfs. Based on these results, we suggest that gas-rich, nonnucleated dwarfs can be transformed into nucleated ones as a result of dissipative gasdynamics in their central regions. We discuss the origin of the observed correlations between physical properties of stellar galactic nuclei and those of their host galaxies.